Difference between revisions of "Introduction to Python"

From FreeCAD Documentation
Jump to navigation Jump to search
m
 
(163 intermediate revisions by 15 users not shown)
Line 1: Line 1:
This is a short tutorial made for who is totally new to Python. [http://en.wikipedia.org/wiki/Python_%28programming_language%29 Python] is an open-source, multiplatform [http://en.wikipedia.org/wiki/Programming_language programming language]. Python has several features that make it very different than other common programming languages, and very accessible to new users like yourself:
+
<languages/>
 +
<translate>
  
*It has been designed specially to be easy to read by human beings, and so it is very easy to learn and understand.
+
<!--T:183-->
*It is interpreted, that is, unlike compiled languages like C, your program doesn't need to be compiled before it is executed. The code you write can be immediately executed, line by line if you want so. This makes it extremely easy to learn and to find errors in your code, because you go slowly, step-by-step.
+
{{Docnav
*It can be embedded in other programs to be used as scripting language. FreeCAD has an embedded Python interpreter, so you can write Python code in FreeCAD, that will manipulate parts of FreeCAD, for example to create geometry. This is extremely powerful, because instead of just clicking a button labeled "create sphere", that a programmer has placed there for you, you have the freedom to create easily your own tool to create exactly the geometry you want.
+
|[[Macros|Macros]]
*It is extensible, you can easily plug new modules in your Python installation and extend its functionality. For example, you have modules that allow Python to read and write jpg images, to communicate with twitter, to schedule tasks to be performed by your operating system, etc.
+
|[[Python_scripting_tutorial|Python scripting tutorial]]
 +
}}
  
So, hands on! Be aware that what will come next is a very simple introduction, by no means a complete tutorial. But my hope is that after that you'll get enough basics to explore deeper into the FreeCAD mechanisms.
+
</translate>
 +
{{TOCright}}
 +
<translate>
  
==The interpreter==
+
==Introduction== <!--T:187-->
  
Usually, when writing computer programs, you simply open a text editor or your special programming environment which is in most case a text editor with several tools around it, write your program, then compile it and execute it. Most of the time you made errors while writing, so your program won't work, and you will get an error message telling you what went wrong. Then you go back to your text editor, correct the mistakes, run again, and so on until your program works fine.
+
<!--T:87-->
 +
This is a short tutorial for those new to [https://en.wikipedia.org/wiki/Python_%28programming_language%29 Python]. Python is an open-source, multiplatform [https://en.wikipedia.org/wiki/Programming_language programming language]. It has several features that make it different from other programming languages, and very accessible to new users:
  
That whole process, in Python, can be done transparently inside the Python interpreter. The interpreter is a Python window with a command prompt, where you can simply type Python code. If you install Python on your computer (download it from the [http://www.python.org Python website] if you are on Windows or Mac, install it from your package repository if you are on GNU/Linux), you will have a Python interpreter in your start menu. But FreeCAD also has a Python interpreter in its bottom part:
+
<!--T:88-->
 +
*It has been designed to be to readable by human beings, making it relatively easy to learn and understand.
 +
*It is interpreted, this means that programs do not need to be compiled before they can be executed. Python code can be executed immediately, even line by line if you wish.
 +
*It can be embedded in other programs as a scripting language. FreeCAD has an embedded Python interpreter. You can write Python code to manipulate parts of FreeCAD. This is very powerful, it means you can build your very own tools.
 +
*It is extensible, you can easily plug new modules into your Python installation and extend its functionality. For example, there are modules that allow Python to read and write images, to communicate with Twitter, to schedule tasks to be performed by your operating system, etc.
  
[[Image:Screenshot_pythoninterpreter.jpg]]
+
<!--T:89-->
 +
The following is a very basic introduction, and by no means a complete tutorial. But hopefully it will provide a good starting point for further exploration into FreeCAD and its mechanisms. We strongly encourage you to enter the code snippets below into a Python interpreter.
  
(If you don't have it, click on View → Views → Python console.)
+
==The interpreter== <!--T:90-->
  
The interpreter shows the Python version, then a >>> symbol, which is the command prompt, that is, where you enter Python code. Writing code in the interpreter is simple: one line is one instruction. When you press Enter, your line of code will be executed (after being instantly and invisibly compiled). For example, try writing this:
+
<!--T:91-->
 +
Usually when writing computer programs, you open a text editor or your special programming environment (which is basically a text editor with some additional tools), write your program, then compile and execute. Often one or more errors were made during entry, so your program won't work. You may even get an error message telling you what went wrong. Then you go back to your text editor, correct the mistakes, run again, repeating until your program works as intended.
  
print "hello"
+
<!--T:92-->
 +
In Python that whole process can be done transparently inside the Python interpreter. The interpreter is a Python window with a command prompt, where you can simply type Python code. If you have installed Python on your computer (download it from the [https://www.python.org/ Python website] if you are on Windows or Mac, install it from your package repository if you are on GNU/Linux), you will have a Python interpreter in your start menu. But, as already mentioned, FreeCAD also has a built-in Python interpreter: the [[Python_console|Python console]].
  
<code>print</code> is a special Python keyword that means, obviously, to print something on the screen. When you press Enter, the operation is executed, and the message "hello" is printed. If you make an error, for example let's write:
+
</translate>
 +
[[Image:FreeCAD_Python_console.png]]
 +
<translate>
 +
<!--T:93-->
 +
{{Caption|The FreeCAD Python console}}
  
print hello
+
<!--T:94-->
 +
If you don't see it, click on {{MenuCommand|View → Panels → Python console}}. The Python console can be resized and also undocked.
  
Python will tell us that it doesn't know what hello is. The " characters specify that the content is a string, which is simply, in programming jargon, a piece of text. Without the ", the print command believed hello was not a piece of text but a special Python keyword. The important thing is, you immediately get notified that you made an error. By pressing the up arrow (or, in the FreeCAD interpreter, CTRL+up arrow), you can go back to the last command you wrote and correct it.
+
<!--T:95-->
 +
The interpreter shows the Python version, then a {{incode|>>>}} symbol which is the command prompt. Writing code in the interpreter is simple: one line is one instruction. When you press {{KEY|Enter}}, your line of code will be executed (after being instantly and invisibly compiled). For example, try writing this:
  
The Python interpreter also has a built-in help system. Try typing:
+
</translate>
 +
{{Code|code=
 +
print("hello")
 +
}}
 +
<translate>
  
help
+
<!--T:96-->
 +
{{incode|print()}} is a Python command that, obviously, prints something on the screen. When you press {{KEY|Enter}}, the operation is executed, and the message {{incode|"hello"}} is printed. If you make an error, for example let's write:
  
or, for example, let's say we don't understand what went wrong with our print hello command above, we want specific information about the "print" command:
+
</translate>
 +
{{Code|code=
 +
print(hello)
 +
}}
 +
<translate>
  
help("print")
+
<!--T:97-->
 +
Python will immediately tell you so. In this case Python doesn't know what {{incode|hello}} is. The {{incode|" "}} characters specify that the content is a string, programming jargon for a piece of text. Without these the {{incode|print()}} command doesn't recognize {{incode|hello}}. By pressing the up arrow you can go back to the last line of code and correct it.
  
You'll get a long and complete description of everything the print command can do.
+
<!--T:98-->
 +
The Python interpreter also has a built-in help system. Let's say we don't understand what went wrong with {{incode|print(hello)}} and we want specific information about the command:
  
Now we dominate totally our interpreter, we can begin with serious stuff.
+
</translate>
 +
{{Code|code=
 +
help("print")
 +
}}
 +
<translate>
  
==Variables==
+
<!--T:100-->
 +
You'll get a long and complete description of everything the {{incode|print()}} command can do.
  
Of course, printing "hello" is not very interesting. More interesting is printing stuff you don't know before, or let Python find for you. That's where the concept of variable comes in. A variable is simply a value that you store under a name. For example, type this:
+
<!--T:101-->
 +
Now that you understand the Python interpreter, we can continue with the more serious stuff.
  
a = "hello"
+
<!--T:188-->
print a
+
[[#top|top]]
  
I guess you understood what happened, we "saved" the string "hello" under the name a. Now, a is not an unknown name anymore! We can use it anywhere, for example in the print command. We can use any name we want, just respecting simple rules, like not using spaces or punctuation. For example, we could very well write:
+
==Variables== <!--T:102-->
  
hello = "my own version of hello"
+
<!--T:103-->
print hello
+
Very often in programming you need to store a value under a name. That's where variables come in. For example, type this:
  
See? now hello is not an undefined word anymore. What if, by terrible bad luck, we choosed a name that already exists in Python? Let's say we want to store our string under the name "print":
+
</translate>
 +
{{Code|code=
 +
a = "hello"
 +
print(a)
 +
}}
 +
<translate>
  
print = "hello"
+
<!--T:104-->
 +
You probably understand what happened here, we saved the string {{incode|"hello"}} under the name {{incode|a}}. Now that {{incode|a}} is known we can use it anywhere, for example in the {{incode|print()}} command. We can use any name we want, we just need to follow some simple rules, such as not using spaces or punctuation and not using Python keywords. For example, we can write:
  
Python is very intelligent and will tell us that this is not possible. It has some "reserved" keywords that cannot be modified. But our own variables can be modified anytime, that's exactly why they are called variables, the contents can vary. For example:
+
</translate>
 +
{{Code|code=
 +
hello = "my own version of hello"
 +
print(hello)
 +
}}
 +
<translate>
  
myVariable = "hello"
+
<!--T:105-->
print myVariable
+
Now {{incode|hello}} is not an undefined any more. Variables can be modified at any time, that's why they are called variables, their content can vary. For example:
myVariable = "good bye"
 
print myVariable
 
  
We changed the value of myVariable. We can also copy variables:
+
</translate>
 +
{{Code|code=
 +
myVariable = "hello"
 +
print(myVariable)
 +
myVariable = "good bye"
 +
print(myVariable)
 +
}}
 +
<translate>
  
var1 = "hello"
+
<!--T:107-->
var2 = var1
+
We changed the value of {{incode|myVariable}}. We can also copy variables:
print var2
 
  
Note that it is interesting to give good names to your variables, because when you'll write long programs, after a while you won't remember what your variable named "a" is for. But if you named it for example myWelcomeMessage, you'll remember easily what it is used for when you'll see it.
+
</translate>
 +
{{Code|code=
 +
var1 = "hello"
 +
var2 = var1
 +
print(var2)
 +
}}
 +
<translate>
  
==Numbers==
+
<!--T:108-->
 +
It is advisable to give meaningful names to your variables. After a while you won't remember what your variable named {{incode|a}} represents. But if you named it, for example, {{incode|myWelcomeMessage}} you'll easily remember its purpose. Plus your code is a step closer to being self-documenting.
  
Of course you must know that programming is useful to treat all kind of data, and especially numbers, not only text strings. One thing is important, Python must know what kind of data it is dealing with. We saw in our print hello example, that the print command recognized our "hello" string. That is because by using the ", we told specifically the print command that what it would come next is a text string.
+
<!--T:179-->
 +
Case is very important, {{incode|myVariable}} is not the same as {{incode|myvariable}}. If you were to enter {{incode|print(myvariable)}} it would come back with an error as not defined.
  
We can always check what data type is the contents of a variable with the special Python keyword type:
+
<!--T:189-->
 +
[[#top|top]]
  
myVar = "hello"
+
==Numbers== <!--T:109-->
type(myVar)
 
  
It will tell us the contents of myVar is 'str', or string in Python jargon. We have also other basic types of data, such as integer and float numbers:
+
<!--T:110-->
 +
Of course Python programs can deal with all kinds of data, not just text strings. One thing is important, Python must know what kind of data it is dealing with. We saw in our print hello example, that the {{incode|print()}} command recognized our {{incode|"hello"}} string. By using {{incode|" "}} characters, we specified that what follows is a text string.
  
firstNumber = 10
+
<!--T:111-->
secondNumber = 20
+
We can always check the data type of a variable with the {{incode|type()}} command:
print firstNumber + secondNumber
 
type(firstNumber)
 
  
This is already much more interesting, isn't it? Now we already have a powerful calculator! Look well at how it worked, Python knows that 10 and 20 are integer numbers. So they are stored as "int", and Python can do with them everything it can do with integers. Look at the results of this:
+
</translate>
 +
{{Code|code=
 +
myVar = "hello"
 +
type(myVar)
 +
}}
 +
<translate>
  
firstNumber = "10"
+
<!--T:112-->
secondNumber = "20"
+
It will tell us the content of {{incode|myVar}} is a {{incode|'str'}}, which is short for string. We also have other basic data types such as integer and float numbers:
print firstNumber + secondNumber
 
  
See? We forced Python to consider that our two variables are not numbers but mere pieces of text. Python can add two pieces of text together, but it won't try to find out any sum. But we were talking about integer numbers. There are also float numbers. The difference is that integer numbers don't have decimal part, while foat numbers can have a decimal part:
+
</translate>
 +
{{Code|code=
 +
firstNumber = 10
 +
secondNumber = 20
 +
print(firstNumber + secondNumber)
 +
type(firstNumber)
 +
}}
 +
<translate>
  
var1 = 13
+
<!--T:113-->
var2 = 15.65
+
Python knows that 10 and 20 are integer numbers, so they are stored as {{incode|'int'}}, and Python can do with them everything it can do with integers. Look at the results of this:
print "var1 is of type ", type(var1)
 
print "var2 is of type ", type(var2)
 
  
Int and Floats can be mixed together without problem:
+
</translate>
 +
{{Code|code=
 +
firstNumber = "10"
 +
secondNumber = "20"
 +
print(firstNumber + secondNumber)
 +
}}
 +
<translate>
  
total = var1 + var2
+
<!--T:114-->
print total
+
Here we forced Python to consider that our two variables are not numbers but pieces of text. Python can add two pieces of text together, although in that case, of course, it won't perform any arithmetic. But we were talking about integer numbers. There are also float numbers. The difference is float numbers can have a decimal part and integer numbers do not:
print type(total)
 
  
Of course the total has decimals, right? Then Python automatically decided that the result is a float. In several cases such as this one, Python automatically decides what type to give to something. In other cases it doesn't. For example:
+
</translate>
 +
{{Code|code=
 +
var1 = 13
 +
var2 = 15.65
 +
print("var1 is of type ", type(var1))
 +
print("var2 is of type ", type(var2))
 +
}}
 +
<translate>
  
varA = "hello 123"
+
<!--T:115-->
varB = 456
+
Integers and floats can be mixed together without problems:
print varA + varB
 
  
This will give us an error, varA is a string and varB is an int, and Python doesn't know what to do. But we can force Python to convert between types:
+
</translate>
 +
{{Code|code=
 +
total = var1 + var2
 +
print(total)
 +
print(type(total))
 +
}}
 +
<translate>
  
varA = "hello"
+
<!--T:116-->
varB = 123
+
Because {{incode|var2}} is a float Python automatically decides that the result must also be a float. But there are cases where Python does not knows what type to use. For example:
print varA + str(varB)
 
  
Now both are strings, the operation works! Note that we "stringified" varB at the time of printing, but we didn't change varB itself. If we wanted to turn varB permanently into a string, we would need to do this:
+
</translate>
 +
{{Code|code=
 +
varA = "hello 123"
 +
varB = 456
 +
print(varA + varB)
 +
}}
 +
<translate>
  
varB = str(varB)
+
<!--T:117-->
 +
This results in an error, {{incode|varA}} is a string and {{incode|varB}} is an integer, and Python doesn't know what to do. However, we can force Python to convert between types:
  
We can also use int() and float() to convert to int and float if we want:
+
</translate>
 +
{{Code|code=
 +
varA = "hello"
 +
varB = 123
 +
print(varA + str(varB))
 +
}}
 +
<translate>
  
varA = "123"
+
<!--T:118-->
print int(varA)
+
Now that both variables are strings the operation works. Note that we "stringified" {{incode|varB}} at the time of printing, but we didn't change {{incode|varB}} itself. If we wanted to turn {{incode|varB}} permanently into a string, we would need to do this:
print float(varA)
 
  
'''Note on Python commands'''
+
</translate>
 +
{{Code|code=
 +
varB = str(varB)
 +
}}
 +
<translate>
  
You must have noticed that in this section we used the print command in several ways. We printed variables, sums, several things separated by commas, and even the result of other Python command such as type(). Maybe you also saw that doing those two commands:
+
<!--T:119-->
 +
We can also use {{incode|int()}} and {{incode|float()}} to convert to integer and float if we want:
  
type(varA)
+
</translate>
print type(varA)
+
{{Code|code=
 +
varA = "123"
 +
print(int(varA))
 +
print(float(varA))
 +
}}
 +
<translate>
  
have exactly the same result. That is because we are in the interpreter, and everything is automatically printed on screen. When we'll write more complex programs that run outside the interpreter, they won't print automatically everything on screen, so we'll need to use the print command. But from now on, let's stop using it here, it'll go faster. So we can simply write:
+
<!--T:121-->
 +
You must have noticed that we have used the {{incode|print()}} command in several ways. We printed variables, sums, several things separated by commas, and even the result of another Python command. Maybe you also saw that these two commands:
  
myVar = "hello friends"
+
</translate>
myVar
+
{{Code|code=
 +
type(varA)
 +
print(type(varA))
 +
}}
 +
<translate>
  
You must also have seen that most of the Python commands (or keywords) we already know have parenthesis used to tell them on what contents the command must work: type(), int(), str(), etc. Only exception is the print command, which in fact is not an exception, it also works normally like this: print("hello"), but, since it is used often, the Python programmers made a simplified version.
+
<!--T:122-->
 +
have the same result. This is because we are in the interpreter, and everything is automatically printed. When we write more complex programs that run outside the interpreter, they won't print automatically, so we'll need to use the {{incode|print()}} command. With that in mind let's stop using it here. From now on we will simply write:
  
==Lists==
+
</translate>
 +
{{Code|code=
 +
myVar = "hello friends"
 +
myVar
 +
}}
 +
<translate>
  
Another interesting data type is lists. A list is simply a list of other data. The same way as we define a text string by using " ", we define lists by using [ ]:
+
<!--T:190-->
 +
[[#top|top]]
  
myList = [1,2,3]
+
==Lists== <!--T:124-->
type(myList)
 
myOtherList = ["Bart", "Frank", "Bob"]
 
myMixedList = ["hello", 345, 34.567]
 
  
You see that it can contain any type of data. Lists are very useful because you can group variables together. You can then do all kind of things within that groups, for example counting them:
+
<!--T:125-->
 +
Another useful data type is a list. A list is a collection of other data. To define a list we use {{incode|[ ]}}:
  
len(myOtherList)
+
</translate>
 +
{{Code|code=
 +
myList = [1, 2, 3]
 +
type(myList)
 +
myOtherList = ["Bart", "Frank", "Bob"]
 +
myMixedList = ["hello", 345, 34.567]
 +
}}
 +
<translate>
  
or retrieving one item of a list:
+
<!--T:126-->
 +
As you can see a list can contain any type of data. You can do many things with a list. For example, count its items:
  
myName = myOtherList[0]
+
</translate>
myFriendsName = myOtherList[1]
+
{{Code|code=
 +
len(myOtherList)
 +
}}
 +
<translate>
  
You see that while the len() command returns the total number of items in a list, their "position" in the list begins with 0. The first item in a list is always at position 0, so in our myOtherList, "Bob" will be at position 2. We can do much more stuff with lists such as you can read [http://diveintopython.org/native_data_types/lists.html here], such as sorting contents, removing or adding elements.
+
<!--T:127-->
 +
Or retrieve one item:
  
A funny and interesting thing for you: a text string is very similar to a list of characters! Try doing this:
+
</translate>
 +
{{Code|code=
 +
myName = myOtherList[0]
 +
myFriendsName = myOtherList[1]
 +
}}
 +
<translate>
  
myvar = "hello"
+
<!--T:128-->
len(myvar)
+
While the {{incode|len()}} command returns the total number of items in a list, the first item in a list is always at position {{incode|0}}, so in our {{incode|myOtherList}} {{incode|"Bob"}} will be at position {{incode|2}}. We can do much more with lists such as sorting items and removing or adding items.
myvar[2]
 
  
Usually all you can do with lists can also be done with strings. In fact both lists and strings are sequences.
+
<!--T:129-->
 +
Interestingly a text string is very similar to a list of characters in Python. Try doing this:
  
Outside strings, ints, floats and lists, there are more built-in data types, such as [http://www.diveintopython.org/getting_to_know_python/dictionaries.html dictionnaries], or you can even create your own data types with [http://www.freenetpages.co.uk/hp/alan.gauld/tutclass.htm classes].
+
</translate>
 +
{{Code|code=
 +
myvar = "hello"
 +
len(myvar)
 +
myvar[2]
 +
}}
 +
<translate>
  
==Indentation==
+
<!--T:130-->
 +
Usually what you can do with lists can also be done with strings. In fact both lists and strings are sequences.
  
One big cool use of lists is also browsing through them and do something with each item. For example look at this:
+
<!--T:131-->
 +
Apart from strings, integers, floats and lists, there are more built-in data types, such as dictionaries, and you can even create your own data types with classes.
  
alldaltons = ["Joe", "William", "Jack", "Averell"]
+
<!--T:191-->
for dalton in alldaltons:
+
[[#top|top]]
    print dalton + " Dalton"
 
  
We iterated (programming jargon again!) through our list with the "for ... in ..." command and did something with each of the items. Note the special syntax: the for command terminates with : which indicates that what will comes after will be a block of one of more commands. Immediately after you enter the command line ending with :, the command prompt will change to ... which means Python knows that a :-ended line has happened and that what will come next will be part of it.
+
==Indentation== <!--T:132-->
  
How will Python know how many of the next lines will be to be executed inside the for...in operation? For that, Python uses indentation. That is, your next lines won't begin immediately. You will begin them with a blank space, or several blank spaces, or a tab, or several tabs. Other programming languages use other methods, like putting everythin inside parenthesis, etc.
+
<!--T:133-->
As long as you write your next lines with the '''same''' indentation, they will be considered part of the for-in block. If you begin one line with 2 spaces and the next one with 4, there will be an error.
+
One important use of lists is the ability to "browse" through them and do something with each item. For example look at this:
When you finished, just write another line without indentation, or simply press Enter to come back from the for-in block
 
  
Indentation is cool because if you make big ones (for example use tabs instead of spaces because it's larger), when you write a big program you'll have a clear view of what is executed inside what. We'll see that many other commands than for-in can have indented blocks of code too.
+
</translate>
 +
{{Code|code=
 +
alldaltons = ["Joe", "William", "Jack", "Averell"]
 +
for dalton in alldaltons:
 +
    print(dalton + " Dalton")
 +
}}
 +
<translate>
  
For-in commands can be used for many things that must be done more than once. It can for example be combined with the range() command:
+
<!--T:134-->
 +
We iterated (programming jargon) through our list with the {{incode|for in}} command and did something with each of the items. Note the special syntax: the {{incode|for}} command terminates with {{incode|:}} indicating the following will be a block of one of more commands. In the interpreter, immediately after you enter the command line ending with {{incode|:}}, the command prompt will change to {{incode|...}} which means Python knows that there is more to come.
  
serie = range(1,11)
+
<!--T:135-->
total = 0
+
How will Python know how many of the next lines will need to be executed inside the {{incode|for in}} operation? For that, Python relies on indentation. The next lines must begin with a blank space, or several blank spaces, or a tab, or several tabs. And as long as the indentation stays the same the lines will be considered part of the {{incode|for in}} block. If you begin one line with 2 spaces and the next one with 4, there will be an error. When you have finished, just write another line without indentation, or press {{KEY|Enter}} to come back from the {{incode|for in}} block
print "sum"
+
 
for number in serie:
+
<!--T:136-->
     print number
+
Indentation also aids in program readability. If you use large indentations (for example use tabs instead of spaces) when you write a big program, you'll have a clear view of what is executed inside what. We'll see that other commands use indented blocks of code as well.
 +
 
 +
<!--T:137-->
 +
The {{incode|for in}} command can be used for many things that must be done more than once. It can, for example, be combined with the {{incode|range()}} command:
 +
 
 +
</translate>
 +
{{Code|code=
 +
serie = range(1, 11)
 +
total = 0
 +
print("sum")
 +
for number in serie:
 +
     print(number)
 
     total = total + number
 
     total = total + number
print "----"
+
print("----")
print total
+
print(total)
 +
}}
 +
<translate>
  
Or more complex things like this:
+
<!--T:173-->
 +
If you have been running the code examples in an interpreter by copy-pasting, you will find the previous block of text will throw an error. Instead, copy to the end of the indented block, i.e. the end of the line {{incode|total <nowiki>=</nowiki> total + number}} and then paste in the interpreter. In the interpreter press {{KEY|Enter}} until the three dot prompt disappears and the code runs. Then copy the final two lines followed by another {{KEY|Enter}}. The final answer should appear.
  
alldaltons = ["Joe", "William", "Jack", "Averell"]
+
<!--T:180-->
for n in range(4):
+
If you type into the interpreter {{incode|help(range)}} you will see:
    print alldaltons[n], " is Dalton number ", n
 
  
You see that the range() command also has that strange particularity that it begins with 0 (if you don't specify the starting number) and that its last number will be one less than the ending number you specify. That is, of course, so it works well with other Python commands. For example:
+
</translate>
 +
{{Code|code=
 +
range(...)
 +
    range(stop) -> list of integers
 +
    range(start, stop[, step]) -> list of integers
 +
}}
 +
<translate>
  
alldaltons = ["Joe", "William", "Jack", "Averell"]
+
<!--T:192-->
total = len(alldaltons)
+
Here the square brackets denote an optional parameter. However all are expected to be integers. Below we will force the step parameter to be an integer using {{incode|int()}}:
for n in range(total):
 
    print alldaltons[n]
 
  
Another interesting use of indented blocks is with the if command. If executes a code block only if a certain condition is met, for example:
+
</translate>
 +
{{Code|code=
 +
number = 1000
 +
for i in range(0, 180 * number, int(0.5 * number)):
 +
    print(float(i) / number)
 +
}}
 +
<translate>
  
alldaltons = ["Joe", "William", "Jack", "Averell"]
+
<!--T:138-->
if "Joe" in alldaltons:
+
Another {{incode|range()}} example:
    print "We found that Dalton!!!"
 
  
Of course this will always print the first sentence, but try replacing the second line by:
+
</translate>
 +
{{Code|code=
 +
alldaltons = ["Joe", "William", "Jack", "Averell"]
 +
for n in range(4):
 +
    print(alldaltons[n], " is Dalton number ", n)
 +
}}
 +
<translate>
  
if "Lucky" in alldaltons:
+
<!--T:139-->
 +
The {{incode|range()}} command also has that strange particularity that it begins with {{incode|0}} (if you don't specify the starting number) and that its last number will be one less than the ending number you specify. That is, of course, so it works well with other Python commands. For example:
  
Then nothing is printed. We can also specify an else: statement:
+
</translate>
 +
{{Code|code=
 +
alldaltons = ["Joe", "William", "Jack", "Averell"]
 +
total = len(alldaltons)
 +
for n in range(total):
 +
    print(alldaltons[n])
 +
}}
 +
<translate>
  
alldaltons = ["Joe", "William", "Jack", "Averell"]
+
<!--T:140-->
if "Lucky" in alldaltons:
+
Another interesting use of indented blocks is with the {{incode|if}} command. This command executes a code block only if a certain condition is met, for example:
    print "We found that Dalton!!!"
 
else:
 
    print "Such Dalton doesn't exist!"
 
  
==Functions==
+
</translate>
 +
{{Code|code=
 +
alldaltons = ["Joe", "William", "Jack", "Averell"]
 +
if "Joe" in alldaltons:
 +
    print("We found that Dalton!!!")
 +
}}
 +
<translate>
  
The [http://docs.python.org/reference/lexical_analysis.html#identifiers standard Python commands] are not many. In current version of Python there are about 30, and we already know several of them. But imagine if we could invent our own commands? Well, we can, and it's extremely easy. In fact, most the additional modules that you can plug into your Python installation do just that, they add commands that you can use. A custom command in Python is called a function and is made like this:
+
<!--T:141-->
 +
Of course this will always print the sentence, but try replacing the second line with:
  
def printsqm(myValue):
+
</translate>
    print str(myValue)+" square meters"
+
{{Code|code=
+
if "Lucky" in alldaltons:
printsqm(45)
+
}}
 +
<translate>
  
Extremely simple: the def() command defines a new function. You give it a name, and inside the parenthesis you define arguments that we'll use in our function. Arguments are data that will be passed to the function. For example, look at the len() command. If you just write len() alone, Python will tell you it needs an argument. That is, you want len() of something, right? Then, for example, you'll write len(myList) and you'll get the length of myList. Well, myList is an argument that you pass to the len() function. The len() function is defined in such a way that it knows what to do with what is passed to it. Same as we did here.
+
<!--T:142-->
 +
Then nothing is printed. We can also specify an {{incode|else}} statement:
  
The "myValue" name can be anything, and it will be used only inside the function. It is just a name you give to the argument so you can do something with it, but it also serves so the function knows how many arguments to expect. For example, if you do this:
+
</translate>
 +
{{Code|code=
 +
alldaltons = ["Joe", "William", "Jack", "Averell"]
 +
if "Lucky" in alldaltons:
 +
    print("We found that Dalton!!!")
 +
else:
 +
    print("Such Dalton doesn't exist!")
 +
}}
 +
<translate>
  
printsqm(45,34)
+
<!--T:193-->
 +
[[#top|top]]
  
There will be an error. Our function was programmed to receive just one argument, but it received two, 45 and 34. We could instead do something like this:
+
==Functions== <!--T:143-->
  
def sum(val1,val2):
+
<!--T:144-->
 +
There are very few [https://docs.python.org/3/reference/lexical_analysis.html#identifiers standard Python commands] and we already know several of them. But you can create your own commands. In fact, most of the additional modules that you can plug into your Python installation do just that, they add commands that you can use. A custom command in Python is called a function and is made like this:
 +
 
 +
</translate>
 +
{{Code|code=
 +
def printsqm(myValue):
 +
    print(str(myValue) + " square meters")
 +
 
 +
printsqm(45)
 +
}}
 +
<translate>
 +
 
 +
<!--T:181-->
 +
The {{incode|def()}} command defines a new function, you give it a name, and inside the parenthesis you define the arguments that the function will use. Arguments are data that will be passed to the function. For example, look at the {{incode|len()}} command. If you just write {{incode|len()}}, Python will tell you it needs an argument. Which is obvious: you want to know the length of something. If you write {{incode|len(myList)}} then {{incode|myList}} is the argument that you pass to the {{incode|len()}} function. And the {{incode|len()}} function is defined in such a way that it knows what to do with this argument. We have done the same thing with our {{incode|printsqm}} function.
 +
 
 +
<!--T:146-->
 +
The {{incode|myValue}} name can be anything, and it will only be used inside the function. It is just a name you give to the argument so you can do something with it. By defining arguments you also to tell the function how many to expect. For example, if you do this:
 +
 
 +
</translate>
 +
{{Code|code=
 +
printsqm(45, 34)
 +
}}
 +
<translate>
 +
 
 +
<!--T:147-->
 +
there will be an error. Our function was programmed to receive just one argument, but it received two, {{incode|45}} and {{incode|34}}. Let's try another example:
 +
 
 +
</translate>
 +
{{Code|code=
 +
def sum(val1, val2):
 
     total = val1 + val2
 
     total = val1 + val2
 
     return total
 
     return total
  
sum(45,34)
+
myTotal = sum(45, 34)
myTotal = sum(45,34)
+
}}
 +
<translate>
 +
 
 +
<!--T:148-->
 +
Here we made a function that receives two arguments, sums them, and returns that value. Returning something is very useful, because we can do something with the result, such as store it in the {{incode|myTotal}} variable.
 +
 
 +
<!--T:194-->
 +
[[#top|top]]
 +
 
 +
==Modules== <!--T:151-->
  
We made a function that receives two arguments, sums them, and returns that value. Returning something is very useful, because we can do something with the result, such as store it in the myTotal variable. Of course, since we are in the interpreter and everything is printed, doing:
+
<!--T:152-->
 +
Now that you have a good idea of how Python works, you will need to know one more thing: How to work with files and modules.
  
sum(45,34)
+
<!--T:153-->
 +
Until now, we have written Python instructions line by line in the interpreter. This method is obviously not suitable for larger programs. Normally the code for Python programs is stored in files with the {{FileName|.py}} extension. Which are just plain text files and any text editor (Linux gedit, emacs, vi or even Windows Notepad) can be used to create and edit them.
  
will print the result on the screen, but outside the interpreter, since there is no more print command inside the function, nothing would appear on the screen. You would need to do:
+
<!--T:154-->
 +
There are several of ways to execute a Python program. In Windows, simply right-click your file, open it with Python, and execute it. But you can also execute it from the Python interpreter itself. For this, the interpreter must know where your program is. In FreeCAD the easiest way is to place your program in a folder that FreeCAD's Python interpreter knows by default, such as FreeCAD's user {{FileName|Mod}} folder:
 +
* On Linux it is usually {{FileName|/home/<username>/.FreeCAD/Mod/}}.
 +
* On Windows it is {{FileName|%APPDATA%\FreeCAD\Mod\}}, which is usually {{FileName|C:\Users\<username>\Appdata\Roaming\FreeCAD\Mod\}}.
 +
* On Mac OSX it is usually {{FileName|/Users/<username>/Library/Preferences/FreeCAD/Mod/}}.
 +
Let's add a subfolder there called {{FileName|scripts}} and then write a file like this:
  
print sum(45,34)
+
</translate>
 +
{{Code|code=
 +
def sum(a,b):
 +
    return a + b
  
to have something printed. Read more about functions [http://www.penzilla.net/tutorials/python/functions/ here].
+
print("myTest.py succesfully loaded")
 +
}}
 +
<translate>
  
==Modules==
+
<!--T:155-->
 +
Save the file as {{FileName|myTest.py}} in the {{FileName|scripts}} folder, and in the interpreter window write:
  
Now that we have a good idea of how Python works, we'll need one last thing: How to work with files and modules.
+
</translate>
 +
{{Code|code=
 +
import myTest
 +
}}
 +
<translate>
  
Until now, we wrote Python instructions line by line in the interpreter, right? What if we could write several lines together, and have them executed all at once? It would certainly be handier for doing more complex things. And we could save our work too. Well, that too, is extremely easy. Simply open a text editor (such as the windows notepad), and write all your Python lines, the same way as you write them in the interpreter, with indentations, etc. Then, save that file somewhere, preferably with a .py extension. That's it, you have a complete Python program. Of course, there are much better editors than notepad, but it is just to show you that a Python program is nothing else than a text file.
+
<!--T:156-->
 +
without the {{FileName|.py}} extension. This will execute the contents of the file, line by line, just as if we had written it in the interpreter. The sum function will be created, and the message will be printed. Files containing functions, like ours, are called modules.
  
To make Python execute that program, there are hundreds of ways. In windows, simply right-click your file, open it with Python, and execute it. But you can also execute it from the Python interpreter itself. For this, the interpreter must know where your .py program is. In FreeCAD, the easiest way is to place your program in a place that FreeCAD's Python interpreter knows by default, such as FreeCAD's bin folder, or any of the Mod folders. Suppose we write a file like this:
+
<!--T:157-->
<pre>
+
When we write a {{incode|sum()}} function in the interpreter, we execute it like this:
def sum(a,b):
 
    return a + b
 
  
print "test.py succesfully loaded"
+
</translate>
</pre>
+
{{Code|code=
 +
sum(14, 45)
 +
}}
 +
<translate>
  
and we save it as test.py in our FreeCAD/bin directory. Now, let's start FreeCAD, and in the interpreter window, write:
+
<!--T:158-->
 +
But when we import a module containing a {{incode|sum()}} function the syntax is a bit different:
  
import test
+
</translate>
 +
{{Code|code=
 +
myTest.sum(14, 45)
 +
}}
 +
<translate>
  
without the .py extension. This will simply execute the contents of the file, line by line, just as if we had written it in the interpreter. The sum function will be created, and the message will be printed. There is one big difference: the import command is made not only to execute programs written in files, like ours, but also to load the functions inside, so they become available in the interpreter. Files containing functions, like ours, are called modules.
+
<!--T:159-->
 +
That is, the module is imported as a "container", and all its functions are inside that container. This is very useful, because we can import a lot of modules, and keep everything well organized. Basically when you see {{incode|something.somethingElse}}, with a dot in between, then this means {{incode|somethingElse}} is inside {{incode|something}}.
  
Normally when we write a sum() function in the interpreter, we execute it simply like that:
+
<!--T:160-->
 +
We can also import our sum() function directly into the main interpreter space:
  
sum(14,45)
+
</translate>
 +
{{Code|code=
 +
from myTest import *
 +
sum(12, 54)
 +
}}
 +
<translate>
  
Like we did earlier. When we import a module containing our sum() function, the syntax is a bit different. We do:
+
<!--T:161-->
 +
Almost all modules do that: they define functions, new data types and classes that you can use in the interpreter or in your own Python modules, because nothing prevents you from importing other modules inside your module!
  
test.sum(14,45)
+
<!--T:162-->
 +
How do we know what modules we have, what functions are inside and how to use them (that is, what kind of arguments they need)? We have already seen that Python has a {{incode|help()}} function. Doing:
  
That is, the module is imported as a "container", and all its functions are inside. This is extremely useful, because we can import a lot of modules, and keep everything well organized. So, basically, everywhere you see something.somethingElse, with a dot in between, that means somethingElse is inside something.
+
</translate>
 +
{{Code|code=
 +
help("modules")
 +
}}
 +
<translate>
  
We can also throw out the test part, and import our sum() function directly into the main interpreter space, like this:
+
<!--T:163-->
 +
will give us a list of all available modules. We can import any of them and browse their content with the {{incode|dir()}} command:
  
from test import *
+
</translate>
sum(12,54)
+
{{Code|code=
 +
import math
 +
dir(math)
 +
}}
 +
<translate>
  
Basically all modules behave like that. You import a module, then you can use its functions like that: module.function(argument). Almost all modules do that: they define functions, new data types and classes that you can use in the interpreter or in your own Python modules, because nothing prevents you to import modules inside your module!
+
<!--T:164-->
 +
We'll see all the functions contained in the {{incode|math}} module, as well as strange stuff named {{incode|__doc__}}, {{incode|__file__}}, {{incode|__name__}}. Every function in a well made module has a {{incode|__doc__}} that explains how to use it. For example, we see that there is a {{incode|sin()}} function inside the math module. Want to know how to use it?
 +
</translate>
 +
{{Code|code=
 +
print(math.sin.__doc__)
 +
}}
 +
<translate>
  
One last extremely useful thing. How do we know what modules we have, what functions are inside and how to use them (that is, what kind of arguments they need)? We saw already that Python has a help() function. Doing:
+
<!--T:165-->
 +
It may not be evident, but on either side of {{incode|doc}} are two underscore characters.
  
help()
+
<!--T:174-->
modules
+
And finally one last tip: When working on new or existing code, it is better to not use the FreeCAD macro file extension, {{FileName|.FCMacro}}, but instead use the standard {{FileName|.py}} extension. This is because Python doesn't recognize the {{FileName|.FCMacro}} extension. If you use {{FileName|.py}} your code can be easily loaded with {{incode|import}}, as we have already seen, and also reloaded with {{incode|importlib.reload()}}:
  
Will give us a list of all available modules. We can now type q to get out of the interactive help, and import any of them. We can even browse their content with the dir() command
+
</translate>
 +
{{Code|code=
 +
import importlib
 +
importlib.reload(myTest)
 +
}}
 +
<translate>
  
import math
+
<!--T:176-->
dir(math)
+
There is however an alternative:
  
We'll see all the functions contained in the math module, as well as strange stuff named __doc__, __file__, __name__. The __doc__ is extremely useful, it is a documentation text. Every function of (well-made) modules has a __doc__ that explains how to use it. For example, we see that there is a sin function in side the math module. Want to know how to use it?
+
</translate>
 +
{{Code|code=
 +
exec(open("C:/PathToMyMacro/myMacro.FCMacro").read())
 +
}}
 +
<translate>
  
print math.sin.__doc__
+
<!--T:195-->
 +
[[#top|top]]
  
==Starting with FreeCAD==
+
==Starting with FreeCAD== <!--T:167-->
  
Well, I think you must know have a good idea of how Python works, and you can start exploring what FreeCAD has to offer. FreeCAD's Python functions are all well organized in different modules. Some of them are already loaded (imported) when you start FreeCAD. So, just do
+
<!--T:168-->
 +
Hopefully you now have a good idea of how Python works, and you can start exploring what FreeCAD has to offer. FreeCAD's Python functions are all well organized in different modules. Some of them are already loaded (imported) when you start FreeCAD. Just try:
  
dir()
+
</translate>
 +
{{Code|code=
 +
dir()
 +
}}
 +
<translate>
  
and read on to [[FreeCAD Scripting Basics]]...
+
<!--T:196-->
 +
[[#top|top]]
  
Of course, we saw here only a very small part of the Python world. There are many important concepts that we didn't mention here. There are two very important Python reference documents on the net:
+
==Notes== <!--T:197-->
* the [http://docs.python.org/reference/ official Python reference]
 
* the [http://www.diveintopython.org/toc/index.html Dive into Python] wikibook
 
Be sure to bookmark them!
 
  
 +
<!--T:198-->
 +
* FreeCAD was originally designed to work with Python 2. Since Python 2 reached the end of its life in 2020, future development of FreeCAD will be done exclusively with Python 3, and backwards compatibility will not be supported.
 +
* Much more information about Python can be found in the [https://docs.python.org/3/tutorial/index.html official Python tutorial]  and the [https://docs.python.org/3/reference/ official Python reference].
  
{{docnav|Macros|FreeCAD Scripting Basics}}
+
<!--T:199-->
 +
[[#top|top]]
  
{{languages |{{es|Introduction to python/es}} {{se|Introduction to python/se}} {{pl|Introduction to python/pl}} {{ru|Introduction to python/ru}} }}
+
<!--T:171-->
 +
{{Docnav
 +
|[[Macros|Macros]]
 +
|[[Python_scripting_tutorial|Python scripting tutorial]]
 +
}}
  
[[Category:Poweruser Documentation]]
+
</translate>
 +
{{Powerdocnavi{{#translation:}}}}
 +
[[Category:Developer Documentation{{#translation:}}]]
 +
[[Category:Python Code{{#translation:}}]]
 +
{{clear}}

Latest revision as of 13:54, 3 June 2020

Other languages:
Bahasa Indonesia • ‎Deutsch • ‎English • ‎Türkçe • ‎español • ‎français • ‎italiano • ‎polski • ‎português • ‎português do Brasil • ‎română • ‎svenska • ‎čeština • ‎русский • ‎中文(中国大陆)‎ • ‎日本語 • ‎한국어

Introduction

This is a short tutorial for those new to Python. Python is an open-source, multiplatform programming language. It has several features that make it different from other programming languages, and very accessible to new users:

  • It has been designed to be to readable by human beings, making it relatively easy to learn and understand.
  • It is interpreted, this means that programs do not need to be compiled before they can be executed. Python code can be executed immediately, even line by line if you wish.
  • It can be embedded in other programs as a scripting language. FreeCAD has an embedded Python interpreter. You can write Python code to manipulate parts of FreeCAD. This is very powerful, it means you can build your very own tools.
  • It is extensible, you can easily plug new modules into your Python installation and extend its functionality. For example, there are modules that allow Python to read and write images, to communicate with Twitter, to schedule tasks to be performed by your operating system, etc.

The following is a very basic introduction, and by no means a complete tutorial. But hopefully it will provide a good starting point for further exploration into FreeCAD and its mechanisms. We strongly encourage you to enter the code snippets below into a Python interpreter.

The interpreter

Usually when writing computer programs, you open a text editor or your special programming environment (which is basically a text editor with some additional tools), write your program, then compile and execute. Often one or more errors were made during entry, so your program won't work. You may even get an error message telling you what went wrong. Then you go back to your text editor, correct the mistakes, run again, repeating until your program works as intended.

In Python that whole process can be done transparently inside the Python interpreter. The interpreter is a Python window with a command prompt, where you can simply type Python code. If you have installed Python on your computer (download it from the Python website if you are on Windows or Mac, install it from your package repository if you are on GNU/Linux), you will have a Python interpreter in your start menu. But, as already mentioned, FreeCAD also has a built-in Python interpreter: the Python console.

FreeCAD Python console.png

The FreeCAD Python console


If you don't see it, click on View → Panels → Python console. The Python console can be resized and also undocked.

The interpreter shows the Python version, then a >>> symbol which is the command prompt. Writing code in the interpreter is simple: one line is one instruction. When you press Enter, your line of code will be executed (after being instantly and invisibly compiled). For example, try writing this:

print("hello")

print() is a Python command that, obviously, prints something on the screen. When you press Enter, the operation is executed, and the message "hello" is printed. If you make an error, for example let's write:

print(hello)

Python will immediately tell you so. In this case Python doesn't know what hello is. The " " characters specify that the content is a string, programming jargon for a piece of text. Without these the print() command doesn't recognize hello. By pressing the up arrow you can go back to the last line of code and correct it.

The Python interpreter also has a built-in help system. Let's say we don't understand what went wrong with print(hello) and we want specific information about the command:

help("print")

You'll get a long and complete description of everything the print() command can do.

Now that you understand the Python interpreter, we can continue with the more serious stuff.

top

Variables

Very often in programming you need to store a value under a name. That's where variables come in. For example, type this:

a = "hello"
print(a)

You probably understand what happened here, we saved the string "hello" under the name a. Now that a is known we can use it anywhere, for example in the print() command. We can use any name we want, we just need to follow some simple rules, such as not using spaces or punctuation and not using Python keywords. For example, we can write:

hello = "my own version of hello"
print(hello)

Now hello is not an undefined any more. Variables can be modified at any time, that's why they are called variables, their content can vary. For example:

myVariable = "hello"
print(myVariable)
myVariable = "good bye"
print(myVariable)

We changed the value of myVariable. We can also copy variables:

var1 = "hello"
var2 = var1
print(var2)

It is advisable to give meaningful names to your variables. After a while you won't remember what your variable named a represents. But if you named it, for example, myWelcomeMessage you'll easily remember its purpose. Plus your code is a step closer to being self-documenting.

Case is very important, myVariable is not the same as myvariable. If you were to enter print(myvariable) it would come back with an error as not defined.

top

Numbers

Of course Python programs can deal with all kinds of data, not just text strings. One thing is important, Python must know what kind of data it is dealing with. We saw in our print hello example, that the print() command recognized our "hello" string. By using " " characters, we specified that what follows is a text string.

We can always check the data type of a variable with the type() command:

myVar = "hello"
type(myVar)

It will tell us the content of myVar is a 'str', which is short for string. We also have other basic data types such as integer and float numbers:

firstNumber = 10
secondNumber = 20
print(firstNumber + secondNumber)
type(firstNumber)

Python knows that 10 and 20 are integer numbers, so they are stored as 'int', and Python can do with them everything it can do with integers. Look at the results of this:

firstNumber = "10"
secondNumber = "20"
print(firstNumber + secondNumber)

Here we forced Python to consider that our two variables are not numbers but pieces of text. Python can add two pieces of text together, although in that case, of course, it won't perform any arithmetic. But we were talking about integer numbers. There are also float numbers. The difference is float numbers can have a decimal part and integer numbers do not:

var1 = 13
var2 = 15.65
print("var1 is of type ", type(var1))
print("var2 is of type ", type(var2))

Integers and floats can be mixed together without problems:

total = var1 + var2
print(total)
print(type(total))

Because var2 is a float Python automatically decides that the result must also be a float. But there are cases where Python does not knows what type to use. For example:

varA = "hello 123"
varB = 456
print(varA + varB)

This results in an error, varA is a string and varB is an integer, and Python doesn't know what to do. However, we can force Python to convert between types:

varA = "hello"
varB = 123
print(varA + str(varB))

Now that both variables are strings the operation works. Note that we "stringified" varB at the time of printing, but we didn't change varB itself. If we wanted to turn varB permanently into a string, we would need to do this:

varB = str(varB)

We can also use int() and float() to convert to integer and float if we want:

varA = "123"
print(int(varA))
print(float(varA))

You must have noticed that we have used the print() command in several ways. We printed variables, sums, several things separated by commas, and even the result of another Python command. Maybe you also saw that these two commands:

type(varA)
print(type(varA))

have the same result. This is because we are in the interpreter, and everything is automatically printed. When we write more complex programs that run outside the interpreter, they won't print automatically, so we'll need to use the print() command. With that in mind let's stop using it here. From now on we will simply write:

myVar = "hello friends"
myVar

top

Lists

Another useful data type is a list. A list is a collection of other data. To define a list we use [ ]:

myList = [1, 2, 3]
type(myList)
myOtherList = ["Bart", "Frank", "Bob"]
myMixedList = ["hello", 345, 34.567]

As you can see a list can contain any type of data. You can do many things with a list. For example, count its items:

len(myOtherList)

Or retrieve one item:

myName = myOtherList[0]
myFriendsName = myOtherList[1]

While the len() command returns the total number of items in a list, the first item in a list is always at position 0, so in our myOtherList "Bob" will be at position 2. We can do much more with lists such as sorting items and removing or adding items.

Interestingly a text string is very similar to a list of characters in Python. Try doing this:

myvar = "hello"
len(myvar)
myvar[2]

Usually what you can do with lists can also be done with strings. In fact both lists and strings are sequences.

Apart from strings, integers, floats and lists, there are more built-in data types, such as dictionaries, and you can even create your own data types with classes.

top

Indentation

One important use of lists is the ability to "browse" through them and do something with each item. For example look at this:

alldaltons = ["Joe", "William", "Jack", "Averell"]
for dalton in alldaltons:
    print(dalton + " Dalton")

We iterated (programming jargon) through our list with the for in command and did something with each of the items. Note the special syntax: the for command terminates with : indicating the following will be a block of one of more commands. In the interpreter, immediately after you enter the command line ending with :, the command prompt will change to ... which means Python knows that there is more to come.

How will Python know how many of the next lines will need to be executed inside the for in operation? For that, Python relies on indentation. The next lines must begin with a blank space, or several blank spaces, or a tab, or several tabs. And as long as the indentation stays the same the lines will be considered part of the for in block. If you begin one line with 2 spaces and the next one with 4, there will be an error. When you have finished, just write another line without indentation, or press Enter to come back from the for in block

Indentation also aids in program readability. If you use large indentations (for example use tabs instead of spaces) when you write a big program, you'll have a clear view of what is executed inside what. We'll see that other commands use indented blocks of code as well.

The for in command can be used for many things that must be done more than once. It can, for example, be combined with the range() command:

serie = range(1, 11)
total = 0
print("sum")
for number in serie:
    print(number)
    total = total + number
print("----")
print(total)

If you have been running the code examples in an interpreter by copy-pasting, you will find the previous block of text will throw an error. Instead, copy to the end of the indented block, i.e. the end of the line total = total + number and then paste in the interpreter. In the interpreter press Enter until the three dot prompt disappears and the code runs. Then copy the final two lines followed by another Enter. The final answer should appear.

If you type into the interpreter help(range) you will see:

range(...)
    range(stop) -> list of integers
    range(start, stop[, step]) -> list of integers

Here the square brackets denote an optional parameter. However all are expected to be integers. Below we will force the step parameter to be an integer using int():

number = 1000
for i in range(0, 180 * number, int(0.5 * number)):
    print(float(i) / number)

Another range() example:

alldaltons = ["Joe", "William", "Jack", "Averell"]
for n in range(4):
    print(alldaltons[n], " is Dalton number ", n)

The range() command also has that strange particularity that it begins with 0 (if you don't specify the starting number) and that its last number will be one less than the ending number you specify. That is, of course, so it works well with other Python commands. For example:

alldaltons = ["Joe", "William", "Jack", "Averell"]
total = len(alldaltons)
for n in range(total):
    print(alldaltons[n])

Another interesting use of indented blocks is with the if command. This command executes a code block only if a certain condition is met, for example:

alldaltons = ["Joe", "William", "Jack", "Averell"]
if "Joe" in alldaltons:
    print("We found that Dalton!!!")

Of course this will always print the sentence, but try replacing the second line with:

if "Lucky" in alldaltons:

Then nothing is printed. We can also specify an else statement:

alldaltons = ["Joe", "William", "Jack", "Averell"]
if "Lucky" in alldaltons:
    print("We found that Dalton!!!")
else:
    print("Such Dalton doesn't exist!")

top

Functions

There are very few standard Python commands and we already know several of them. But you can create your own commands. In fact, most of the additional modules that you can plug into your Python installation do just that, they add commands that you can use. A custom command in Python is called a function and is made like this:

def printsqm(myValue):
    print(str(myValue) + " square meters")

printsqm(45)

The def() command defines a new function, you give it a name, and inside the parenthesis you define the arguments that the function will use. Arguments are data that will be passed to the function. For example, look at the len() command. If you just write len(), Python will tell you it needs an argument. Which is obvious: you want to know the length of something. If you write len(myList) then myList is the argument that you pass to the len() function. And the len() function is defined in such a way that it knows what to do with this argument. We have done the same thing with our printsqm function.

The myValue name can be anything, and it will only be used inside the function. It is just a name you give to the argument so you can do something with it. By defining arguments you also to tell the function how many to expect. For example, if you do this:

printsqm(45, 34)

there will be an error. Our function was programmed to receive just one argument, but it received two, 45 and 34. Let's try another example:

def sum(val1, val2):
    total = val1 + val2
    return total

myTotal = sum(45, 34)

Here we made a function that receives two arguments, sums them, and returns that value. Returning something is very useful, because we can do something with the result, such as store it in the myTotal variable.

top

Modules

Now that you have a good idea of how Python works, you will need to know one more thing: How to work with files and modules.

Until now, we have written Python instructions line by line in the interpreter. This method is obviously not suitable for larger programs. Normally the code for Python programs is stored in files with the .py extension. Which are just plain text files and any text editor (Linux gedit, emacs, vi or even Windows Notepad) can be used to create and edit them.

There are several of ways to execute a Python program. In Windows, simply right-click your file, open it with Python, and execute it. But you can also execute it from the Python interpreter itself. For this, the interpreter must know where your program is. In FreeCAD the easiest way is to place your program in a folder that FreeCAD's Python interpreter knows by default, such as FreeCAD's user Mod folder:

  • On Linux it is usually /home/<username>/.FreeCAD/Mod/.
  • On Windows it is %APPDATA%\FreeCAD\Mod\, which is usually C:\Users\<username>\Appdata\Roaming\FreeCAD\Mod\.
  • On Mac OSX it is usually /Users/<username>/Library/Preferences/FreeCAD/Mod/.

Let's add a subfolder there called scripts and then write a file like this:

def sum(a,b):
    return a + b

print("myTest.py succesfully loaded")

Save the file as myTest.py in the scripts folder, and in the interpreter window write:

import myTest

without the .py extension. This will execute the contents of the file, line by line, just as if we had written it in the interpreter. The sum function will be created, and the message will be printed. Files containing functions, like ours, are called modules.

When we write a sum() function in the interpreter, we execute it like this:

sum(14, 45)

But when we import a module containing a sum() function the syntax is a bit different:

myTest.sum(14, 45)

That is, the module is imported as a "container", and all its functions are inside that container. This is very useful, because we can import a lot of modules, and keep everything well organized. Basically when you see something.somethingElse, with a dot in between, then this means somethingElse is inside something.

We can also import our sum() function directly into the main interpreter space:

from myTest import *
sum(12, 54)

Almost all modules do that: they define functions, new data types and classes that you can use in the interpreter or in your own Python modules, because nothing prevents you from importing other modules inside your module!

How do we know what modules we have, what functions are inside and how to use them (that is, what kind of arguments they need)? We have already seen that Python has a help() function. Doing:

help("modules")

will give us a list of all available modules. We can import any of them and browse their content with the dir() command:

import math
dir(math)

We'll see all the functions contained in the math module, as well as strange stuff named __doc__, __file__, __name__. Every function in a well made module has a __doc__ that explains how to use it. For example, we see that there is a sin() function inside the math module. Want to know how to use it?

print(math.sin.__doc__)

It may not be evident, but on either side of doc are two underscore characters.

And finally one last tip: When working on new or existing code, it is better to not use the FreeCAD macro file extension, .FCMacro, but instead use the standard .py extension. This is because Python doesn't recognize the .FCMacro extension. If you use .py your code can be easily loaded with import, as we have already seen, and also reloaded with importlib.reload():

import importlib
importlib.reload(myTest)

There is however an alternative:

exec(open("C:/PathToMyMacro/myMacro.FCMacro").read())

top

Starting with FreeCAD

Hopefully you now have a good idea of how Python works, and you can start exploring what FreeCAD has to offer. FreeCAD's Python functions are all well organized in different modules. Some of them are already loaded (imported) when you start FreeCAD. Just try:

dir()

top

Notes

  • FreeCAD was originally designed to work with Python 2. Since Python 2 reached the end of its life in 2020, future development of FreeCAD will be done exclusively with Python 3, and backwards compatibility will not be supported.
  • Much more information about Python can be found in the official Python tutorial and the official Python reference.

top