Introduction to Python: Difference between revisions

From FreeCAD Documentation
(Revising...)
m (updated: Mac OSX…/Preferences/… to macOS …/Application Support/…)
 
(77 intermediate revisions by 2 users not shown)
Line 1: Line 1:
<languages/>
<languages/>

{{Page in progress}}

<translate>
<translate>


<!--T:183-->
<!--T:183-->
{{docnav
{{Docnav
|[[Macros|Macros]]
|[[Scripts|Scripts]]
|[[Python_scripting_tutorial|Python scripting tutorial]]
|[[Python_scripting_tutorial|Python scripting tutorial]]
}}
}}
Line 15: Line 12:
<translate>
<translate>


==Introduction==
==Introduction== <!--T:187-->


<!--T:87-->
<!--T:87-->
This is a short tutorial for those new to [http://en.wikipedia.org/wiki/Python_%28programming_language%29 Python]. Python is an open-source, multiplatform [http://en.wikipedia.org/wiki/Programming_language programming language]. It has several features that make it very different from other programming languages, and very accessible to new users:
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:


<!--T:88-->
<!--T:88-->
*It has been designed to be to readable by human beings, making it easy to learn and understand.
*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, line by line if you wish.
*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 in FreeCAD that will manipulate parts of FreeCAD, for example to create geometry. This is extremely powerful, instead of just clicking a button labeled "create sphere" that some programmer has coded, you have the freedom to build your own tools.
*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 jpg images, to communicate with twitter, to schedule tasks to be performed by your operating system, etc.
*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.


<!--T:89-->
<!--T:89-->
The following is a very basic introduction, and by no means a complete tutorial. But our hope is that this 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. Hands on please!
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.

==Python version information==

<!--T:182-->
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.


==The interpreter== <!--T:90-->
==The interpreter== <!--T:90-->


<!--T:91-->
<!--T:91-->
Usually, when writing computer programs, you open a text editor or your special programming environment (which is basically a text editor with several 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.
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.


<!--T:92-->
<!--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 [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, as already mentioned, FreeCAD also has a built-in Python interpreter.
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]].


</translate>
</translate>
[[Image:Screenshot_pythoninterpreter.jpg]]
[[Image:FreeCAD_Python_console.png]]
<translate>
<translate>
<!--T:93-->
<!--T:93-->
{{Caption|The Python console}}
{{Caption|The FreeCAD Python console}}


<!--T:94-->
<!--T:94-->
Line 52: Line 44:


<!--T:95-->
<!--T:95-->
The interpreter shows the Python version, then a {{incode|>>>}} symbol, which is the command prompt, and 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:
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:


</translate>
</translate>
Line 61: Line 53:


<!--T:96-->
<!--T:96-->
{{incode|print}} 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:
{{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:


</translate>
</translate>
Line 70: Line 62:


<!--T:97-->
<!--T:97-->
Python will tell us that it doesn't know what hello is. The {{incode|"}} characters specify that the content is a string, which is, in programming jargon, a piece of text. Without these the print command doesn't recognize {{incode|hello}}. The important thing is, you immediately get notified that you have made an error. By pressing the up arrow (or, in the FreeCAD interpreter, CTRL+up arrow), you can go back to the last line of code and correct it.
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.


<!--T:98-->
<!--T:98-->
The Python interpreter also has a built-in help system. Try typing:
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:

</translate>
{{Code|code=
help()
}}
<translate>

<!--T:99-->
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 {{incode|print}} command:


</translate>
</translate>
Line 91: Line 74:


<!--T:100-->
<!--T:100-->
You'll get a long and complete description of everything the print command can do.
You'll get a long and complete description of everything the {{incode|print()}} command can do.


<!--T:101-->
<!--T:101-->
Now that you understand the Python interpreter, we can begin with the serious stuff.
Now that you understand the Python interpreter, we can continue with the more serious stuff.

</translate>{{Top}}<translate>


==Variables== <!--T:102-->
==Variables== <!--T:102-->


<!--T:103-->
<!--T:103-->
Very often in programming you need to store a value under a name. That's where the concept of the variable comes in. For example, type this:
Very often in programming you need to store a value under a name. That's where variables come in. For example, type this:


</translate>
</translate>
Line 109: Line 94:


<!--T:104-->
<!--T:104-->
I guess you understood what happened, 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 print command. We can use any name we want, just follow some simple rules, like not using spaces or punctuation. For example, we can write:
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:


</translate>
</translate>
Line 119: Line 104:


<!--T:105-->
<!--T:105-->
Now hello is not an undefined word any more. Variables can be modified at any time, that's why they are called variables, the contents can vary. For example:
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:


</translate>
</translate>
Line 142: Line 127:


<!--T:108-->
<!--T:108-->
Note that it is important 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.
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.


<!--T:179-->
<!--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.
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.

</translate>{{Top}}<translate>


==Numbers== <!--T:109-->
==Numbers== <!--T:109-->


<!--T:110-->
<!--T:110-->
Of course Python programs can deal with all kinds of data, 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 {{incode|print}} command recognized our {{incode|"hello"}} string. By using {{incode|"}} characters, we told specifically that what follows next is a text string.
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.


<!--T:111-->
<!--T:111-->
We can always check the data type of a variable with the special Python keyword type:
We can always check the data type of a variable with the {{incode|type()}} command:


</translate>
</translate>
Line 163: Line 150:


<!--T:112-->
<!--T:112-->
It will tell us the contents of {{incode|myVar}} is {{incode|'str'}}, which is short for string. We have also other basic types of data, such as integer and float numbers:
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:


</translate>
</translate>
Line 175: Line 162:


<!--T:113-->
<!--T:113-->
This is much more interesting, isn't it? Now we have a powerful calculator! Look at how well it worked, 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:
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:


</translate>
</translate>
Line 186: Line 173:


<!--T:114-->
<!--T:114-->
See? We forced Python to consider that our two variables are not numbers but pieces of text instead. 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 float numbers can have a decimal part and integer numbers don't:
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:


</translate>
</translate>
Line 198: Line 185:


<!--T:115-->
<!--T:115-->
Integers and Floats can be mixed together without problems:
Integers and floats can be mixed together without problems:


</translate>
</translate>
Line 209: Line 196:


<!--T:116-->
<!--T:116-->
Of course the total has decimals, and therefore Python automatically decides that the result must be a float. But there are also cases where Python does not knows what type to use. For example:
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:


</translate>
</translate>
Line 220: Line 207:


<!--T:117-->
<!--T:117-->
This will give us 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:
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:


</translate>
</translate>
Line 231: Line 218:


<!--T:118-->
<!--T:118-->
Now that both 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:
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:


</translate>
</translate>
Line 240: Line 227:


<!--T:119-->
<!--T:119-->
We can also use {{incode|int()}} and {{incode|float()}} to convert to int and float if we want:
We can also use {{incode|int()}} and {{incode|float()}} to convert to integer and float if we want:


</translate>
</translate>
Line 249: Line 236:
}}
}}
<translate>
<translate>

<!--T:120-->
'''Note on Python commands'''


<!--T:121-->
<!--T:121-->
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 another Python command such as type(). Maybe you also saw that these two commands:
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:


</translate>
</translate>
Line 264: Line 248:


<!--T:122-->
<!--T:122-->
have the same result. Tis 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. From now on, let's stop using it here, it'll go faster. So we can simply write:
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:


</translate>
</translate>
Line 272: Line 256:
}}
}}
<translate>
<translate>

</translate>{{Top}}<translate>


==Lists== <!--T:124-->
==Lists== <!--T:124-->


<!--T:125-->
<!--T:125-->
Another interesting data type is a list. A list is a collection of other data. The same way that we define a text string by using {{incode|" "}}, we define a list by using {{incode|[ ]}}:
Another useful data type is a list. A list is a collection of other data. To define a list we use {{incode|[ ]}}:


</translate>
</translate>
Line 288: Line 274:


<!--T:126-->
<!--T:126-->
You see that it can contain any type of data. Lists are very useful because you can group values together. You can then do all kinds of things within that group, for example counting them:
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:


</translate>
</translate>
Line 297: Line 283:


<!--T:127-->
<!--T:127-->
or retrieving one item of a list:
Or retrieve one item:


</translate>
</translate>
Line 307: Line 293:


<!--T:128-->
<!--T:128-->
You see that while the {{incode|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 with lists such as sorting contents, removing or adding elements.
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.


<!--T:129-->
<!--T:129-->
Line 324: Line 310:


<!--T:131-->
<!--T:131-->
Outside strings, integers, floats and lists, there are more built-in data types, such as dictionaries, or you can even create your own data types with classes.
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.

</translate>{{Top}}<translate>


==Indentation== <!--T:132-->
==Indentation== <!--T:132-->


<!--T:133-->
<!--T:133-->
One important use of lists is the ability to browse through them and do something with each item. For example look at this:
One important use of lists is the ability to "browse" through them and do something with each item. For example look at this:


</translate>
</translate>
Line 340: Line 328:


<!--T:134-->
<!--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.
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.


<!--T:135-->
<!--T:135-->
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 everything inside parenthesis, etc.
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
As long as you write your next lines with the {{Emphasis|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.
When you have finished, just write another line without indentation, or simply press Enter to come back from the for-in block


<!--T:136-->
<!--T:136-->
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 commands other than for-in, can have indented blocks of code too.
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-->
<!--T:137-->
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 {{incode|range()}} command:
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>
</translate>
{{Code|code=
{{Code|code=
serie = range(1,11)
serie = range(1, 11)
total = 0
total = 0
print("sum")
print("sum")
Line 367: Line 353:


<!--T:173-->
<!--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 "total = total + number" and then paste in the interpreter. In the interpreter issue an Enter until the three dot prompt disappears and the code runs. Then copy the final two lines followed by one or more Enter. The final answer should appear.)
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.


<!--T:180-->
<!--T:180-->
Line 380: Line 366:
<translate>
<translate>


<!--T:192-->
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()}}:
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()}}:


Line 402: Line 389:


<!--T:139-->
<!--T:139-->
You see that the {{incode|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:
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:


</translate>
</translate>
Line 445: Line 432:
}}
}}
<translate>
<translate>

</translate>{{Top}}<translate>


==Functions== <!--T:143-->
==Functions== <!--T:143-->


<!--T:144-->
<!--T:144-->
There are very few [http://docs.python.org/reference/lexical_analysis.html#identifiers standard Python commands]. In the 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 quite 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:
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>
</translate>
Line 464: Line 453:


<!--T:146-->
<!--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, but it also serves to tell the function how many arguments to expect. For example, if you do this:
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>
</translate>
{{Code|code=
{{Code|code=
printsqm(45,34)
printsqm(45, 34)
}}
}}
<translate>
<translate>


<!--T:147-->
<!--T:147-->
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:
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>
</translate>
Line 486: Line 475:


<!--T:148-->
<!--T:148-->
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:
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.


</translate>
</translate>{{Top}}<translate>
{{Code|code=
sum(45,34)
}}
<translate>

<!--T:149-->
will print the result on the screen, but outside the interpreter, since there is no print command inside the function, nothing would appear on the screen. To have something printed you would need to use:

</translate>
{{Code|code=
print(sum(45, 34))
}}
<translate>


==Modules== <!--T:151-->
==Modules== <!--T:151-->
Line 509: Line 485:


<!--T:153-->
<!--T:153-->
Until now, we have written Python instructions line by line in the interpreter. But 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, Linux gedit, emacs, or vi), 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.
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.


<!--T:154-->
<!--T:154-->
To make Python execute that program, there are several 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 {{FileName|bin}} folder, or any of the {{FileName|Mod}} folders. (In Linux, you probably have a directory {{FileName|/home/<username>/.FreeCAD/Mod}}, let's add a subdirectory to that called {{FileName|scripts}} where we will put the text file.) Suppose we write a file like this:
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>/.local/share/FreeCAD/Mod/}} ({{VersionPlus|0.20}}) or {{FileName|/home/<username>/.FreeCAD/Mod/}} ({{VersionMinus|0.19}}).
* On Windows it is {{FileName|%APPDATA%\FreeCAD\Mod\}}, which is usually {{FileName|C:\Users\<username>\Appdata\Roaming\FreeCAD\Mod\}}.
* On macOS it is usually {{FileName|/Users/<username>/Library/Application Support/FreeCAD/Mod/}}.
Let's add a subfolder there called {{FileName|scripts}} and then write a file like this:


</translate>
</translate>
Line 524: Line 504:


<!--T:155-->
<!--T:155-->
and we save it as {{FileName|myTest.py}} in our {{FileName|FreeCAD/bin directory}} (or on Linux to {{FileName|/home/<username>/.FreeCAD/Mod/scripts}}.) Now, let's start FreeCAD, and in the interpreter window, write:
Save the file as {{FileName|myTest.py}} in the {{FileName|scripts}} folder, and in the interpreter window write:


</translate>
</translate>
Line 533: Line 513:


<!--T:156-->
<!--T:156-->
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.
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.


<!--T:157-->
<!--T:157-->
Normally when we write a sum() function in the interpreter, we execute it simply like that:
When we write a {{incode|sum()}} function in the interpreter, we execute it like this:


</translate>
</translate>
{{Code|code=
{{Code|code=
sum(14,45)
sum(14, 45)
}}
}}
<translate>
<translate>


<!--T:158-->
<!--T:158-->
Like we did earlier. When we import a module containing our sum() function, the syntax is a bit different. We do:
But when we import a module containing a {{incode|sum()}} function the syntax is a bit different:


</translate>
</translate>
{{Code|code=
{{Code|code=
myTest.sum(14,45)
myTest.sum(14, 45)
}}
}}
<translate>
<translate>


<!--T:159-->
<!--T:159-->
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 {{incode|something.somethingElse}}, with a dot in between, that means {{incode|somethingElse}} is inside {{incode|something}}.
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}}.


<!--T:160-->
<!--T:160-->
We can also import our sum() function directly into the main interpreter space, like this:
We can also import our sum() function directly into the main interpreter space:


</translate>
</translate>
{{Code|code=
{{Code|code=
from myTest import *
from myTest import *
sum(12,54)
sum(12, 54)
}}
}}
<translate>
<translate>


<!--T:161-->
<!--T:161-->
Basically all modules behave like that. You import a module, then you can use its functions: {{incode|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 from importing other modules inside your module!
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!


<!--T:162-->
<!--T:162-->
One last very 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 {{incode|help()}} function. Doing:
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:


</translate>
</translate>
{{Code|code=
{{Code|code=
help()
help("modules")
modules
}}
}}
<translate>
<translate>


<!--T:163-->
<!--T:163-->
Will give us a list of all available modules. We can now type {{incode|q}} to get out of the interactive help, and import any of them. We can even browse their content with the {{incode|dir()}} command
will give us a list of all available modules. We can import any of them and browse their content with the {{incode|dir()}} command:


</translate>
</translate>
Line 590: Line 569:


<!--T:164-->
<!--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 sin function in side the math module. Want to know how to use it?
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>
</translate>
{{Code|code=
{{Code|code=
print math.sin.__doc__
print(math.sin.__doc__)
}}
}}
<translate>
<translate>


<!--T:165-->
<!--T:165-->
It may not be evident, but on either side of doc are two underscore characters.
It may not be evident, but on either side of {{incode|doc}} are two underscore characters.


<!--T:174-->
<!--T:174-->
And finally one last little goodie: When we work on a new or existing module, it's best to replace the file extension with {{FileName|py}} such as: {{FileName|myModule.FCMacro}} => {{FileName|myModule.py}}. We often want to test it so we will load it as above.
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()}}:


</translate>
</translate>
{{Code|code=
{{Code|code=
import myModule
import importlib
importlib.reload(myTest)
myModule.myTestFunction()
}}
}}
<translate>
<translate>

<!--T:166-->
But what if we see that {{incode|myTestFunction()}} doesn't work correctly? We go back to our editor and make changes. Then, instead of closing and reopening the python interpreter, we can simply update the module like this:

</translate>
{{Code|code=
reload(myModule)
}}
<translate>

<!--T:175-->
This file renaming is because Python doesn't know about the extension {{FileName|FCMacro}}.


<!--T:176-->
<!--T:176-->
Line 631: Line 598:
<translate>
<translate>


</translate>{{Top}}<translate>
<!--T:178-->
To share code across macros, you can access the FreeCAD or FreeCADGui module (or any other Python module) and set any attribute to it. This should survive the execution of the macro.

</translate>
{{Code|code=
import FreeCAD
if hasattr(FreeCAD,"macro2_executed"):
...
else:
FreeCAD.macro2_executed = True # you can assign any value because we only check for the existence of the attribute
... execute macro2
}}
<translate>


==Starting with FreeCAD== <!--T:167-->
==Starting with FreeCAD== <!--T:167-->


<!--T:168-->
<!--T:168-->
Well, I think 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. So, just do:
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:


</translate>
</translate>
Line 656: Line 611:
<translate>
<translate>


</translate>{{Top}}<translate>
<!--T:169-->

and read on to [[FreeCAD Scripting Basics]].
==Notes== <!--T:197-->

<!--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].

</translate>{{Top}}<translate>


<!--T:170-->
Of course, we saw here only a very small part of the Python world. There are many important concepts that we didn't mention. There are three very important Python reference documents on the net:
* the [http://docs.python.org/3/tutorial/index.html official Python tutorial with way more information than this one]
* the [http://docs.python.org/reference/ official Python reference]
Be sure to bookmark them!


<!--T:171-->
<!--T:171-->
{{docnav
{{Docnav
|[[Macros|Macros]]
|[[Scripts|Scripts]]
|[[Python_scripting_tutorial|Python scripting tutorial]]
|[[Python_scripting_tutorial|Python scripting tutorial]]
}}
}}
Line 673: Line 630:
</translate>
</translate>
{{Powerdocnavi{{#translation:}}}}
{{Powerdocnavi{{#translation:}}}}
[[Category:Python_Code{{#translation:}}]]
[[Category:Developer Documentation{{#translation:}}]]
[[Category:Python Code{{#translation:}}]]
{{clear}}

Latest revision as of 17:12, 16 February 2023

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.

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>/.local/share/FreeCAD/Mod/ (version 0.20 and above) or /home/<username>/.FreeCAD/Mod/ (version 0.19 and below).
  • On Windows it is %APPDATA%\FreeCAD\Mod\, which is usually C:\Users\<username>\Appdata\Roaming\FreeCAD\Mod\.
  • On macOS it is usually /Users/<username>/Library/Application Support/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