Introduction to Python

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 very different from other programming languages, and very accessible to new users:


 * It has been designed to be to readable by human beings, making it 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 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 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.

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!

Python version information
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
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.

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.



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

The interpreter shows the Python version, then a 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:

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:

Python will tell us that it doesn't know what hello is. The 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. 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.

The Python interpreter also has a built-in help system. Try typing:

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 command:

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

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

Variables
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:

I guess you understood what happened, we saved the string under the name. Now that 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:

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:

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

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

Case is very important. is not the same as. If you were to enter it would come back with an error as not defined.

Numbers
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 command recognized our  string. By using characters, we told specifically that what follows next is a text string.

We can always check the data type of a variable with the special Python keyword type:

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

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, and Python can do with them everything it can do with integers. Look at the results of this:

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:

Integers and Floats can be mixed together without problems:

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:

This will give us an error, is a string and  is an integer, and Python doesn't know what to do. However, we can force Python to convert between types:

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

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

Note on Python commands

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 doing those two commands,

have exactly the same result. That 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:

Lists
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, we define a list by using :

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:

or retrieving one item of a list:

You see that while the 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.

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

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

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.

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

We iterated (programming jargon) through our list with the command and did something with each of the items. Note the special syntax: the 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 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. As long as you write your next lines with the 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

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.

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 command:

(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 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 type into the interpreter you will see:

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 :

Another example:

You see that the 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:

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

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

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

Functions
There are few 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 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:

(Another copy and paste error, only copy through the end of the indented section i.e. " square meters" Paste to the interpreter, and issue until the three dot prompt goes a way, then copy and paste the final line.)

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.

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 to tell the function how many arguments to expect. For example, if you do this:

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:

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:

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. You would need to:

to have something printed. Read more about functions here.

Modules
Now that we have a good idea of how Python works, we'll need one last thing: How to work with files and modules.

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, 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.

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. (In Linux, you probably have a directory /home/ /.FreeCAD/Mod, let's add a subdirectory to that called scripts where we will put the text file.) Suppose we write a file like this:

and we save it as myTest.py in our FreeCAD/bin directory (or on Linux to /home/ /.FreeCAD/Mod/scripts.) Now, let's start FreeCAD, and in the interpreter window, write:

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.

Normally when we write a sum function in the interpreter, we execute it simply like that:

Like we did earlier. When we import a module containing our sum function, the syntax is a bit different. We do:

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.

We can also import our sum function directly into the main interpreter space, like this:

Basically all modules behave like that. You import a module, then you can use its functions: 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!

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:

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

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?

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

And finally one last little goodie: When we work on a new or existing module, it's best to replace the file extension with py such as: myModule.FCMacro => myModule.py. We often want to test it so we will load it as above.

But what if we see that 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:

This file renaming is because Python doesn't know about the extension FCMacro.

However, there are two alternates: Inside the one macro use Python's exec or execfile functions.

or

For Python 3.xxx replace this code with:

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.

Starting with FreeCAD
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

and read on to FreeCAD Scripting Basics.

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: Be sure to bookmark them!
 * the official Python tutorial with way more information than this one
 * the official Python reference
 * the Dive into Python wikibook/ book.