Introduction to Python/fr

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


 * It has been designed specially to be easy to read by human beings, and so it is very easy to learn and understand.
 * 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 wish. Because you can go slowly, step-by-step, it is extremely easy to learn and to find errors in your code.
 * It can be embedded in other programs to be used as 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 easily build your own tool, creating exactly the geometry you want, in a manner or shape that the programmer may not foresee.
 * 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.

We strongly encourage you to enter the code snippets below into a Python interpreter. For many of our discussions, the important point is the line after the snippet is run, the reveal. Not running the code would be all build up, without a punch line. So, hands on! The following is a very simple introduction, and by no means a complete tutorial. But our hope is that this will provide enough basics to explore deeper into the FreeCAD mechanisms.

L’interpréteur
Usually, when writing computer programs, you simply open a text editor or your special programming environment, (which is usually a text editor with several additional tools) write your program, then compile and execute. Usually, 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.

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 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 lower window:



(SI vous ne l'avez pas, cliquez sur le menu Affichage --> Panneaux --> Console Python).

L’interpréteur affiche la version de Python installée, puis le symbole >>>, qui est l'invite de commande pour entrer votre code Python. L'écriture du code dans l'interpréteur est très simple: une ligne, est une instruction. Lorsque vous appuyez sur, votre ligne de code est exécuté (après avoir été instantanément compilé et cela de manière transparente pour vous).

Par exemple, écrivez ce code:

Ici print est une commande spéciale de Python qui signifie: affiche ce que je te demande. Lorsque vous pressez, l'opération s’exécute et le message "bonjour" s'affiche à l'écran. Si vous effectuez une erreur, par exemple, écrivez:

Python vous dira qu'il ne sait pas ce qu'est bonjour. Les caractères " (guillemets) spécifient que le contenu est une chaîne de caractères qui doit être affichée. Sans les " (guillemets), la commande d'affichage de bonjour n'est pas reconnue comme du texte, mais comme un mot-réservé spécial de Python. L'important est, que vous obtenez immédiatement une notification d'erreur. En appuyant sur la (ou, dans l'interpréteur FreeCAD, ), vous pouvez revenir à la dernière commande que vous avez écrite et la corriger.

L'interpréteur Python dispose également d'un système d'aide intégré. Voulez vous taper:

ou, par exemple, nous n'avons pas compris ce qui n'allait pas avec notre commande d'affichage "help" ci-dessus, et nous allons demander des informations spécifiques sur la commande "print": tapez

Nous voilà devant une longue description sur la commande "print".

Now that we totally dominate our interpreter, we can begin with the serious stuff.

Les Variables
Of course, printing "hello" is not very interesting. More interesting is printing stuff you didn't know before, or let Python find for you. That's where the concept of the variable comes in. A variable is simply a value that you store under a name. For example, type this:

I guess you understood what happened, we "saved" the string "hello" under the name "a." Now, "a" is not an unknown name any more! 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 could write:

See? now hello is not an undefined word any more. What if, by terrible bad luck, we choose a name that already exists in Python? Let's say we want to store our string under the name "print":

Python is very intelligent and will tell us that this is not possible. It has some "reserved" keywords that cannot be modified. But our variables can be modified any time, that's why they are called variables, the contents can vary. For example:

Nous venons de changer la valeur de myVariable. Nous pouvons également copier des variables:

Note that it is important 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, the difference in the upper/lower case v. If you were to enter print myvariable it would come back with an error as not defined.

Les Nombres
Of course you must know that programming is useful to treat all kinds 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 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 myVar is 'str', short for string in Python jargon. 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 "int", 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 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 float numbers can have a decimal part:

Les types entier et à virgule flottante, Int et Float peuvent être mélangés sans problème:

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 use. In other cases it doesn't. For example:

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

Maintenant, l'opération fonctionne, pourquoi ! Vous avez noté, que nous avons converti varB en "string" au moment de l'affichage avec la commande str, mais nous n'avons pas modifié le type de varB qui reste un int. Si nous voulons convertir varB de façon permanente en une chaîne de caractères pour les besoins futur du programme, nous aurons besoin de faire:

Nous pouvons également utiliser les commandes int et float pour convertir une chaîne de caractères str en un int ou float Pour la conversion, il faut faire:

Note au sujet des commandes Python

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,

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:

You must have seen that most of the Python commands (or keywords) type, int, str, etc. have parenthesis to limit the command contents. The only exception is the print command, which in fact is not really an exception, as it also works normally: print("hello"). However, since it is used often, the Python designers allowed a simpler version.

Les Listes (Tableaux)
Another interesting data type is a list. A list is simply 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 variables together. You can then do all kinds of things within that group, for example counting them:

ou récupérer un objet de cette liste:

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 with lists, you can read here, such as sorting contents, removing or adding elements.

A funny and interesting thing: a text string is very similar to a list of characters! 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, ints, floats and lists, there are more built-in data types, such as dictionaries, or you can even create your own data types with classes.

L'Indentation
Une manière pratique et élégante d'afficher chaque élément de la liste, est de naviguer à l’intérieur de cette liste.

Entrez ce code dans la console:

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 a colon ended line has happened and more is coming.

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 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 finished, just write another line without indentation, or simply press Enter to come back from the for-in block

Indentation is cool because it aids in program readability. If you use large indentations (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 commands other than for-in, can have indented blocks of code too.

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:

(If you have been running the code examples in an interpreter by Copying and 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 to the interpreter.  In the interpreter issue an until the three dot prompt disappears and the code runs.  Then copy the final two lines into the interpreter followed by one or more The final answer should appear.)

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

Here the square brackets denote an optional parameter. However all are expected to be integers. Below we will force the range parameters to be an integer using int

Ou des choses plus complexes comme ceci:

Vous voyez que la commande range a également la particularité de commencer à 0 (si vous ne spécifiez pas un nombre de départ) et que son dernier nombre sera le nombre que vous aurez spécifié moins un. Bien sûr, cette commande fonctionne parfaitement avec les autres commandes Python.

Par exemple:

Une autre fonction intéressante utilisée dans un bloc indenté est la commande de condition if (si). Avec " if " la suite de la procédure sera exécutée uniquement si la condition est remplie.

C'est bien, ce code affiche "OK il c'est bien un Dalton !!!" car la condition est exacte. Mais maintenant essayons cette ligne:

Il ne c'est rien affiché car la condition n'était pas remplie. Nous pouvons alors lui demander else (si la condition n'est pas remplie alors):

Les Fonctions
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:

Cette commande affichera l'erreur "TypeError: printsqm takes exactly 1 argument (2 given)" car la fonction "def printsqm(myValue):" ne demande qu'un seul argument, "myValue" et, nous lui en avons donné deux, 45 et 34.

Maintenant, écrivez cette fonction:

Nous avons créé une fonction qui demande deux arguments, les exécutes, et nous renvoie le résultat. Le retour du résultat est très utile car nous pouvons l'utiliser pour l'afficher ou le stocker dans une variable myTotal (pour notre exemple mais n'importe quel nom conviendra) ou les deux. Comme nous sommes dans l'interpréteur de Python, le résultat s'affiche en faisant:

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:

Voilà c'est affiché.

Pour plus de renseignements sur les autres possibilités des fonctions.

Les Modules
Maintenant, vous avez une idée du fonctionnement de Python: mais comment faire pour travailler avec les fichiers et les 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:

sans l'extension .py.

Le contenu du fichier sera tout simplement exécuté, ligne par ligne, comme si nous l'avions écrit dans l'interpréteur. La fonction somme a été créée, et le message "test.py a bien été chargé" sera affiché. Il ya une grande différence: la commande import est faite non seulement pour exécuter des programmes écrits dans des fichiers comme le nôtre, mais aussi de charger des fonctions dans Python, de sorte qu'elles deviennent disponibles dans l'interpréteur. Les fichiers contenant des fonctions, comme le nôtre, sont appelés modules.

Normalement, lorsque nous écrivons une fonction sum dans l'interpréteur, nous l'exécutons simplement comme ceci,

comme nous l'avons fait plus haut.

Mais quand nous importons un module contenant une fonction comme sum(a,b), la syntaxe est un peu différente. Nous ferons:

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!

Encore une chose extrêmement utile. Comment connaître les modules disponibles ? quelles sont les fonctions contenues dans ces modules et comment les utiliser (c'est à dire quels arguments sont demandés par la fonction)? Nous avons vu que Python a une fonction d'aide.

Alors, dans l'interpréteur Python de FreeCad faisons:

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

Nous voyons maintenant toutes les fonctions contenues dans le module math, ainsi que des trucs étranges comme:'' __ doc__, __ FILE__, __ name__. . . ''.

Le __ doc__ est extrêmement utile, il s'agit d'un texte de documentation. Dans les modules, chaque fonction de fait a une __ doc__ qui explique comment l'utiliser. Par exemple, nous voyons qu'il ya une fonction sin dans le module math.

Vous voulez savoir comment utiliser cette fonction ? alors:

(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.

ou

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.

Démarrer avec 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

et lire dans l’interpréteur tous les modules chargés dans FreeCad, voir Scripts de base dans FreeCad...

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.