Introduction to Python/zh-cn

本简短教程可供Python新手参阅. Python是一种开源、支持多平台的 编程语言. Python有几种不同于其他常见语言的特性，使之与众不同，而且还易于对此毫无经验的人使用：


 * Python被设计为一种易于常人阅读的语言，因此方便学习且代码容易理解.
 * Python是一种解释型语言，也就是它并非C那样的编译型语言，在程序运行前无需进行编译. 您所编写的代码将如您所愿即刻逐行执行. 由于代码可放慢脚步逐行执行，因此更便于学习与查找代码中错误.
 * 可将Python嵌入其他程序用作脚本语言. FreeCAD中配有一个内嵌的Python解释器；您可以在FreeCAD中编写Python代码来控制FreeCAD的各种组件，比如用来创建一个几何图形等等. 这是一种极其强大的工具，程序员可通过它编写代码，以此取代仅能点击标有"创建球体"按钮的机械式操作；您可以借此自由方便地构建自己的工具，创建精准的几何图形，但程序员可能无法预览待创建的图形.
 * Python具有可扩展性，您可以轻松地为它接入新的模块来拓展其功能. 例如，您可以借助特定模块使Python可以读写jpg图片、与twitter进行交互，甚至调度任务在操作系统上的执行，等等.

我们墙裂鼓励您亲自将代码输入至Python解释器. 经过我们的激烈讨论，认为重点在于令代码跑起来，并显示出结果. 不运行代码根本不会理解Python. 因此，快动手吧！以下是相关简介，而并非完整的教程. 但是我们希望这能为您深入理解FreeCAD中的内部机制而提供相关的基础知识.

解释器
用户在写程序时通常会打开文本编辑器或特定的编程环境(一般是配有其他辅助工具的文本编辑器)，借此编写代码、编译并执行. 当代码中存在一个或多个错误时，程序往往就跑不起来了. 这时，您可以根据报错信息定位错误的根源. 再返回编辑器，纠正错误，再次运行，如此往复直到程序可按预期执行为止.

在整个处理流程中，Python解释器的内部工作原理对用户而言都是透明的. 所谓的解释器是一种附有命令提示符的Python窗口，在此，您可以方便地输入Python代码. 如果您在自己的计算机上安装了Python(如果系统为Windows或Mac，从Python website下载；如果系统是GNU/Linux，则借助软件包资料库（package repository）进行安装), 您可从开始菜单中开启Python解释器. 但是，FreeCAD中的下方窗口中也集成了Python解释器：



(If you don't have it, click on View --> Panels --> Python console.)

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

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 simply, in programming jargon, a piece of text. Without the ", the print command believed hello was not a piece of text but a special Python keyword. The important thing is, you immediately get notified that you made an error. By pressing the up arrow (or, in the FreeCAD interpreter, CTRL+up arrow), you can go back to the last command you wrote and correct it.

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 "print" command:

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

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

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:

We changed the value of myVariable. 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 "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.

Numbers
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:

Int and Floats can be mixed together without problem:

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:

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

We can also use int and float 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 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.

Lists
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:

or retrieving one item of a list:

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.

Indentation
One big cool use of lists is also browsing through them and do something with each item. For example look at this:

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

Or more complex things like this:

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

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

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

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