6. Modules
If you quit from the Python interpreter and enter it again, the definitions you have made (functions and variables) are lost. Therefore, if you want to write a somewhat longer program, you are better off using a text editor to prepare the input for the interpreter and running it with that file as input instead. This is known as creating ascript. As your program gets longer, you may want to split it into several files for easier maintenance. You may also want to use a handy function that you’ve written in several programs without copying its definition into each program.
上诉几乎是废话，意思就是Module是一个方便的方法，方便你把写几个程序不用每个程序的拷贝它们的definition.
To support this, Python has a way to put definitions in a file and use them in a script or in an interactive instance of the interpreter. Such a file is called a module; definitions from a module can be imported into other modules or into the main module (the collection of variables that you have access to in a script executed at the top level and in calculator mode).
讲的是python对module机制的支持，可以call一个module出来。imoport到其他module和主要的module。
A module is a file containing Python definitions and statements. The file name is the module name with the suffix .pyappended. Within a module, the module’s name (as a string) is available as the value of the global variable__name__. For instance, use your favorite text editor to create a file called fibo.py in the current directory with the following contents:
一个module是一文件，就是一个file。名称带后缀.py。在一个模块内，模块名也可以作为全局变量。
# Fibonacci numbers module
def fib(n): # write Fibonacci series up to n
a, b = 0, 1
while b < n:
print b,
a, b = b, a+b
def fib2(n): # return Fibonacci series up to n
result = []
a, b = 0, 1
while b < n:
result.append(b)
a, b = b, a+b
return result
Now enter the Python interpreter and import this module with the following command:
This does not enter the names of the functions defined in fibo directly in the current symbol table; it only enters the module name fibo there. Using the module name you can access the functions:
If you intend to use a function often you can assign it to a local name:
6.1. More on Modules
A module can contain executable statements as well as function definitions. These statements are intended to initialize the module. They are executed only the first time the module is imported somewhere. [1]
Each module has its own private symbol table, which is used as the global symbol table by all functions defined in the module. Thus, the author of a module can use global variables in the module without worrying about accidental clashes with a user’s global variables. On the other hand, if you know what you are doing you can touch a module’s global variables with the same notation used to refer to its functions, modname.itemname.
上述一大段话要说明的其实就是私有和公有问题，有些module在一些环境中，不用提供全部的function，有些是自己内部的。所以有下面的import。
Modules can import other modules. It is customary but not required to place all import statements at the beginning of a module (or script, for that matter). The imported module names are placed in the importing module’s global symbol table.
There is a variant of the import statement that imports names from a module directly into the importing module’s symbol table. For example:
This does not introduce the module name from which the imports are taken in the local symbol table (so in the example, fibo is not defined).
There is even a variant to import all names that a module defines:
6.1.1. Executing modules as scripts
下面举例让module像脚本一样跑
When you run a Python module with
python fibo.py <arguments>
the code in the module will be executed, just as if you imported it, but with the __name__ set to "__main__". That means that by adding this code at the end of your module:
you can make the file usable as a script as well as an importable module, because the code that parses the command line only runs if the module is executed as the “main” file:
$ python fibo.py 50 1 1 2 3 5 8 13 21 34
If the module is imported, the code is not run:
This is often used either to provide a convenient user interface to a module, or for testing purposes (running the module as a script executes a test suite).
6.1.2. The Module Search Path
When a module named spam is imported, the interpreter searches for a file named spam.py in the directory containing the input script and then in the list of directories specified by the environment variable PYTHONPATH. This has the same syntax as the shell variable PATH, that is, a list of directory names. When PYTHONPATH is not set, or when the file is not found there, the search continues in an installation-dependent default path; on Unix, this is usually.:/usr/local/lib/python.
讲了module的搜索路径，有个环境变量交PYTHONPATH，同样的语法作为shell 的各种PATH，其实是目录列表。如果这个环境变量没设置，会从默认位置找。
Actually, modules are searched in the list of directories given by the variable sys.path which is initialized from the directory containing the input script (or the current directory), PYTHONPATH and the installation- dependent default. This allows Python programs that know what they’re doing to modify or replace the module search path. Note that because the directory containing the script being run is on the search path, it is important that the script not have the same name as a standard module, or Python will attempt to load the script as a module when that module is imported. This will generally be an error. See section Standard Modules for more information.
这个讲要注意module名词不要和standard modules重复
6.2. Standard Modules
Python comes with a library of standard modules, described in a separate document, the Python Library Reference (“Library Reference” hereafter). Some modules are built into the interpreter; these provide access to operations that are not part of the core of the language but are nevertheless built in, either for efficiency or to provide access to operating system primitives such as system calls. The set of such modules is a configuration option which also depends on the underlying platform For example, the winreg module is only provided on Windows systems. One particular module deserves some attention: sys, which is built into every Python interpreter. The variables sys.ps1and sys.ps2 define the strings used as primary and secondary prompts:
These two variables are only defined if the interpreter is in interactive mode.
The variable sys.path is a list of strings that determines the interpreter’s search path for modules. It is initialized to a default path taken from the environment variable PYTHONPATH, or from a built-in default ifPYTHONPATH is not set. You can modify it using standard list operations:
用这个sys.path可以修改PYTHONPATH的设置
6.3. The dir() Function
The built-in function dir() is used to find out which names a module defines. It returns a sorted list of strings:
这个dir用来找出module里边的定义。并且返回他们的字符串
Without arguments, dir() lists the names you have defined currently:
Note that it lists all types of names: variables, modules, functions, etc.
dir() does not list the names of built-in functions and variables. If you want a list of those, they are defined in the standard module __builtin__:
6.4. Packages
packages 是一个module命名空间的结构。其实就是把其他module封装起来方便调用而已。
Packages are a way of structuring Python’s module namespace by using “dotted module names”. For example, the module name A.B designates a submodule named B in a package named A. Just like the use of modules saves the authors of different modules from having to worry about each other’s global variable names, the use of dotted module names saves the authors of multi-module packages like NumPy or the Python Imaging Library from having to worry about each other’s module names.
Suppose you want to design a collection of modules (a “package”) for the uniform handling of sound files and sound data. There are many different sound file formats (usually recognized by their extension, for example: .wav, .aiff,.au), so you may need to create and maintain a growing collection of modules for the conversion between the various file formats. There are also many different operations you might want to perform on sound data (such as mixing, adding echo, applying an equalizer function, creating an artificial stereo effect), so in addition you will be writing a never-ending stream of modules to perform these operations. Here’s a possible structure for your package (expressed in terms of a hierarchical filesystem):
sound/ Top-level package
__init__.py Initialize the sound package
formats/ Subpackage for file format conversions
__init__.py
wavread.py
wavwrite.py
aiffread.py
aiffwrite.py
auread.py
auwrite.py
...
effects/ Subpackage for sound effects
__init__.py
echo.py
surround.py
reverse.py
...
filters/ Subpackage for filters
__init__.py
equalizer.py
vocoder.py
karaoke.py
...
When importing the package, Python searches through the directories on sys.path looking for the package subdirectory.
The __init__.py files are required to make Python treat the directories as containing packages; this is done to prevent directories with a common name, such as string, from unintentionally hiding valid modules that occur later on the module search path. In the simplest case, __init__.py can just be an empty file, but it can also execute initialization code for the package or set the __all__ variable, described later.
Users of the package can import individual modules from the package, for example:
This loads the submodule sound.effects.echo. It must be referenced with its full name.
An alternative way of importing the submodule is:
This also loads the submodule echo, and makes it available without its package prefix, so it can be used as follows:
Yet another variation is to import the desired function or variable directly:
Again, this loads the submodule echo, but this makes its function echofilter() directly available:
Note that when using from package import item, the item can be either a submodule (or subpackage) of the package, or some other name defined in the package, like a function, class or variable. The import statement first tests whether the item is defined in the package; if not, it assumes it is a module and attempts to load it. If it fails to find it, an ImportError exception is raised.
Contrarily, when using syntax like import item.subitem.subsubitem, each item except for the last must be a package; the last item can be a module or a package but can’t be a class or function or variable defined in the previous item.
6.4.2. Intra-package References
个人感觉下面这句话比较有价值
These explicit relative imports use leading dots to indicate the current and parent packages involved in the relative import. From the surround module for example, you might use:
6.4.3. Packages in Multiple Directories
Packages support one more special attribute, __path__. This is initialized to be a list containing the name of the directory holding the package’s __init__.py before the code in that file is executed. This variable can be modified; doing so affects future searches for modules and subpackages contained in the package.
While this feature is not often needed, it can be used to extend the set of modules found in a package.
Packages 支持更多的特殊属性，__path__。初始化列表在__init__.py里边，在代码执行前都会执行它。所以我们可以根据自己的需要修改一下。但是这个特点不经常需要，它经常被用来扩展package里边的modules设置。