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Python from novice to pro
Modules, like everything else in Python, are objects. Once imported, you can always get a reference to a module through the global dictionary sys.modules.
Example 6.12. Introducing sys.modules
>>> import sys >>> print '\n'.join(sys.modules.keys()) win32api os.path os exceptions __main__ ntpath nt sys __builtin__ site signal UserDict stat
This example demonstrates how to use sys.modules.
Example 6.13. Using sys.modules
>>> import fileinfo >>> print '\n'.join(sys.modules.keys()) win32api os.path os fileinfo exceptions __main__ ntpath nt sys __builtin__ site signal UserDict stat >>> fileinfo <module 'fileinfo' from 'fileinfo.pyc'> >>> sys.modules["fileinfo"] <module 'fileinfo' from 'fileinfo.pyc'>
The next example shows how to use the __module__ class attribute with the sys.modules dictionary to get a reference to the module in which a class is defined.
Example 6.14. The __module__ Class Attribute
>>> from fileinfo import MP3FileInfo >>> MP3FileInfo.__module__ 'fileinfo' >>> sys.modules[MP3FileInfo.__module__] <module 'fileinfo' from 'fileinfo.pyc'>
|Every Python class has a built-in class attribute __module__, which is the name of the module in which the class is defined.|
|Combining this with the sys.modules dictionary, you can get a reference to the module in which a class is defined.|
Now you're ready to see how sys.modules is used in fileinfo.py, the sample program introduced in Chapter 5. This example shows that portion of the code.
Example 6.15. sys.modules in fileinfo.py
def getFileInfoClass(filename, module=sys.modules[FileInfo.__module__]): "get file info class from filename extension" subclass = "%sFileInfo" % os.path.splitext(filename).upper()[1:] return hasattr(module, subclass) and getattr(module, subclass) or FileInfo
|This is a function with two arguments; filename is required, but module is optional and defaults to the module that contains the FileInfo class. This looks inefficient, because you might expect Python to evaluate the sys.modules expression every time the function is called. In fact, Python evaluates default expressions only once, the first time the module is imported. As you'll see later, you never call this function with a module argument, so module serves as a function-level constant.|
|You'll plow through this line later, after you dive into the os module. For now, take it on faith that subclass ends up as the name of a class, like MP3FileInfo.|
|You already know about getattr, which gets a reference to an object by name. hasattr is a complementary function that checks whether an object has a particular attribute; in this case, whether a module has a particular class (although it works for any object and any attribute, just like getattr). In English, this line of code says, “If this module has the class named by subclass then return it, otherwise return the base class FileInfo.”|
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