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To find out what a module contains, you can use the dir function, which lists all the attributes of an object (and therefore all functions, classes, variables, and so on of a module). If you print out dir(copy), you get a long list of names. (Go ahead, try it.) Several of these names begin with an underscore a hint (by convention) that they aren t meant to be used outside the module. So let s filter them out with a little list comprehension (check the section on list comprehension in 5 if you don t remember how this works): >>> [n for n in dir(copy) if not n.startswith('_')] ['Error', 'PyStringMap', 'copy', 'deepcopy', 'dispatch_table', 'error', 'name', 't'] The list comprehension is the list consisting of all the names from dir(copy) that don t have an underscore as their first letter. This list is much less confusing than the full listing.
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1. The term grok is hackerspeak, meaning to understand fully, taken from Robert A. Heinlein s novel Stranger in a Strange Land (Ace Books, reissue 1995).
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The __all__ Variable
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What I did with the little list comprehension in the previous section was to make a guess about what I was supposed to see in the copy module. However, you can get the correct answer directly from the module itself. In the full dir(copy) list, you may have noticed the name __all__. This is a variable containing a list similar to the one I created with list comprehension, except that this list has been set in the module itself. Let s see what it contains: >>> copy.__all__ ['Error', 'copy', 'deepcopy'] My guess wasn t all that bad. I got only a few extra names that weren t intended for my use. But where did this __all__ list come from, and why is it really there The first question is easy to answer. It was set in the copy module, like this (copied directly from copy.py): __all__ = ["Error", "copy", "deepcopy"] So why is it there It defines the public interface of the module. More specifically, it tells the interpreter what it means to import all the names from this module. So if you use this: from copy import * you get only the four functions listed in the __all__ variable. To import PyStringMap, for example, you would need to be explicit, by either importing copy and using copy.PyStringMap, or by using from copy import PyStringMap. Setting __all__ like this is actually a useful technique when writing modules, too. Because you may have a lot of variables, functions, and classes in your module that other programs might not need or want, it is only polite to filter them out. If you don t set __all__, the names exported in a starred import defaults to all global names in the module that don t begin with an underscore.
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Until now, you ve been using your ingenuity and knowledge of various Python functions and special attributes to explore the copy module. The interactive interpreter is a very powerful tool for this sort of exploration because your mastery of the language is the only limit to how deeply you can probe a module. However, there is one standard function that gives you all the information you would normally need. That function is called help. Let s try it on the copy function: >>> help(copy.copy) Help on function copy in module copy: copy(x) Shallow copy operation on arbitrary Python objects. See the module's __doc__ string for more info. >>>
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This tells you that copy takes a single argument x, and that it is a shallow copy operation. But it also mentions the module s __doc__ string. What s that You may remember that I mentioned docstrings in 6. A docstring is simply a string you write at the beginning of a function to document it. That string may then be referred to by the function attribute __doc__. As you may understand from the preceding help text, modules may also have docstrings (they are written at the beginning of the module), as may classes (they are written at the beginning of the class). Actually, the preceding help text was extracted from the copy function s docstring: >>> print copy.copy.__doc__ Shallow copy operation on arbitrary Python objects. See the module's __doc__ string for more info. The advantage of using help over just examining the docstring directly like this is that you get more information, such as the function signature (that is, the arguments it takes). Try to call help(copy) (on the module itself) and see what you get. It prints out a lot of information, including a thorough discussion of the difference between copy and deepcopy (essentially that deepcopy(x) makes copies of the values found in x as attributes and so on, while copy(x) just copies x, binding the attributes of the copy to the same values as those of x).
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