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CHAPTER 10 ■ BATTERIES INCLUDED 213
>>> 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.
This is interesting: it 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 Chapter 6. A docstring is simply a string you write at
the beginning of a function to document it. That string is then stored in 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 info, such as the function signature (that is, what 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 stored 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).
Documentation
A natural source for information about a module is, of course, its documentation. I’ve postponed
the discussion of documentation because it’s often much quicker to just examine the module
a bit yourself first. For example, you may wonder, “What were the arguments to range again?”
Instead of searching through a Python book or the standard Python documentation for a
description of range, you can just check it directly:
>>> print range.__doc__
range([start,] stop[, step]) -> list of integers
Return a list containing an arithmetic progression of integers.
range(i, j) returns [i, i+1, i+2,..., j-1]; start (!) defaults to 0.
When step is given, it specifies the increment (or decrement).
For example, range(4) returns [0, 1, 2, 3]. The end point is omitted!
These are exactly the valid indices for a list of 4 elements.