[docs] map objects are not lists

Aaron Leanhardt aaron at leanhardtingenuities.com
Thu Sep 20 21:14:36 CEST 2012

5.1.3. List Comprehensions<http://docs.python.org/dev/tutorial/datastructures.html#list-comprehensions>

List comprehensions provide a concise way to create lists. Common
applications are to make new lists where each element is the result of some
operations applied to each member of another sequence or iterable, or to
create a subsequence of those elements that satisfy a certain condition.

For example, assume we want to create a list of squares, like:

>>> squares = []>>> for x in range(10):...     squares.append(x**2)...>>> squares[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

We can obtain the same result with:

squares = [x**2 for x in range(10)]

This is also equivalent to squares = map(lambda x: x**2, range(10)), but
it’s more concise and readable.

I think that the last sentence above should read:

squares = list(map(lambda x: x**2, range(10)))

In other words, the map function returns a map object, not a list object,
so the list() function needs to be used to convert it to something that is
truly equivalent to the previous definitions of "squares".  (In case it
matters, I am using Python-3.3.0rc2 on RHEL 6.3.)


Aaron E. Leanhardt, http://www.leanhardtingenuities.com/
*  --being ingenious is more valuable than being a genius!*
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