Richard Richard - 4 months ago 35
Python Question

How to use Filter, Map, and Reduce in Python 3.3.0

Filter, map, and reduce work perfectly in Python 2.7.3. Here is an example:

>>> def f(x): return x % 2 != 0 and x % 3 != 0

>>> filter(f, range(2, 25))

[5, 7, 11, 13, 17, 19, 23]
>>> def cube(x): return x*x*x

>>> map(cube, range(1, 11))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
>>> def add(x,y): return x+y

>>> reduce(add, range(1, 11))
55


But in Python 3.3.0, I receive the following outputs:

>>> def f(x): return x % 2 != 0 and x % 3 != 0

>>> filter(f, range(2, 25))
<filter object at 0x0000000002C14908>
>>> def cube(x): return x*x*x

>>> map(cube, range(1, 11))
<map object at 0x0000000002C82B70>
>>> def add(x,y): return x+y

>>> reduce(add, range(1, 11))
Traceback (most recent call last):
File "<pyshell#8>", line 1, in <module>
reduce(add, range(1, 11))
NameError: name 'reduce' is not defined


I would appreciate if someone could explain to me why this is.

Screenshot of code for further clarity:

http://i.imgur.com/WekeD.png

Answer

You can read about the changes in What's New In Python 3.0. You should read it thoroughly when you move from 2.x to 3.x since a lot has been changed.

The whole answer here are quotes from the documentation.

Views And Iterators Instead Of Lists

Some well-known APIs no longer return lists:

  • [...]
  • map() and filter() return iterators. If you really need a list, a quick fix is e.g. list(map(...)), but a better fix is often to use a list comprehension (especially when the original code uses lambda), or rewriting the code so it doesn’t need a list at all. Particularly tricky is map() invoked for the side effects of the function; the correct transformation is to use a regular for loop (since creating a list would just be wasteful).
  • [...]

Builtins

  • [...]
  • Removed reduce(). Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable.
  • [...]
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