`filter()` 在 Python 2.x 和 3.x 之间是否发生了变化？

Question

I tried many different examples for filter() with anonymous functions, but always get strange results as long as I use it on strings. Below is an example:

>>>print(filter(lambda x: x.isdigit(), "aas30dsa20"))
<filter object at 0x00000000035DE470>

If not strings, everything works fine. Eg;

>>> print(list(filter(lambda x: x >= 30 and x <= 70, [x**2 for x in range(1,11)])))
[36, 49, 64]

By the way, if I remove the list() function part, the problem similar to string case appears:

>>> print(filter(lambda x: x >= 30 and x <= 70, [x**2 for x in range(1,11)]))
<filter object at 0x00000000037BFDD8>

I am using Python 3.4.1 on Windows 7.

Answer 1

Yes. Several functional tools (most notably filter() and map() ) were changed to return iterators instead of sequences for 3.x.

Answer 2

In Python 2, the filter() function returned a list, the result of filtering a sequence through a function that returned True or False for each item in the sequence. In Python 3, the filter() function returns an iterator, not a list. Source: diveintopython3.net

2to3 tool will in some cases place a list() call around the call to filter() to ensure that the result is still a list. If you need code that runs in both Python 2 and Python 3 without 2to3 conversion and you need the result to be a list, you can do the same.