Python filter() function

Question

filter(function,  an_iter)
*If the iterable an_iter is a sequence, then the returned value is of that same type, 
otherwise the returned value is a list.* 

I came across the above description as part of the definition for the filter(func, a_sequence) function in Python.

I understand how filter works on a sequence type (lists, strings, tuples). However, can you give me situations where a non-sequence type is the an_iter parameter and what kind of result would form?

Answer 1

When it says 'non-sequence', it basically means generators or unordered iterables. Here is an example with xrange:

>>> filter(lambda n: n % 2, xrange(10))
[1, 3, 5, 7, 9]

And with a set:

>>> filter(lambda n: n % 2, {0, 1, 2, 3, 4, 5, 6, 7, 8, 9})
[1, 3, 5, 7, 9]

Answer 2

For python 3, the definition is changed.

From doc

filter(function, iterable)

Construct an iterator from those elements of iterable for which function returns true. iterable may be either a sequence, a container which supports iteration, or an iterator. If function is None, the identity function is assumed, that is, all elements of iterable that are false are removed.

Example:

>>> filter(lambda x: x in 'hello buddy!', 'hello world')
<filter object at 0x000002ACBEEDCB00> # filter returns object !important

>>> ''.join([i for i in filter(lambda x: x in 'hello buddy!', 'hello world')])
'hello old'

>>> [i for i in filter(lambda n: n % 2, {0, 1, 2, 3, 4, 5, 6, 7, 8, 9})]
[1, 3, 5, 7, 9]