CODEWITHSUNDEEP
pythoninputfilter
nsamuel
nsamuel

Reputation: 105

Python filter() function

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?

Upvotes: 4

Views: 5255

Answers (3)

Asclepius
Asclepius

Reputation: 63282

It may help to see the effective implementation of filter:

def filtered(function, iterable):
    if function is None:
        return (item for item in iterable if item)
    return (item for item in iterable if function(item))

Another way to implement it (with the same result) is:

def filtered(function, iterable):
    if function is None:
        function = bool
    return (item for item in iterable if function(item))

The result always is iterable.

Upvotes: 0

shaik moeed
shaik moeed

Reputation: 5785

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]

Upvotes: 5

anon582847382
anon582847382

Reputation: 20351

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]

Upvotes: 5

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