filter

Function filter() applies function to all sequence elements and returns an iterator with those objects for which function has returned True.

For example, return only those strings that contain numbers:

In [1]: list_of_strings = ['one', 'two', 'list', '', 'dict', '100', '1', '50']

In [2]: filter(str.isdigit, list_of_strings)
Out[2]: <filter at 0xb45eb1cc>

In [3]: list(filter(str.isdigit, list_of_strings))
Out[3]: ['100', '1', '50']

From the list of numbers leave only odd:

In [3]: list(filter(lambda x: x % 2, [10, 111, 102, 213, 314, 515]))
Out[3]: [111, 213, 515]

Similarly, only even ones:

In [4]: list(filter(lambda x: not x % 2, [10, 111, 102, 213, 314, 515]))
Out[4]: [10, 102, 314]

From the list of words leave only those with more than two letters:

In [5]: list_of_words = ['one', 'two', 'list', '', 'dict']

In [6]: list(filter(lambda x: len(x) > 2, list_of_words))
Out[6]: ['one', 'two', 'list', 'dict']

List comprehension instead of filter

As a rule, you can use list comprehension instead of filter.

Examples similar to those above in list comprehension version.

Return only those strings that contain numbers:

In [7]: list_of_strings = ['one', 'two', 'list', '', 'dict', '100', '1', '50']

In [8]: [s for s in list_of_strings if s.isdigit()]
Out[8]: ['100', '1', '50']

Odd/even numbers:

In [9]: nums = [10, 111, 102, 213, 314, 515]

In [10]: [n for n in nums if n % 2]
Out[10]: [111, 213, 515]

In [11]: [n for n in nums if not n % 2]
Out[11]: [10, 102, 314]

From the list of words leave only those with more than two letters:

In [12]: list_of_words = ['one', 'two', 'list', '', 'dict']

In [13]: [word for word in list_of_words if len(word) > 2]
Out[13]: ['one', 'two', 'list', 'dict']