{"id":1867,"date":"2019-12-29T11:51:43","date_gmt":"2019-12-29T11:51:43","guid":{"rendered":"https:\/\/www.askpython.com\/?p=1867"},"modified":"2022-08-06T13:09:14","modified_gmt":"2022-08-06T13:09:14","slug":"python-filter-function","status":"publish","type":"post","link":"https:\/\/www.askpython.com\/python\/built-in-methods\/python-filter-function","title":{"rendered":"Python filter() function"},"content":{"rendered":"\n

Python’s filter<\/strong>()<\/code> function is used to filter the elements of an iterable (sequence) with the help of a predicate that tests each element on the iterable.<\/p>\n\n\n\n

A predicate is a function that always returns True<\/strong><\/code> or False<\/strong><\/code>. We cannot use a generic function with filter()<\/code>, since it returns all elements only if a suitable condition is met. This means that the filtering function must always return a boolean value, and thus, the filter function is a predicate.<\/p>\n\n\n\n

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Basic Format of filter()<\/h2>\n\n\n\n

Since this is a function that operates on a Python iterable, the iterable is one of the parameters. And since it tests a predicate on each element, the function is also another parameter which is required.<\/p>\n\n\n\n

And since it filters out elements from the sequence, it must also return an iterable, which only consists of the elements which satisfy the filtering function.<\/p>\n\n\n\n

But in this case, since we work with objects, Python returns us a filter object<\/em><\/strong> as an iterable, which will prove to be handy for conversion to other types, using methods like list()<\/code> and dict()<\/code>.<\/p>\n\n\n\n

Simple, isn’t it? Let us look at how we apply this and create working programs using filter()<\/code>.<\/p>\n\n\n\n

Format: filter_object = filter(predicate, iterable)<\/code><\/p>\n\n\n\n

Here is a very simple example that filters a list with a function that tests whether a number is odd or even.<\/p>\n\n\n

\na = [1, 2, 3, 4, 5]\n\n# We filter using a lambda function predicate.\n# This predicate returns true\n# only if the number is even.\nfilter_obj_even = filter(lambda x: x%2 == 0, a)\n\nprint(type(filter_obj_even))\n\n# Convert to a list using list()\nprint('Even numbers:', list(filter_obj_even))\n\n# We can also use define the predicate using def()\ndef odd(num):\n    return (num % 2) != 0\n\nfilter_obj_odd = filter(odd, a)\nprint('Odd numbers:', list(filter_obj_odd))\n<\/pre><\/div>\n\n\n
Output<\/p>\n\n\n
\n<class 'filter'>\nEven numbers: [2, 4]\nOdd numbers: [1, 3, 5]\n<\/pre><\/div>\n\n\n
Note that we can get the individual elements of the filter object by iterating through it, since it is an iterable:<\/p>\n\n\n
\nfor item in filter_obj_odd:\n    print(item)\n<\/pre><\/div>\n\n\n
Output<\/p>\n\n\n
\n1\n3\n5\n<\/pre><\/div>\n\n\n
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filter() and None<\/h2>\n\n\n\n
We can also use None<\/code> as a predicate with filter()<\/code>. None<\/code> returns True<\/code> if the object has a boolean value of True<\/code>, and False<\/code> otherwise.<\/p>\n\n\n\n
This means that objects like 0<\/code>, None<\/code>, ''<\/code>, []<\/code> etc, are all filtered out by None<\/code> predicate, since they are empty element objects.<\/p>\n\n\n
\na = [0, 1, 'Hello', '', [], [1,2,3], 0.1, 0.0]\n\nprint(list(filter(None, a)))\n<\/pre><\/div>\n\n\n
Output<\/p>\n\n\n
\n[1, 'Hello', [1, 2, 3], 0.1]\n<\/pre><\/div>\n\n\n
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Conclusion<\/h2>\n\n\n\n
We learned about the filter()<\/code> function that Python provides us with, for applying a predicate on an iterable.<\/p>\n\n\n\n
The conciseness and readability of filter<\/code> makes it a very popular function amongst Developers for modern Python codebases.<\/p>\n\n\n\n
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References<\/h2>\n\n\n\n