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first: The function you always missed in Python

https://secure.travis-ci.org/hynek/first.png

first is an MIT licensed Python package with a simple function that returns the first true value from an iterable, or None if there is none. If you need more power, you can also supply a key function that is used to judge the truth value of the element or a default value if None doesn’t fit your use case.

I’m using the term “true” consistently with Python docs for any() and all() — it means that the value evaluates to true like: True, 1, "foo" or [None]. But not: None, False or 0. In JavaScript, they call this “truthy”.

Examples

A simple example to get started:

>>> from first import first
>>> first([0, None, False, [], (), 42])
42

However, it’s especially useful for dealing with regular expressions in if/elif/else branches:

import re

from first import first


re1 = re.compile('b(.*)')
re2 = re.compile('a(.*)')

m = first(regexp.match('abc') for regexp in [re1, re2])
if not m:
   print('no match!')
elif m.re is re1:
   print('re1', m.group(1))
elif m.re is re2:
   print('re2', m.group(1))

The optional key function gives you even more selection power. If you want to return the first even number from a list, just do the following:

>>> from first import first
>>> first([1, 1, 3, 4, 5], key=lambda x: x % 2 == 0)
4

default on the other hand allows you to specify a value that is returned if none of the elements is true:

>>> from first import first
>>> first([0, None, False, [], ()], default=42)
42

Usage

The package consists of one module consisting of one function:

from first import first

first(iterable, default=None, key=None)

This function returns the first element of iterable that is true if key is None. If there is no true element, the value of default is returned, which is None by default.

If a callable is supplied in key, the result of key(element) is used to judge the truth value of the element, but the element itself is returned.

first has no dependencies and should work with any Python available. Of course, it works with the awesome Python 3 everybody should be using.

Alternatives

first brings nothing to the table that wasn’t possible before. However the existing solutions aren’t very idiomatic for such a common and simple problem.

The following constructs are equivalent to first(seq) and work since Python 2.6:

next(itertools.ifilter(None, seq), None)
next(itertools.ifilter(bool, seq), None)
next((x for x in seq if x), None)

None of them is as pretty as I’d like them to be. The re example from above would look like the following:

next(itertools.ifilter(None, (regexp.match('abc') for regexp in [re1, re2])), None)
next((regexp.match('abc') for regexp in [re1, re2] if regexp.match('abc')), None)

Note that in the second case you have to call regexp.match() twice. For comparison, one more time the first-version:

first(regexp.match('abc') for regexp in [re1, re2])

Idiomatic, clear and readable. Pythonic. :)

Background

The idea for first goes back to a discussion I had with Łukasz Langa about how the re example above is painful in Python. We figured such a function is missing Python, however it’s rather unlikely we’d get it in and even if, it wouldn’t get in before 3.4 anyway, which is years away as of yours truly is writing this.

So I decided to release it as a package for now. If it proves popular enough, it may even make it into Python’s stdlib in the end.

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