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The function you always missed in Python: return the first true value of an iterable.
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first: The function you always missed in Python

first is a 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 predicate whose result will be returned iff true.

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”.


A simple example first:

>> from first import first
>> first([0, None, False, [], (), 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 is re1:
elif is re2:

The optional predicate gives you even more power. If you want to return the square of the first even number from a list, just do the following:

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


The package consists of one module consisting of one function:

from first import first

first(iterable, pred=None)

This function returns the first element of iterable that is true if pred is None. If there is no true element, None is returned.

If a callable is supplied in pred, the result of pred(element) is returned, if the result it true.

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


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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