Why is there no first(iterable) built-in function in Python?

Question

I'm wondering if there's a reason that there's no first(iterable) in the Python built-in functions, somewhat similar to any(iterable) and all(iterable) (it may be tucked in a stdlib module somewhere, but I don't see it in itertools). first would perform a short-circuit generator evaluation so that unnecessary (and a potentially infinite number of) operations can be avoided; i.e.

def identity(item):     return item  def first(iterable, predicate=identity):     for item in iterable:         if predicate(item):             return item     raise ValueError('No satisfactory value found') 

This way you can express things like:

denominators = (2, 3, 4, 5) lcd = first(i for i in itertools.count(1)     if all(i % denominators == 0 for denominator in denominators)) 

Clearly you can't do list(generator)[0] in that case, since the generator doesn't terminate.

Or if you have a bunch of regexes to match against (useful when they all have the same groupdict interface):

match = first(regex.match(big_text) for regex in regexes) 

You save a lot of unnecessary processing by avoiding list(generator)[0] and short-circuiting on a positive match.

Answer 1

In Python 2, if you have an iterator, you can just call its next method. Something like:

>>> (5*x for x in xrange(2,4)).next() 10 

In Python 3, you can use the next built-in with an iterator:

>>> next(5*x for x in range(2,4)) 10 

Answer 2

There's a Pypi package called “first” that does this:

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

Here's how you would use to return the first odd number, for example:

>> first([2, 14, 7, 41, 53], key=lambda x: x % 2 == 1) 7 

If you just want to return the first element from the iterator regardless of whether is true or not, do this:

>>> first([0, None, False, [], (), 42], key=lambda x: True) 0 

It's a very small package: it only contains this function, it has no dependencies, and it works on Python 2 and 3. It's a single file, so you don't even have to install it to use it.

In fact, here's almost the entire source code (from version 2.0.1, by Hynek Schlawack, released under the MIT licence):

def first(iterable, default=None, key=None):     if key is None:         for el in iterable:             if el:                 return el     else:         for el in iterable:             if key(el):                 return el     return default