zip iterators asserting for equal length in python

I am looking for a nice way to zip several iterables raising an exception if the lengths of the iterables are not equal.

In the case where the iterables are lists or have a len method this solution is clean and easy:

def zip_equal(it1, it2):
    if len(it1) != len(it2):
        raise ValueError("Lengths of iterables are different")
    return zip(it1, it2)

However, if it1 and it2 are generators, the previous function fails because the length is not defined TypeError: object of type 'generator' has no len().

I imagine the itertools module offers a simple way to implement that, but so far I have not been able to find it. I have come up with this home-made solution:

def zip_equal(it1, it2):
    exhausted = False
    while True:
        try:
            el1 = next(it1)
            if exhausted: # in a previous iteration it2 was exhausted but it1 still has elements
                raise ValueError("it1 and it2 have different lengths")
        except StopIteration:
            exhausted = True
            # it2 must be exhausted too.
        try:
            el2 = next(it2)
            # here it2 is not exhausted.
            if exhausted:  # it1 was exhausted => raise
                raise ValueError("it1 and it2 have different lengths")
        except StopIteration:
            # here it2 is exhausted
            if not exhausted:
                # but it1 was not exhausted => raise
                raise ValueError("it1 and it2 have different lengths")
            exhausted = True
        if not exhausted:
            yield (el1, el2)
        else:
            return

The solution can be tested with the following code:

it1 = (x for x in ['a', 'b', 'c'])  # it1 has length 3
it2 = (x for x in [0, 1, 2, 3])     # it2 has length 4
list(zip_equal(it1, it2))           # len(it1) < len(it2) => raise
it1 = (x for x in ['a', 'b', 'c'])  # it1 has length 3
it2 = (x for x in [0, 1, 2, 3])     # it2 has length 4
list(zip_equal(it2, it1))           # len(it2) > len(it1) => raise
it1 = (x for x in ['a', 'b', 'c', 'd'])  # it1 has length 4
it2 = (x for x in [0, 1, 2, 3])          # it2 has length 4
list(zip_equal(it1, it2))                # like zip (or izip in python2)

Am I overlooking any alternative solution? Is there a simpler implementation of my zip_equal function?

Update:

  • Requiring python 3.10 or newer, see Asocia's answer
  • Thorough performance benchmarking and best performing solution on python<3.10: Stefan's answer
  • Simple answer without external dependencies: Martijn Pieters' answer (please check the comments for a bugfix in some corner cases)
  • More complex than Martijn's, but with better performance: cjerdonek's answer
  • If you don't mind a package dependency, see pylang's answer

An optional boolean keyword argument, strict, is introduced for the built-in zip function in PEP 618.

Quoting What’s New In Python 3.10:

The zip() function now has an optional strict flag, used to require that all the iterables have an equal length.

When enabled, a ValueError is raised if one of the arguments is exhausted before the others.

>>> list(zip('ab', range(3)))
[('a', 0), ('b', 1)]
>>> list(zip('ab', range(3), strict=True))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: zip() argument 2 is longer than argument 1

I can think of a simpler solution, use itertools.zip_longest() and raise an exception if the sentinel value used to pad out shorter iterables is present in the tuple produced:

from itertools import zip_longest

def zip_equal(*iterables):
    sentinel = object()
    for combo in zip_longest(*iterables, fillvalue=sentinel):
        if sentinel in combo:
            raise ValueError('Iterables have different lengths')
        yield combo

Unfortunately, we can't use zip() with yield from to avoid a Python-code loop with a test each iteration; once the shortest iterator runs out, zip() would advance all preceding iterators and thus swallow the evidence if there is but one extra item in those.