zip iterators asserting for equal length in python

Question

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

Answer 1

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