Ensure that an argument can be iterated twice

Suppose I have the following function:

def print_twice(x):
    for i in x: print(i)
    for i in x: print(i)

When I run:

print_twice([1,2,3])

or:

print_twice((1,2,3))

I get the expected result: the numbers 1,2,3 are printed twice.

But when I run:

print_twice(zip([1,2,3],[4,5,6]))

the pairs (1,4),(2,5),(3,6) are printed only once. Probably, this is because the zip returns a generator that terminates after one pass.

How can I modify the function print_twice such that it will correctly handle all inputs?

I could insert a line at the beginning of the function: x = list(x). But this might be inefficient in case x is already a list, a tuple, a range, or any other iterator that can be iterated more than once. Is there a more efficient solution?

Answer

I could insert a line at the beginning of the function: x = list(x). But this might be inefficient in case x is already a list, a tuple, a range, or any other iterator that can be iterated more than once. Is there a more efficient solution?

Copying single-use iterables to a list is perfectly adequate, and reasonably efficient even for multi-use iterables.

The list (and to some extend tuple) type is one of the most optimised data structures in Python. Common operations such as copying a list or tuple to a list are internally optimised;¹ even for iterables that are not special-cased, copying them to a list is significantly faster than any realistic work done by two (or more) loops.

def print_twice(x):
    x = list(x)
    for i in x: print(i)
    for i in x: print(i)

Copying indiscriminately can also be advantageous in the context of concurrency, when the iterable may be modified while the function is running. Common cases are threading and weakref collections.

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In case one wants to avoid needless copies, checking whether the iterable is a Collection is a reasonable guard.

from collections.abc import Collection

x = list(x) if not isinstance(x, Collection) else x

Alternatively, one can check whether the iterable is in fact an iterator, since this implies statefulness and thus single-use.

from collections.abc import Iterator

x = list(x) if isinstance(x, Iterator) else x
x = list(x) if iter(x) is x else x

Notably, the builtins zip, filter, map, … and generators all are iterators.

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¹Copying a list of 128 items is roughly as fast as checking whether it is a Collection.