An easy hole to fall into using Python iterators

pitfall.py

# coding: utf-8
x = range(4)
y = zip('1234', 'abcd')

def has_four_unique_elements(seq):
    return len(set(seq)) == 4

# This works fine.
print(1, has_four_unique_elements(x))  # -> True
# Still works fine.
print(2, has_four_unique_elements(x))  # -> True
print()

# This works too.
print(3, has_four_unique_elements(y))  # -> True
# ...but this doesn't.
print(4, has_four_unique_elements(y))  # -> False
print()

# Why? Because zips can only be used once. This makes sense, (if
# reusable, zip implementation would have to store whole seq) but it's
# easy to forget when you're writing something bigger with many
# iterators.

y = zip('1234', 'abcd')
print(5, list(y))  # -> [('1', 'a'), ('2', 'b')', ...]
print(6, list(y))  # -> []
print()

# In contrast, range is reusable:
print(7, list(x))  # -> [0, 1, 2, 3]
print(8, list(x))  # -> [0, 1, 2, 3]