Performing len on list of a zip object clears zip [duplicate]

Question

I am seeing a strange behavior when working with the zip() function. When I perform the following operation len(list(z)) where z is a zip object, the result is 0 (which seems wrong to me), and the action seems to clear out the zip object. Can someone please help me understand what is going on.

# python3
Python 3.2.3 (default, Sep 30 2012, 16:41:36) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> w = [11, 22, 33, 44, 55, 66]
>>> x = [1, 2, 3, 4]
>>> y = ['a', 'b', 'c']
>>> z = zip(x, y, w)
>>> z
<zip object at 0x7f854f613cb0>
>>> list(z)
[(1, 'a', 11), (2, 'b', 22), (3, 'c', 33)]
>>> len(list(z))
0
>>> list(z)
[]
>>> z
<zip object at 0x7f854f613cb0>
>>> 

Thank you, Ahmed.

Answer 1

In Python 3 zip is a generator. The generator is being exhausted when you do list(z). You can create a list from the values returned by the generator and operate on that.

l = list(z)
len(l)
# -> 3
l
# -> [(1, 'a', 11), (2, 'b', 22), (3, 'c', 33)]

---

Generators are a good thing. They allow us to write memory-efficient code in nearly the same way we would write code that deals with lists. To use an example from the linked wiki:

def double(L):
    return [x*2 for x in L]

Could be rewritten as a generator to avoid creating another list in memory:

def double(L):
    for x in L:
        yield x*2

Answer 2

To complete the previous answer, there is a Python library called cardinality for getting the size of iterables.

http://cardinality.readthedocs.io/en/latest/