Python Zip Object 'disappears' After Iterating Through?

Total noob question here but I really want to know the answer. I have no idea why the zip object simply 'disappears' after I attempt to iterate through it in its list form: eg. >

Solution 1:

There was a change of behavior between Python2 to Python3: in python2, zip returns a list of tuples while in python3 it returns an iterator.

The nature of iterator is that once it's done iterating the data - it points to an empty collection and that's the behavior you're experiencing.

Python2:

Python 2.7.9 (default, Jan 292015, 06:28:58)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)] on darwin
Type"help", "copyright", "credits"or"license"for more information.
>>> A=[1,2,3]
>>> B=['A','B','C']
>>> Z=zip(A,B)
>>> Z
[(1, 'A'), (2, 'B'), (3, 'C')]
>>> list(Z)
[(1, 'A'), (2, 'B'), (3, 'C')]
>>> list(Z)
[(1, 'A'), (2, 'B'), (3, 'C')]
>>> list(Z)
[(1, 'A'), (2, 'B'), (3, 'C')]
>>> {p:q for (p,q) in Z}
{1: 'A', 2: 'B', 3: 'C'}
>>> Z
[(1, 'A'), (2, 'B'), (3, 'C')]
>>> Z
[(1, 'A'), (2, 'B'), (3, 'C')]

Python3:

Python 3.4.2 (v3.4.2:ab2c023a9432, Oct  52014, 20:42:22)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type"help", "copyright", "credits"or"license"for more information.
>>> A=[1,2,3]
>>> B=['A','B','C']
>>> Z=zip(A,B)
>>> list(Z)
[(1, 'A'), (2, 'B'), (3, 'C')]
>>> list(Z)
[]
>>>

Solution 2:

zip creates an object for iterating once over the results. This also means it's exhausted after one iteration:

>>>a = [1,2,3]>>>b = [4,5,6]>>>z = zip(a,b)>>>list(z)
[(1, 4), (2, 5), (3, 6)]
>>>list(z)
[]

You need to call zip(a,b) every time you wish to use it or store the list(zip(a,b)) result and use that repeatedly instead.

Solution 3:

For python 2 and 3, I use

xzip = zip
zip = lambda *x: list(xzip(*x))

Then zip returns always a list rather than an iterator.

IMO, xzip would have been a better name for zip in python 3.