Performing len on list of a zip object clears zip

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 >>> list(z) [(1, 'a', 11), (2, 'b', 22), (3, 'c', 33)] >>> len(list(z)) 0 >>> list(z) [] >>> z >>> ``` Thank you, Ahmed.

Accepted Answer

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
```

Performing len on list of a zip object clears zip

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