Calling list() empties my iterable object?

Question

a = range(1, 3)
a = iter(a)
list(a)
a = list(a)

a evaluates to [ ].

a = range(1, 3)
a = iter(a)
a = list(a)

a evaluates to [1, 2].

The first result is unexpected to me. What semantics are going on here?

Answer 1

The issue is not list() but iter() which as documented returns a single-use iterator. Once something has accessed the iterator's elements, the iterator is permanently empty. The more commonly used iterable type is (normally) reusable, and the two types shouldn't be confused.

Note that you don't need iter() in order to turn a range into a list, because list() takes an iterable as an argument:

>>> a = range(1, 3)
>>> list(a)
[1, 2]
>>> list(a)
[1, 2]

And it is only the iterator returned by iter() that is single-use:

>>> b = iter(a)
>>> list(b)
[1, 2]
>>> list(b)
[]
>>> list(a)
[1, 2]

Answer 2

Let's examine what happens:

>>> a = range(1, 3)
>>> a is iter(a)
False

as you can see, iter gives a new iterator object, which is not a itself

>>>> a = iter(a)

the name a now corresponds to the distinct iterator object iter gave us (just as if iter(a) had returned itself, e.g. as it happens with zip and with files)

>>> list(a)
[1, 2]

exhausts the iterator, therefore

>>> list(a)
[]

gives nothing as the iterator has been used (iterated on) already

Here are a few more experiments you can try to fully grasp what happens:

>>> a = range(1, 3)
>>> a
range(1, 3)
>>> type(a)
<class 'range'>
>>> b = iter(a)
>>> b
<range_iterator object at 0x7f331a6d96c0>
>>> type(b)
<class 'range_iterator'>
>>> a is b
False
>>> list(b)
[1, 2]
>>> list(b)
[]
>>> list(a)
[1, 2]
>>> list(a)
[1, 2]
>>> a
range(1, 3)
>>> next(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'range' object is not an iterator
>>> b=iter(a)
>>> next(b)
1
>>> next(b)
2
>>> next(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> a=[1,2,3]
>>> b=[4,5,6]
>>> z=zip(a,b)
>>> iter(z) is z
True
>>> with open('words.txt') as f:
...  iter(f) is f
... 
True

Note: on Python 2 quite a few functions return lists instead of iterators (e.g. zip)