Groupby object disappears after list operation

Question

I am trying the run-length encoding problem, and after running a groupby & list operation, my groupby object somehow disappeared.

import itertools
s = 'AAAABBBCCDAA'
for c, group in itertools.groupby(s):
    print(list(group))
    print(list(group))

My output is

['A', 'A', 'A', 'A']
[]
['B', 'B', 'B']
[]
['C', 'C']
[]
['D']
[]
['A', 'A']
[]

so for each loop, the 2 print commands yield different results.

Can anybody help to explain what I did wrong?

Answer 1

Because there generators, after they're used they're gone:

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

To keep them:

import itertools
s = 'AAAABBBCCDAA'
for c, group in itertools.groupby(s):
    l = list(group)
    print(l)
    print(l)

Output:

['A', 'A', 'A', 'A']
['A', 'A', 'A', 'A']
['B', 'B', 'B']
['B', 'B', 'B']
['C', 'C']
['C', 'C']
['D']
['D']
['A', 'A']
['A', 'A']