From the docs:
The returned group is itself an iterator that shares the underlying iterable with
groupby(). Because the source is shared, when thegroupby()object is advanced, the previous group is no longer visible. So, if that data is needed later, it should be stored as a list[.]
And you are storing iterators in obj and materializing them later.
In [21]: dict((k, list(g)) for k, g in groupby(range(10), lambda x : x > 5))
Out[21]: {False: [0, 1, 2, 3, 4, 5], True: [6, 7, 8, 9]}