Question

>>from itertools import groupby
>>keyfunc = lambda x : x > 500
>>obj = dict(groupby(range(1000), keyfunc))
>>list(obj[True])
[999]
>>list(obj[False])
[]

range(1000) is obviously sorted by default for the condition (x > 500).

I was expecting the numbers from 0 to 999 to be grouped in a dict by the condition (x > 500). But the resulting dictionary had only 999.

where are the other numbers?.
Can any one explain what is happening here?

Answer 1

From the docs:

The returned group is itself an iterator that shares the underlying iterable with groupby(). Because the source is shared, when the groupby() 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]}