The urlencode documentation states that:
The value element in itself can be a sequence and in that case, if the optional parameter doseq is evaluates to True, individual key=value pairs separated by '&' are generated for each element of the value sequence for the key.

Passing a generator as the value gives unexpected results:
>>> from urllib.parse import urlencode
>>> def gen():
...     yield from range(2)
>>> urlencode({'a': gen()}, doseq=True)
'a=%3Cgenerator+object+gen+at+0x7f35ff78db48%3E'

A list gives:
>>> urlencode({'a': [0, 1]}, doseq=True)
'a=0&a=1'