Why does this nested list comprehension generate an error?

conn_request, weights = zip(*[
    ((conn, request), request[2])
    for conn in unchoked_conns
    for request in conn.peer_requests
])

Generates:

    for conn in unchoked_conns
ValueError: need more than 0 values to unpack

I cannot work out what I'm doing wrong. I thi开发者_运维技巧nk it has some thing to do with the dependence of request on conn?

>>> a, b = zip(*[])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 0 values to unpack

Your LC produces an empty list. Try verifying that unchoked_conns has any elements.

zipping an empty list returns a single empty list, hence the assignment fails. That is consistent with the zip help, which clearly says:

The returned list is truncated in length to the length of the shortest argument sequence

In your case, the shortest argument sequence has length 0.