Question

I noticed something I didn’t expect when writing a script this morning. I tried to use a list comprehension and sort it all in one statement and got a surprising result. The following code summarizes my general use case, but is simplified for this question:

Transaction = namedtuple('Transaction', ['code', 'type'])

my_list = [Transaction('code1', 'AAAAA'), Transaction('code2', 'BBBBB'), Transaction('code3', 'AAAAA')]

types = ['AAAAA', 'CCCCC']

result = [trans for trans in my_list if trans.type in types].sort(key = lambda x: x.code)

print result

Output:

None

If I create the list using the comprehension, then sort it after the fact, everything is fine. I’m curious why this happens?

Answer 1

The method list.sort() is sorting the list in place, and as all mutating methods it returns None. Use the built-in function sorted() to return a new sorted list.

result = sorted((trans for trans in my_list if trans.type in types),
                key=lambda x: x.code)

Instead of lambda x: x.code, you could also use the slightly faster operator.attrgetter("code").