list.sort() returns None
sort() sorts in place and gives back None. Assigning its result throws the list away.
What it means
Python marks methods that mutate by returning None. list.sort(), list.reverse() and list.append() all do this, and the None return is a deliberate signal that the object changed rather than a new one being produced.
Common causes
1. Assigning the result of sort()
The list is sorted correctly, then replaced with None.
Breaks
names = names.sort()Works
names.sort() # in place
# or
names = sorted(names) # new list2. Chaining off a mutating method
The chain continues on None, so the next call fails.
Breaks
first = names.sort()[0]Works
first = sorted(names)[0]How to find it in your own code
Remember the pair: sort/sorted, reverse/reversed. The bare verb mutates and returns None; the -ed form returns a new list. If a variable mysteriously becomes None, check whether the last thing assigned to it was a mutating method.
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