The API I'm working with can return empty [] lists.
The following conditional statements aren't working as expected:
if myList is not None: #not working
pass
if myList is not []: #not working
pass
What will work?
3 Answers
if not myList:
print "Nothing here"
1 Comment
thyu
This is the way recommended by PEP8 as Chris Lacasse said in another comment.
I like Zarembisty's answer. Although, if you want to be more explicit, you can always do:
if len(my_list) == 0:
print "my_list is empty"
6 Comments
Chris Lacasse
You can do that, but it would violate pep 8, which says: - For sequences, (strings, lists, tuples), use the fact that empty sequences are false. Yes: if not seq: if seq: No: if len(seq) if not len(seq)
inspectorG4dget
Thank you for pointing this out to me, Chris Lacasse. I had no known about pep8, earlier
Ad N
It would also be a general performance pessimisation: do not spend time counting elements of potentially long collections, if all you need to know is if they are empty.
inspectorG4dget
@AdN:
len is an O(1) operation, so that's not really an issue. However, it may be faster (I'm not sure) to simply check if len(myList). Of course, if we head down that way, we may as well do if myList.
rkachach
And the above will crash if my_list is None
|
Empty lists evaluate to False in boolean contexts (such as if some_list:).
!=instead ofis notwould have made this work, though theif myListform is preferred.if myList == []: print("The list is empty!")!=instead ofis notso it'd work