Why does slice [:-0] return empty list in Python

问题:

Stumbled upon something slightly perplexing today while writing some unittests:

blah = ['a', 'b', 'c']
blah[:-3] # []
blah[:-2] # ['a']
blah[:-1] # ['a', 'b']
blah[:-0] # []

Can't for the life of me figure out why blah[:-0] # [] should be the case, the pattern definitely seems to suggest that it should be ['a', 'b', 'c']. Can anybody help to shed some light on why that is the case? Haven't been able to find mention in the docs as to why that is the case.

回答1:

-0 is 0, and a slice that goes from the beginning of a list inclusive to index 0 non-inclusive is an empty list.

回答2:

Python doesn't treat -0 any different from 0.

blah[:0]

means all elements up to but not including the first one. This is an empty list, and blah[:-0] is exactly the same thing.

You can test this by checking that

blah[0:]

is the whole list, starting from the first element.