syntax - What does :-1 mean in python?

It is list indexing, it returns all elements
[:] except the last one -1. Similar
question href="https://stackoverflow.com/questions/509211/the-python-slice-notation">here

For example,

>>> a =
            [1,2,3,4,5,6]
>>> a[:-1]
[1, 2, 3, 4,
            5]

It works like this

a[start:end]

>>>
            a[1:2]
[2]

a[start:]

>>>
            a[1:]

[2, 3, 4, 5,
            6]

a[:end] />Your case

>>> a =
            [1,2,3,4,5,6]
>>> a[:-1]
[1, 2, 3, 4,
            5]

a[:]

>>>
            a[:]
[1, 2, 3, 4, 5, 6]