Is there any difference between Python list slicing [-1:] and [-1]?

Question

I have read a snippet of code like this:

s = self.buffer_file.readline()
if s[-1:] == "\n":
    return s

And if I do this:

s = 'abc'
In [78]: id(s[-1:]), id(s[-1])
Out[78]: (140419827715248, 140419827715248)

In [79]: id(s[-1:]) is id(s[-1])
Out[79]: False

In [80]: id(s[-1:]) == id(s[-1])
Out[80]: True

It doesn't make sense to me, the ID numbers are same, but the IDs are different. So they are different for some reason.

Answer 1

The difference is that the result of slicing a list is a list

x = [1, 2, 3]

print(x[-1])  # --> 3
print(x[-1:]) # --> [3]

The second case just happens to be a list of one element, but it's still a list.

Note however that Python doesn't have a char type distinct from the str type and this means that both element access and slicing on str objects return another str object:

print("abcd"[-1])  # --> "d"
print("abcd"[-1:]) # --> "d"

The only advantage of using for example s[-1:] or s[:1] with a string instead of s[-1] and s[0] is that the slice expressions will not raise a runtime error when acting on an empty string (something that element access does)... this may allow code simplification:

if len(s) > 0 and s[0] == '*': ...
if s[:1] == '*': ...

Answer 2

The key practical benefit to s[-1:] rather than s[-1] is that the former will produce an empty iterable rather than stopping with a traceback.

>>> 'hi'[-1:]
'i'
>>> 'hi'[-1]
'i'
>>> ''[-1:]
''
>>> ''[-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

This allows if s[-1:] == "\n": to handle an empty line s without preceding it with if s: by simply evaluating to False instead of generating an error that would need to be handled with a try..except structure.