What are the default slice indices in Python *really*?

问题

From the python documentation docs.python.org/tutorial/introduction.html#strings:

Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.

For the standard case, this makes a lot of sense:

>>> s = 'mystring'
>>> s[1:]
'ystring'
>>> s[:3]
'mys'
>>> s[:-2]
'mystri'
>>> s[-1:]
'g'
>>> 

So far, so good. However, using a negative step value seems to suggest slightly different defaults:

>>> s[:3:-1]
'gnir'
>>> s[0:3:-1]
''
>>> s[2::-1]
'sym'

Fine, perhaps if the step is negative, the defaults reverse. An ommitted first index defaults to the size of the string being sliced, an omitted second index defaults to zero:

>>> s[len(s):3:-1]
'gnir'

Looking good!

>>> s[2:0:-1]
'sy'

Whoops. Missed that 'm'.

Then there is everyone's favorite string reverse statement. And sweet it is:

>>> s[::-1]
'gnirtsym'

However:

>>> s[len(s):0:-1]
'gnirtsy'

The slice never includes the value of the second index in the slice. I can see the consistency of doing it that way.

So I think I am beginning to understand the behavior of slice in its various permutations. However, I get the feeling that the second index is somewhat special, and that the default value of the second index for a negative step can not actually be defined in terms of a number.

Can anyone concisely define the default slice indices that can account for the provided examples? Documentation would be a huge plus.

回答1:

There actually aren't any defaults; omitted values are treated specially.

However, in every case, omitted values happen to be treated in exactly the same way as None. This means that, unless you're hacking the interpreter (or using the parser, ast, etc. modules), you can just pretend that the defaults are None (as recursive's answer says), and you'll always get the right answers.

The informal documentation cited isn't quite accurate—which is reasonable for something that's meant to be part of a tutorial. For the real answers, you have to turn to the reference documentation.

For 2.7.3, Sequence Types describes slicing in notes 3, 4, and 5.

For [i:j]:

… If i is omitted or None, use 0. If j is omitted or None, use len(s).

And for [i:j:k]:

If i or j are omitted or None, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k is None, it is treated like 1.

For 3.3, Sequence Types has the exact same wording as 2.7.3.

回答2:

I don't have any documentation, but I think the default is [None:None:None]

>>> "asdf"[None:None:None]
'asdf'
>>> "asdf"[None:None:-1]
'fdsa'

回答3:

The notes in the reference documentation for sequence types explains this in some detail:

(5.) The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that 0 <= n < (j-i)/k. In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j). If i or j is greater than len(s), use len(s). If i or j are omitted or None, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k is None, it is treated like 1.

So you can get the following behaviour:

>>> s = "mystring"
>>> s[2:None:-1]
'sym'

回答4:

The end value is always exclusive, thus the 0 end value means include index 1 but not 0. Use None instead (since negative numbers have a different meaning):

>>> s[len(s)-1:None:-1]
'gnirtsym'

Note the start value as well; the last character index is at len(s) - 1; you may as well spell that as -1 (as negative numbers are interpreted relative to the length):

>>> s[-1:None:-1]
'gnirtsym'

回答5:

Actually it is logical ...

if you look to the end value, it always points to the index after the last index. So, using 0 as the end value, means it gets till element at index 1. So, you need to omit that value .. so that it returns the string you want.

>>> s = '0123456789'
>>> s[0], s[:0]
('0', '')
>>> s[1], s[:1]
('1', '0')
>>> s[2], s[:2]
('2', '01')
>>> s[3], s[:3]
('3', '012')
>>> s[0], s[:0:-1]
('0', '987654321')

回答6:

Useful to know if you are implementing __getslice__: j defaults to sys.maxsize (https://docs.python.org/2/reference/datamodel.html#object.getslice)

>>> class x(str):
...   def __getslice__(self, i, j):
...     print i
...     print j
...
...   def __getitem__(self, key):
...     print repr(key)
...
>>> x()[:]
0
9223372036854775807
>>> x()[::]
slice(None, None, None)
>>> x()[::1]
slice(None, None, 1)
>>> x()[:1:]
slice(None, 1, None)
>>> import sys
>>> sys.maxsize
9223372036854775807L

来源：https://stackoverflow.com/questions/12521798/what-are-the-default-slice-indices-in-python-really