The documentation implies this has a few useful properties:
word[:2] # The first two characters
word[2:] # Everything except the first two characters
Here’s a useful invariant of slice operations:
s[:i] + s[i:]equalss.For non-negative indices, the length of a slice is the difference of the indices, if both are within bounds. For example, the length of
word[1:3]is2.
I think we can assume that the range functions act the same for consistency.
Answer from Toomai on Stack OverflowThe documentation implies this has a few useful properties:
word[:2] # The first two characters
word[2:] # Everything except the first two characters
Here’s a useful invariant of slice operations:
s[:i] + s[i:]equalss.For non-negative indices, the length of a slice is the difference of the indices, if both are within bounds. For example, the length of
word[1:3]is2.
I think we can assume that the range functions act the same for consistency.
Here's the opinion of Guido van Rossum:
[...] I was swayed by the elegance of half-open intervals. Especially the invariant that when two slices are adjacent, the first slice's end index is the second slice's start index is just too beautiful to ignore. For example, suppose you split a string into three parts at indices i and j -- the parts would be a[:i], a[i:j], and a[j:].
[Google+ is closed, so link doesn't work anymore. Here's an archive link.]
Videos
Regarding list slicing: can anyone help me understand the reasoning behind inclusive vs. exclusive indexing with negative vs. non-negative integers?
What is slicing in Python? - Python - Data Science Dojo Discussions
slice - How slicing in Python works - Stack Overflow
Why slices work the way they are?
Python beginner here, coming from a Matlab and R background... so there's been a bit of a hurdle for me when it comes to understanding indexing in Python.
My current brain fuck has to do with wrapping my head around indexing and list ranges/slicing. And no amount of Google-fu is helping me here.
Let's say I have a simple list:
list = ["a", "b", "c", "d"]
If I want to take a slice of the list, the end of the range is exclusive when using non-negative values:
print(list[0:1]) ['a']
Not super intuitive given my background, but fine. I understand what's going on and generally why it happens.
What I don't understand is why the end of the range is inclusive when using negative indexing:
print(list[0:-1]) ['a', 'b', 'c']
It seems that the slice should only include ['a', 'b']: given how Python indexes a range of non-negative values, I would expect the result to only give the list items up to (but not including) the penultimate item in the list.
What's the rationale behind an exclusive range for non-negative values but an inclusive range for negative values?
The syntax is:
a[start:stop] # items start through stop-1
a[start:] # items start through the rest of the array
a[:stop] # items from the beginning through stop-1
a[:] # a copy of the whole array
There is also the step value, which can be used with any of the above:
a[start:stop:step] # start through not past stop, by step
The key point to remember is that the :stop value represents the first value that is not in the selected slice. So, the difference between stop and start is the number of elements selected (if step is 1, the default).
The other feature is that start or stop may be a negative number, which means it counts from the end of the array instead of the beginning. So:
a[-1] # last item in the array
a[-2:] # last two items in the array
a[:-2] # everything except the last two items
Similarly, step may be a negative number:
a[::-1] # all items in the array, reversed
a[1::-1] # the first two items, reversed
a[:-3:-1] # the last two items, reversed
a[-3::-1] # everything except the last two items, reversed
Python is kind to the programmer if there are fewer items than you ask for. For example, if you ask for a[:-2] and a only contains one element, you get an empty list instead of an error. Sometimes you would prefer the error, so you have to be aware that this may happen.
Relationship with the slice object
A slice object can represent a slicing operation, i.e.:
a[start:stop:step]
is equivalent to:
a[slice(start, stop, step)]
Slice objects also behave slightly differently depending on the number of arguments, similar to range(), i.e. both slice(stop) and slice(start, stop[, step]) are supported.
To skip specifying a given argument, one might use None, so that e.g. a[start:] is equivalent to a[slice(start, None)] or a[::-1] is equivalent to a[slice(None, None, -1)].
While the :-based notation is very helpful for simple slicing, the explicit use of slice() objects simplifies the programmatic generation of slicing.
The Python tutorial talks about it (scroll down a bit until you get to the part about slicing).
The ASCII art diagram is helpful too for remembering how slices work:
+---+---+---+---+---+---+
| P | y | t | h | o | n |
+---+---+---+---+---+---+
0 1 2 3 4 5
-6 -5 -4 -3 -2 -1
One way to remember how slices work is to think of the indices as pointing between characters, with the left edge of the first character numbered 0. Then the right edge of the last character of a string of n characters has index n.
I understand how it works, but it really bothers me why?
Python is sold as intuitive language and yet when you want to get elements a[0] . a[1] and a[2] you don't use a[0:2] which feels natural to me. You have to use a[0:3]
I'm really curious what is justification for it working that way. Is there real use example where it makes more sense that way?
They both behave exactly the same.
For some reason you expect the list's indexes to start from 0 and
the string's indexes to start from 1.
The fact is that they both start at 0.
As @Mark suggested in the comments, strings and lists indexing from Python's documentation.
But it is the same!
Insert indices before each element:
[0=1,1=2,2=3,3=4][2:] means: from index 2 until end
[0=T,1=h,2=i,3=s,4= ,5=i,6=s,...][5:] means: from index 5( in this case from the letter 'i') until the end.