I need a good explanation (references are a plus) on Python's slice notation.
To me, this notation needs a bit of picking up.
It looks extremely powerful, but I haven't quite got my head around it.
It's pretty simple really:
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 the value represents the first value that is not in the selected slice. So, the difference between
start is the number of elements selected (if
step is 1, the default).
The other feature is that
stop maybe 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
step maybe 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 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.
The slicing operator
 is actually being used in the above code with a
slice() object using the
: notation (which is only valid within
is equivalent to:
a[slice(start, stop, step)]
Slice objects also behave slightly differently depending on the number of arguments, similarly to
range(), i.e. both
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)].
:-based notation is very helpful for simple slicing, the explicit use of
slice() objects simplify the programmatic generation of slicing.
answered Jan 20
Python's slice notation is used to return a list or a portion of a list. The basic syntax is as follows:
start_at is the index of the first item to be returned (included),
stop_before is the index of the element before which to stop (not included) and
step is the stride between any two items.
All three of the arguments are optional, meaning you can omit any of them. For example:
nums = [1, 2, 3, 4, 5] nums[1:4] # [2, 3, 4] (start at 0, stop before 4) nums[2:] # [3, 4, 5] (start at 0, stop at end of list) nums[:3] # [1, 2, 3] (start at 0, stop before 3) nums[1:4:2] # [2, 4] (start at 1, stop before 4, every 2nd element) nums[2::2] # [3, 5] (start at 2, stop at end of list, every 2nd element) nums[:3:2] # [1, 3] (start at 0, stop before 3, every 2nd element) nums[::2] # [1, 3, 5] (start at 0, stop at end of list, every 2nd element) nums[::] # [1, 2, 3, 4, 5] (start at 0, stop at end of list)
As you can probably tell from the examples above, the default values are
start_at = 0,
stop_before = len(nums),
step = 1.
An idiomatic way to shallow clone a list would be using
nums_clone = nums[:]).
All three of the arguments also accept negative values. For
stop_before, a negative value means counting from the end of the list instead of counting from the start. For example
-1 would represent the last element,
-2 the second last element etc. For example:
nums = [1, 2, 3, 4, 5] nums[1:-2] # [2, 3] (start at 1, stop before 2nd to last) nums[-3:-1] # [3, 4] (start at 3rd to last, stop before last)
step means that the list is sliced in reverse (from end to start). This also means that
start_at should be greater than
stop_before and that
stop_before in the context of a reverse stride is more like
stop_after if you are looking at the list non-reversed. For example:
nums = [1, 2, 3, 4, 5] nums[::-1] # [5, 4, 3, 2, 1] (reversed) nums[4:1:-1] # [5, 4, 3] (reversed, start at 4, stop after 1) nums[-1:1:-2] # [5, 3] (reversed, start at last, stop after 1, every 2nd)
Bear in mind that slice notation is very forgiving, so you'll get an empty list if the arguments' values are out of the list's range. For example:
nums = [1, 2, 3, 4, 5] nums[6:8] #  nums[:-10] # 
answered Jan 20
And a couple of things that weren't immediately obvious to me when I first saw the slicing syntax:
>>> x = [1,2,3,4,5,6] >>> x[::-1] [6,5,4,3,2,1]
Easy way to reverse sequences!
And if you wanted, for some reason, every second item in the reversed sequence:
>>> x = [1,2,3,4,5,6] >>> x[::-2] [6,4,2]
answered Jan 20
I use the "an index points between elements" method of thinking about it myself, but one way of describing it which sometimes helps others get it is this:
X is the index of the first element you want.
Y is the index of the first element you don't want.
answered Jan 20