Posted By: Simon
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 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 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
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 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 simplifies the programmatic generation of slicing.