Slicing to reverse part of a list in python
I have a list from which I want to extract a sub-layer from end to end. With two lines of code, this is
mylist = [...]
mysublist = mylist[begin:end]
mysublist = mysublist[::-1]
Is there a notation for slicing to get the same effect in one line? it
mysublist = mylist[end:begin:-1]
is incorrect because it includes end
and excludes items begin
. it
mysublist = mylist[end-1:begin-1:-1]
doesn't work when begin
0, because begin-1
now -1
that is interpreted as the index of the last element mylist
.
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Use None
if begin
- 0
:
mysublist = mylist[end - 1:None if not begin else begin - 1:-1]
None
means "default", the same as omitting a value.
You can always put the conditional on a separate line:
start, stop, step = end - 1, None if not begin else begin - 1, -1
mysublist = mylist[start:stop:step]
Demo:
>>> mylist = ['foo', 'bar', 'baz', 'eggs']
>>> begin, end = 1, 3
>>> mylist[end - 1:None if not begin else begin - 1:-1]
['baz', 'bar']
>>> begin, end = 0, 3
>>> mylist[end - 1:None if not begin else begin - 1:-1]
['baz', 'bar', 'foo']
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