ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:
|Raymond Boute <raymond.boute@pandora.be> wrote or quoted:
|>Moreover, using index -1 for the last item is a bad choice
|This feature is the bee's knees, man.

I was just looking for a list with a sort of "max-out" vibe
that turns all negative indices into zeros. My AI wingman
whipped it up for me.

In my code, it goes by "SourceList":

s = list( 'Example' )
t = SourceList( 'Example' )

print( repr( s[ -1: 2 ] ))
print( repr( t[ -1: 2 ] ))

spits out:

[]
['E', 'x']

'cause "t[ -1: 2 ]" is basically the same as "s[ 0: 2 ]".

Here's the whole shebang:

from collections.abc import MutableSequence

class SourceList(MutableSequence):
'''
Interpret all negative indices in subscriptions and slices to mean 0.
'''
def __init__(self, iterable=None):
self._list = list(iterable) if iterable is not None else []

def _normalize_index(self, index):
return max(0, index) if isinstance(index, int) else index

def _normalize_slice(self, key):
start = self._normalize_index(key.start) if key.start is not None else None
stop = self._normalize_index(key.stop) if key.stop is not None else None
return slice(start, stop, key.step)

def __getitem__(self, key):
if isinstance(key, slice):
key = self._normalize_slice(key)
else:
key = self._normalize_index(key)
return self._list[key]

def __setitem__(self, key, value):
if isinstance(key, slice):
key = self._normalize_slice(key)
else:
key = self._normalize_index(key)
self._list[key] = value

def __delitem__(self, key):
if isinstance(key, slice):
key = self._normalize_slice(key)
else:
key = self._normalize_index(key)
del self._list[key]

def __len__(self):
return len(self._list)

def insert(self, index, value):
self._list.insert(self._normalize_index(index), value)

def __str__(self):
return f"SourceList({self._list})"

def __repr__(self):
return self.__str__()

s = list( 'Example' )
t = SourceList( 'Example' )

print( repr( s[ 0: 2 ] ))
print( repr( t[ -1: 2 ] ))

.