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@zacharyvoase
Created June 11, 2012 22:36
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A smarter Python `slice` object.
class SmartSlice(object):
"""
A slice object which represents the mapping between old and new indices.
Given a slice object, you can determine whether a given index will be
present in the sub-list that slice represents. For example:
>>> 3 in SmartSlice(2, 5)
True
>>> 0 in SmartSlice(2, 5)
False
>>> 5 in SmartSlice(2, 5)
False
This even works with a `step` argument:
>>> 2 in SmartSlice(2, 10, 2)
True
>>> 4 in SmartSlice(2, 10, 2)
True
>>> 7 in SmartSlice(2, 10, 2)
False
You can map indices in the existing list to indices in the sub-list:
>>> SmartSlice(2, 10, 2).forward_mapping(2)
0
>>> SmartSlice(2, 10, 2).forward_mapping(4)
1
>>> SmartSlice(2, 10, 2).forward_mapping(8)
3
If an index isn't present in the sub-list, an IndexError will be raised:
>>> SmartSlice(2, 10, 2).forward_mapping(5)
Traceback (most recent call last):
...
IndexError: index 5 is not represented in the slice
This works for reverse mapping as well (mapping indices in the slice to
their original positions):
>>> SmartSlice(2, 10, 2).reverse_mapping(0)
2
>>> SmartSlice(2, 10, 2).reverse_mapping(1)
4
>>> SmartSlice(2, 10, 2).reverse_mapping(3)
8
This may also raise an IndexError:
>>> SmartSlice(2, 10, 2).reverse_mapping(12)
Traceback (most recent call last):
...
IndexError: the slice has no index 12
If you provide a stop index, you can also reverse-map negative indices:
>>> SmartSlice(2, 10, 2).reverse_mapping(-2)
6
And determine the length of the slice:
>>> len(SmartSlice(2, 10, 2))
4
>>> len(SmartSlice(2, 11, 2))
5
But of course some things fail if you don't have a stop index, because
there's no way of calculating them:
>>> len(SmartSlice(2, None, 2))
Traceback (most recent call last):
...
TypeError: slice has no stop, and therefore no length
>>> SmartSlice(2, None, 2).reverse_mapping(-2)
Traceback (most recent call last):
...
TypeError: slice has no stop, so does not support negative indexing
"""
__slots__ = ('start', 'stop', 'step')
def __init__(self, *args):
if len(args) < 1 or len(args) > 3:
raise TypeError("Expects between 1 and 3 arguments")
elif len(args) == 1:
start, stop, step = None, args[0], None
elif len(args) == 2:
start, stop, step = args[0], args[1], None
elif len(args) == 3:
start, stop, step = args[0], args[1], args[2]
self.start = start
self.stop = stop
self.step = step
def __contains__(self, i):
if i < 0:
raise TypeError("Negative indices are not supported for forward mapping")
elif i < (self.start or 0):
return False
elif i >= (self.stop or float('inf')):
return False
return (i - (self.start or 0)) % (self.step or 1) == 0
def __len__(self):
if self.stop is None:
raise TypeError("slice has no stop, and therefore no length")
return ((self.stop + 1) - (self.start or 0)) // (self.step or 1)
def forward_mapping(self, i):
"""Map indices in a list to their new indices in the slice."""
if i not in self:
raise IndexError("index %d is not represented in the slice" % i)
return (i - (self.start or 0)) // (self.step or 1)
def reverse_mapping(self, i):
"""Map indices in the slice to their old positions."""
if (self.stop is not None) and i >= len(self) and i > 0:
raise IndexError("the slice has no index %d" % i)
elif i < 0:
if self.stop is None:
raise TypeError("slice has no stop, so does not support negative indexing")
elif -i > len(self):
raise IndexError("the slice has no index %d" % i)
else:
i = len(self) + i
return (i * self.step) + (self.start or 0)
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