How to raise an IndexError when slice indices are out of range?

The Python Documentation states that

slice indices are silently truncated to fall in the allowed range

and therefor no IndexErrors are risen when slicing a list, regardless what start or stop parameters are used:

>>> egg = [1, "foo", list()]
>>> egg[5:10]
[]

Since the list egg does not contain any indices greater then 2, a egg[5] or egg[10] call would raise an IndexError:

>> egg[5]
Traceback (most recent call last):
IndexError: list index out of range

The question is now, how can we raise an IndexError, when both given slice indices are out of range?

2 Answers

In Python 2 you can override __getslice__ method by this way:

class MyList(list): def __getslice__(self, i, j): len_ = len(self) if i > len_ or j > len_: raise IndexError('list index out of range') return super(MyList, self).__getslice__(i, j)

Then use your class instead of list:

>>> egg = [1, "foo", list()]
>>> egg = MyList(egg)
>>> egg[5:10]
Traceback (most recent call last):
IndexError: list index out of range

There is no silver bullet here; you'll have to test both boundaries:

def slice_out_of_bounds(sequence, start=None, end=None, step=1): length = len(sequence) if start is None: start = 0 if step > 1 else length if start < 0: start = length - start if end is None: end = length if step > 1 else 0 if end < 0: end = length - end if not (0 <= start < length and 0 <= end <= length): raise IndexError()

Since the end value in slicing is exclusive, it is allowed to range up to length.