This was closed over a year ago, but since mark.dickinson was asking for convincing use-cases: I'm breaking up a file into line-delimited chunks.  These chunks are non-overlapping, contiguous, and tend to be fairly large, so I'm just recording the start line of each chunk in a 2-ple:

mapping = [
  (10, 'first chunk'),
  (50, 'second chunk'),
  (60, 'third chunk')
]

Lines 10-49 are in the first chunk, lines 50-59 are in the second, lines 60+ are in the third.  So:

def CategorizeLine(line, mapping):
   loc = bisect.bisect([m[0] for m in mapping], line)
   if loc == 0:
      return None # before first chunk
   return mapping[loc-1][1]

It Would Be Nice if I could write the second line as:

   loc = bisect.bisect(mapping, line, key=lambda m:m[0])

The bisect documentation suggests pre-computing the key list, but it seems messy and error-prone to keep a redundant data structure in sync with its source.  I could also rewrite my "mapping" data structure to be two parallel lists instead of one list of 2-ples, but this data structure is more readable and extensible and handles empty lists more naturally.