How to distinguish between a sequence and a mapping

Question

I would like to perform an operation on an argument based on the fact that it might be a map-like object or a sequence-like object. I understand that no strategy is going to be 100% reliable for type-like checking, but I'm looking for a robust solution.

Based on this answer, I know how to determine whether something is a sequence and I can do this check after checking if the object is a map.

def ismap(arg):
   # How to implement this?

def isseq(arg):
   return hasattr(arg,"__iter__")

def operation(arg):
   if ismap(arg):
      # Do something with a dict-like object
   elif isseq(arg):
      # Do something with a sequence-like object
   else:
      # Do something else

Because a sequence can be seen as a map where keys are integers, should I just try to find a key that is not an integer? Or maybe I could look at the string representation? or...?

UPDATE

I selected SilentGhost's answer because it looks like the most "correct" one, but for my needs, here is the solution I ended up implementing:

if hasattr(arg, 'keys') and hasattr(arg, '__getitem__'):
   # Do something with a map
elif hasattr(arg, '__iter__'):
   # Do something with a sequence/iterable
else:
   # Do something else

Essentially, I don't want to rely on an ABC because there are many custom classes that behave like sequences and dictionary but that still do not extend the python collections ABCs (see @Manoj comment). I thought the keys attribute (mentioned by someone who removed his/her answer) was a good enough check for mappings.

Classes extending the Sequence and Mapping ABCs will work with this solution as well.

Answer 1

Sequences have an __add__ method that implements the + operator. Maps do not have that method, since adding to a map requires both a key and a value, and the + operator only has one right-hand side.

So you may try:

def ismap(arg):
    return isseq(arg) and not hasattr(arg, "__add__")

Answer 2

>>> from collections import Mapping, Sequence
>>> isinstance('ac', Sequence)
True
>>> isinstance('ac', Mapping)
False
>>> isinstance({3:42}, Mapping)
True
>>> isinstance({3:42}, Sequence)
False

collections abstract base classes (ABCs)