Question

I have a function that may take in a number or a list of numbers. Whats the most pythonic way of checking which it is? So far I’ve come up with try/except block checking if i can slice the zero item ie. obj[0:0]

Edit:

I seem to have started a war of words down below by not giving enough info. For completeness let me provide more details so that I may pick and get the best answer for my situation:

I’m running Django on Python 2.6 and I’m writing a function that may take in a Django model instance or a queryset object and perform operations on it one of which involves using the filter ‘in’ that requires a list (the queryset input), or alternately if it is not a list then I would use the ‘get’ filter (the django get filter).

Answer 1

In such situations, you normally need to check for ANY iterable, not just lists — if you’re accepting lists OR numbers, rejecting (e.g) a tuple would be weird. The one kind of iterable you might want to treat as a “scalar” is a string — in Python 2.*, this means str or unicode. So, either:

def isNonStringIterable(x):
  if isinstance(x, basestring):
    return False
  try: iter(x)
  except: return False
  else: return True

or, usually much handier:

def makeNonStringIterable(x):
  if isinstance(x, basestring):
    return (x,)
  try: return iter(x)
  except: return (x,)

where you just go for i in makeNonStringIterable(x): ...