In Python, how do I determine if an object is iterable?

Question

Is there a method like isiterable? The only solution I have found so far is to call

hasattr(myObj, \'__iter__\')

But I am not

Answer 1

1. Checking for __iter__ works on sequence types, but it would fail on e.g. strings in Python 2. I would like to know the right answer too, until then, here is one possibility (which would work on strings, too):

   from __future__ import print_function
   
   try:
       some_object_iterator = iter(some_object)
   except TypeError as te:
       print(some_object, 'is not iterable')
   

   The iter built-in checks for the __iter__ method or in the case of strings the __getitem__ method.

2. Another general pythonic approach is to assume an iterable, then fail gracefully if it does not work on the given object. The Python glossary:

   Pythonic programming style that determines an object's type by inspection of its method or attribute signature rather than by explicit relationship to some type object ("If it looks like a duck and quacks like a duck, it must be a duck.") By emphasizing interfaces rather than specific types, well-designed code improves its flexibility by allowing polymorphic substitution. Duck-typing avoids tests using type() or isinstance(). Instead, it typically employs the EAFP (Easier to Ask Forgiveness than Permission) style of programming.

   ...

   try:
      _ = (e for e in my_object)
   except TypeError:
      print my_object, 'is not iterable'
   

3. The collections module provides some abstract base classes, which allow to ask classes or instances if they provide particular functionality, for example:

   from collections.abc import Iterable
   
   if isinstance(e, Iterable):
       # e is iterable
   

   However, this does not check for classes that are iterable through __getitem__.

Answer 2

The best solution I've found so far:

hasattr(obj, '__contains__')

which basically checks if the object implements the in operator.

Advantages (none of the other solutions has all three):

• it is an expression (works as a lambda, as opposed to the try...except variant)
• it is (should be) implemented by all iterables, including strings (as opposed to __iter__)
• works on any Python >= 2.5

Notes:

• the Python philosophy of "ask for forgiveness, not permission" doesn't work well when e.g. in a list you have both iterables and non-iterables and you need to treat each element differently according to it's type (treating iterables on try and non-iterables on except would work, but it would look butt-ugly and misleading)
• solutions to this problem which attempt to actually iterate over the object (e.g. [x for x in obj]) to check if it's iterable may induce significant performance penalties for large iterables (especially if you just need the first few elements of the iterable, for example) and should be avoided

Answer 3

It's always eluded me as to why python has callable(obj) -> bool but not iterable(obj) -> bool...
surely it's easier to do hasattr(obj,'__call__') even if it is slower.

Since just about every other answer recommends using try/except TypeError, where testing for exceptions is generally considered bad practice among any language, here's an implementation of iterable(obj) -> bool I've grown more fond of and use often:

For python 2's sake, I'll use a lambda just for that extra performance boost...
(in python 3 it doesn't matter what you use for defining the function, def has roughly the same speed as lambda)

iterable = lambda obj: hasattr(obj,'__iter__') or hasattr(obj,'__getitem__')

Note that this function executes faster for objects with __iter__ since it doesn't test for __getitem__.

Most iterable objects should rely on __iter__ where special-case objects fall back to __getitem__, though either is required for an object to be iterable.
(and since this is standard, it affects C objects as well)