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

I often find convenient, inside my scripts, to define an iterable function. (Now incorporates Alfe's suggested simplification):

import collections

def iterable(obj):
    return isinstance(obj, collections.Iterable):

so you can test if any object is iterable in the very readable form

if iterable(obj):
    # act on iterable
else:
    # not iterable

as you would do with thecallable function

EDIT: if you have numpy installed, you can simply do: from numpy import iterable, which is simply something like

def iterable(obj):
    try: iter(obj)
    except: return False
    return True

If you do not have numpy, you can simply implement this code, or the one above.

Answer 2

Duck typing

try:
    iterator = iter(theElement)
except TypeError:
    # not iterable
else:
    # iterable

# for obj in iterator:
#     pass

Type checking

Use the Abstract Base Classes. They need at least Python 2.6 and work only for new-style classes.

from collections.abc import Iterable   # import directly from collections for Python < 3.3

if isinstance(theElement, Iterable):
    # iterable
else:
    # not iterable

However, iter() is a bit more reliable as described by the documentation:

Checking isinstance(obj, Iterable) detects classes that are registered as Iterable or that have an __iter__() method, but it does not detect classes that iterate with the __getitem__() method. The only reliable way to determine whether an object is iterable is to call iter(obj).

Answer 3

This isn't sufficient: the object returned by __iter__ must implement the iteration protocol (i.e. next method). See the relevant section in the documentation.

In Python, a good practice is to "try and see" instead of "checking".

Answer 4

Instead of checking for the __iter__ attribute, you could check for the __len__ attribute, which is implemented by every python builtin iterable, including strings.

>>> hasattr(1, "__len__")
False
>>> hasattr(1.3, "__len__")
False
>>> hasattr("a", "__len__")
True
>>> hasattr([1,2,3], "__len__")
True
>>> hasattr({1,2}, "__len__")
True
>>> hasattr({"a":1}, "__len__")
True
>>> hasattr(("a", 1), "__len__")
True

None-iterable objects would not implement this for obvious reasons. However, it does not catch user-defined iterables that do not implement it, nor do generator expressions, which iter can deal with. However, this can be done in a line, and adding a simple or expression checking for generators would fix this problem. (Note that writing type(my_generator_expression) == generator would throw a NameError. Refer to this answer instead.)

You can use GeneratorType from types:

>>> import types
>>> types.GeneratorType
<class 'generator'>
>>> gen = (i for i in range(10))
>>> isinstance(gen, types.GeneratorType)
True

--- accepted answer by utdemir

(This makes it useful for checking if you can call len on the object though.)

Answer 5

The easiest way, respecting the Python's duck typing, is to catch the error (Python knows perfectly what does it expect from an object to become an iterator):

class A(object):
    def __getitem__(self, item):
        return something

class B(object):
    def __iter__(self):
        # Return a compliant iterator. Just an example
        return iter([])

class C(object):
    def __iter__(self):
        # Return crap
        return 1

class D(object): pass

def iterable(obj):
    try:
        iter(obj)
        return True
    except:
        return False

assert iterable(A())
assert iterable(B())
assert iterable(C())
assert not iterable(D())

Notes:

1. It is irrelevant the distinction whether the object is not iterable, or a buggy __iter__ has been implemented, if the exception type is the same: anyway you will not be able to iterate the object.

2. I think I understand your concern: How does callable exists as a check if I could also rely on duck typing to raise an AttributeError if __call__ is not defined for my object, but that's not the case for iterable checking?

   I don't know the answer, but you can either implement the function I (and other users) gave, or just catch the exception in your code (your implementation in that part will be like the function I wrote - just ensure you isolate the iterator creation from the rest of the code so you can capture the exception and distinguish it from another TypeError.

Answer 6

Not really "correct" but can serve as quick check of most common types like strings, tuples, floats, etc...

>>> '__iter__' in dir('sds')
True
>>> '__iter__' in dir(56)
False
>>> '__iter__' in dir([5,6,9,8])
True
>>> '__iter__' in dir({'jh':'ff'})
True
>>> '__iter__' in dir({'jh'})
True
>>> '__iter__' in dir(56.9865)
False