How to Check if an Object is Iterable in Python

Determining whether an object is iterable (can be used in a loop) is a fundamental task in Python.

This guide explains the most reliable method for checking iterability using the iter() function and try-except blocks. It also discusses why alternative approaches (like checking for __iter__ or using collections.abc.Iterable) are less comprehensive.

Checking for Iterability with iter() (Recommended)

The most reliable and Pythonic way to check if an object is iterable is to use the built-in iter() function within a try-except block:

my_str = 'hello'

try:
    my_iterator = iter(my_str)  # Attempt to get an iterator
    print('The object is iterable')
    for i in my_iterator:  # We can iterate over the object
        print(i)  # Output: h, e, l, l, o
except TypeError as te:
    print('The object is NOT iterable')
    print(te)

• iter(my_str) attempts to get an iterator from the object.
• If the object is not iterable, then iter() will raise a TypeError exception.
• If the try block succeeds (no TypeError), the object is iterable.

Creating a Reusable Function

For convenience, encapsulate this logic in a reusable function:

def is_iterable(value):
    try:
        iter(value)
        return True
    except TypeError:
        return False

print(is_iterable('hello'))  # Output: True
print(is_iterable(100))      # Output: False

• This function can be used for checking any variable if its type is iterable or not.

Why collections.abc.Iterable is Insufficient

You might encounter code that uses collections.abc.Iterable and isinstance():

from collections.abc import Iterable

my_list = ['a', 'b', 'c']
print(isinstance(my_list, Iterable))    # Output: True
print(isinstance(100, Iterable))        # Output: False

However, this approach is not comprehensive. It only checks if the object's class (or a parent class) is registered as a subclass of Iterable. It doesn't detect objects that are iterable via the __getitem__ method (the sequence protocol), which is a valid way to make an object iterable. Therefore, relying solely on isinstance(obj, Iterable) can lead to false negatives.

Most Iterable Built-in Objects Implement the __iter__ Method

Most built-in iterable objects implement the __iter__() method, making it possible to check for iterability using iter() method. Here are a few examples of Python built-in objects that implement the __iter__ method:

print({}.__iter__)
print(().__iter__)
print([].__iter__)
print(''.__iter__)
print({'a', }.__iter__)

Making a Class Iterable

To make your own classes iterable, you need to implement either __iter__() (returning an iterator) or __getitem__ (for sequence-like behavior). The more common and flexible approach is to use __iter__():

class Counter:
   def __init__(self, start, stop):
      self.current = start - 1
      self.stop = stop

   def __iter__(self):
      return self # Return iterator

   def __next__(self):
       self.current += 1
       if self.current < self.stop:
           return self.current
       raise StopIteration # Indicate the iteration is over


   for c in Counter(0, 4):
       print(c) # Output: 0, 1, 2, 3

• This Counter class is iterable because it defines both __iter__ and __next__.