Checking if a class is iterable

Question

I have a class fib given below. It implements __iter__ and __next__. It is an iterable as well as its own iterator.

class fib(object):
    def __init__(self):
        self.prev = 0
        self.curr = 1

    def __iter__(self):
        return self

    def __next__(self):
        value = self.curr
        self.curr += self.prev
        self.prev = value
        return value


from collections import Iterable

print(isinstance(fib, Iterable))

The print statement returns False, I would expect it to return True

`fib` is the *class*, not an instance. So you would use the subclass check `issubclass(fib, Iterable)`. Or you create an instance first: `isinstance(fib(), Iterable)` – That’s what you get for starting your class name with a lower case character ;) — poke, Nov 21 '16 at 20:28
`fib` is not iterable. `fib` is a class whose instances are iterable. Trying to iterate over `fib` itself would be like trying to eat the concept of burgers. — user2357112, Nov 21 '16 at 20:58

Dimitris Fasarakis Hilliard · Accepted Answer · 2018-12-14T09:23:18.577

Checking if an object is iterable is correctly, as you've done, performed with:

isinstance(obj, collections.Iterable)

The problem here is you're supplying a class to isinstance and not an instance. It is False because isinstance will go ahead and check if type(fib) has an __iter__ method defined:

type(fib).__iter__  # AttributeError

This of course, isn't the case. type(fib) is type which doesn't define an __iter__ method.

If you supply an instance to it, it correctly prints True:

isinstance(fib(), Iterable)  # True

because looking in type(fib()) it will find fib.__iter__.

Alternatively, feeding fib to issubclass performs a similar check that, instead, takes a class as a first argument:

issubclass(fib, Iterable)    # True

Two extra minor things to point out:

Using object as an explicit base class is unnecessary in Python 3 (though, if you're developing code that runs on both Py2 and Py3, it is a good thing. (See Python class inherits object for more on this.)
According to PEP 8, class names should follow the CapWords convention, so fib should ideally be named Fib.

Checking if a class is iterable

1 Answers1