Is "... object is not iterable" A Misleading Error For "x in y"?

Question

Consider the following two classes:

class Foo0(object):                                                 
    pass

class Foo1(object):
    def __contains__(self, _):
        return False

Neither of them is iterable, as can be observed by trying for i in Foo1(): pass:

Traceback (most recent call last):
   File "stuff.py", line 11, in <module>
    for i in Foo1(): pass
TypeError: 'Foo1' object is not iterable

Conversely, 3 in Foo1() is valid (as Foo1 has __contains__), whereas 3 in Foo0() is not:

Traceback (most recent call last):
  File "stuff.py", line 9, in <module>
    3 in Foo0()
TypeError: argument of type 'Foo0' is not iterable

---

I find the error misleading. The point is not that Foo0 is not iterable - after all Foo1 is not iterable too, so that's not the inherent reason. Moreover, as @niemmi and @tobias_k correctly note below, if a class does not support __contains__, __iter__ is used as a fallback, which is the direct cause for this error. However this makes things worse, as a user might try to solve this error by implementing __iter__, which is a very inefficient means of implementing __contains__.

Why was this chosen as the error, then?

Answer 1

If the object doesn't have __contains__ method Python automatically tries to iterate over it to see if it can find the element. Since Foo0 isn't an iterable you will see the error.

UPDATE: Python language reference explains this in bit more detail:

For user-defined classes which define the contains() method, x in y is true if and only if y.contains(x) is true.

For user-defined classes which do not define contains() but do define iter(), x in y is true if some value z with x == z is produced while iterating over y. If an exception is raised during the iteration, it is as if in raised that exception.

Lastly, the old-style iteration protocol is tried: if a class defines getitem(), x in y is true if and only if there is a non-negative integer index i such that x == y[i], and all lower integer indices do not raise IndexError exception. (If any other exception is raised, it is as if in raised that exception).