Python; recursive flatten of nested iterables, with proper handling of string elements

flatten.py

#!/usr/bin/env python3
import collections

def flatten(t):
    """
    Generator flattening the structure
    
    >>> list(flatten([2, [2, "test", (4, 5, [7], [2, [6, 2, 6, [6], 4]], 6)]]))
    [2, 2, "test", 4, 5, 7, 2, 6, 2, 6, 6, 4, 6]
    """
    for x in t:
    if isinstance(x, str) or not isinstance(x, collections.Iterable):
        yield x
    else:
        yield from flatten(x)