Python recursive generator example

The following is taken from “Beginning Python”, 3rd edition, 2017 by Magnus Lie Hetland.

The goal of this example is to flatten a series of arbitrarily deep nested lists using a recursive generator.

def flatten(nested):
    try:
        #Don't iterate over string-like objects:
        try:
            nested + " "
        except TypeError:
                pass
        else:
            raise TypeError
        for sublist in nested:
            for element in flatten(sublist):
                yield element

    except TypeError:
        yield nested

print(list(flatten(['foo',1,2,3, ('2','3'),['bar','123',['baz']]])))

The output is ['foo', 1, 2, 3, '2', '3', 'bar', '123', 'baz']

In this case, we don’t want to iterate over string-like objects; we want to treat them as atomic values, not sequences that should be flattened.

To test if an object is string-like, this code try’s to concatenate it with a string (“ “). If the expression nested + " " raises a TypeError, it is ignored; however, if the expression does not raise the TypeError, the else clause of the inner try statement raises a TypeError of its own. This causes the string-like object to be yielded as is (in the outer except clause).

This is one of the few times I’ve seen a pass statement used with purpose in a working code sample, Hah!

Here’s the same function rewritten as a plain function (without a generator):

def flatten(nested):
    result = []
    try:
        #Don't iterate over string-like objects:
        try:
            nested + " "
        except TypeError:
                pass
        else:
            raise TypeError
        for sublist in nested:
            for element in flatten(sublist):
                result.append(element)

    except TypeError:
        result.append(nested)
    return nested