Joining strings. Generator or list comprehension?

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When you call str.join(gen) where gen is a generator, Python does the equivalent of list(gen) before going on to examine the length of the resulting sequence.

Specifically, if you look at the code implementing str.join in CPython, you’ll see this call:

    fseq = PySequence_Fast(seq, "can only join an iterable");

The call to PySequence_Fast converts the seq argument into a list if it wasn’t a list or tuple already.

So, the two versions of your call are handled almost identically. In the list comprehension, you’re building the list yourself and passing it into join. In the generator expression version, the generator object you pass in gets turned into a list right at the start of join, and the rest of the code operates the same for both versions..

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