[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)

Serhiy Storchaka storchaka at gmail.com
Wed Nov 22 08:03:09 EST 2017

From https://stackoverflow.com/questions/45190729/differences-between-generator-comprehension-expressions.

 g = [(yield i) for i in range(3)]

Syntactically this looks like a list comprehension, and g should be a list, right? But actually it is a generator. This code is equivalent to the following code:

 def _make_list(it):
     result = []
     for i in it:
         result.append(yield i)
     return result
 g = _make_list(iter(range(3)))

Due to "yield" in the expression _make_list() is not a function returning a list, but a generator function returning a generator.

This change in semantic looks unintentional to me. It looks like leaking an implementation detail. If a list comprehension would be implemented not via creating and calling an intermediate function, but via an inlined loop (like in Python 2) this would be a syntax error if used outside of a function or would make an outer function a generator function.

 __result = []
 __i = None
 try:
     for __i in range(3):
         __result.append(yield __i)
     g = __result
 finally:
     del __result, __i

I don't see how the current behavior can be useful.

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