Return is Not the End

Created: 2026-05-29 09:13 by Thom

In Python there are some weird stuff happening with the `return` statement. Look at this:

In [1]: def f1():
   ...:     return None
   ...:

In [2]: def f2():
   ...:     return None
   ...:     yield
   ...:

In [3]: print(f1())
None

In [4]: print(f2())


Why?? Why does this happen? Well I found something. It is all to do with flags (CO_GENERATOR) in this case.

In [6]: import dis

In [7]: f2.__code__.co_flags & 0x20
Out[7]: 32

In [8]: f1.__code__.co_flags & 0x20
Out[8]: 0

In [9]:

But this doesn't make the `return` meaningless.

In [9]: gen = f2()

In [10]: next(gen)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
Cell In[10], line 1
----> 1 next(gen)

StopIteration:

So Python does some interesting stuff at compile time to do generators. And they need to be like this because they are lazy.