23.11.17 18:08, Guido van Rossum пише:
This thread is still going over the speed limit. Don't commit anything
without my explicit approval.
I'm not going to write a single line of code while the decision about
this issue is not made. This is not easy issue.
A problem with dropping the "function-ness" of the comprehension by
renaming the variables (as Ivan/Serhiy's plan seems to be?) would be
what does it look like in the debugger -- can I still step through the
loop and print the values of expressions? Or do I have to know the
"renamed" names?
Does the debugger supports stepping inside a comprehension? Isn't the
whole comprehension a single logical line? It this is supported the the
debugger should see a local variable with a strange name ".0". This is a
leaked implementation detail. It wouldn't be surprised if details of
other implementation will be leaked too in the debugger. The debugger
can unmangle the inner variable names and hide local variables in the
outer scope.
And my mind boggles when considering a generator expression containing
yield that is returned from a function. I tried this and cannot say I
expected the outcome:
def f():
return ((yield i) for i in range(3))
print(list(f()))
In both Python 2 and Python 3 this prints
[0, None, 1, None, 2, None]
Even if there's a totally logical explanation for that, I still don't
like it, and I think yield in a comprehension should be banned. From
this it follows that we should also simply ban yield from comprehensions.
This behavior doesn't look correct to me and Ivan. Ivan explained that
this function should be rough equivalent to
def f():
t = [(yield i) for i in range(3)]
return (x for x in t)
which should be equivalent to
def f():
t = []
for i in range(3):
t.append((yield i))
return (x for x in t)
and list(f()) should be [1, 2, 3].
But while I know how to fix yield in comprehensions (just inline the
code with some additions), I have no ideas how to fix yield in
generators. Me and Ivan agreed that SyntaxError in this case is better
than the current behavior. Maybe we will find how to implement the
expected behavior.
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