So the way I envision it is that *in the absence of a nonlocal or global
declaration in the containing scope*, := inside a comprehension or genexpr
causes the compiler to assign to a local in the containing scope, which is
elevated to a cell (if it isn't already). If there is an explicit nonlocal
or global declaration in the containing scope, that is honored.
Examples:
# Simplest case, neither nonlocal nor global declaration
def foo():
[p := q for q in range(10)] # Creates foo-local variable p
print(p) # Prints 9
# There's a nonlocal declaration
def bar():
p = 42 # Needed to determine its scope
def inner():
nonlocal p
[p := q for q in range(10)] # Assigns to p in bar's scope
inner()
print(p) # Prints 9
# There's a global declaration
def baz():
global p
[p := q for q in range(10)]
baz()
print(p) # Prints 9
All these would work the same way if you wrote list(p := q for q in
range(10)) instead of the comprehension.
We should probably define what happens when you write [p := p for p in
range(10)]. I propose that this overwrites the loop control variable rather
than creating a second p in the containing scope -- either way it's
probably a typo anyway.
:= outside a comprehension/genexpr is treated just like any other
assignment (other than in-place assignment), i.e. it creates a local unless
a nonlocal or global declaration exists.
--
--Guido van Rossum (python.org/~guido)
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