[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part)

Chris Barker chris.barker at noaa.gov
Thu Jun 28 00:52:43 EDT 2018

On Wed, Jun 27, 2018 at 12:01 PM, Eric V. Smith <eric at trueblade.com> wrote:

> >>> def test():
> >>>    spam = 1
> >>>    ham = 2
> >>>    vars = [key1+key2 for key1 in locals() for key2 in locals()]
> >>>    return vars
> >>>
> >>> Wanna guess what that's gonna return?

Guessing aside, messing around with locals() isn't really helpful for the
usual case of common code.

But the real problem I see with all of this the distinction between
generator expressions and comprehensions:

an assignment expression occurring in a list, set or dict comprehension or
in a generator expression (below collectively referred to as
"comprehensions") binds the target in the containing scope, honoring a
nonlocal or global declaration for the target in that scope, if one exists.
For the purpose of this rule the containing scope of a nested comprehension
is the scope that contains the outermost comprehension. A lambda counts as
a containing scope.

It seems everyone agrees that scoping rules should be the same for
generator expressions and comprehensions, which is a good reason for
python3's non-leaking comprehensions:


In [*5*]: i = 0

In [*6*]: l = [i *for* i *in* range(3)]

In [*7*]: i

Out[*7*]: 2

In [*8*]: i = 0

In [*9*]: g = (i *for* i *in* range(3))

In [*10*]: i

Out[*10*]: 0

In [*11*]: *for* j *in* g:

    ...:     *pass*


In [*12*]: i

Out[*12*]: 0

so comprehensions and generator expressions behave differently -- not great.


In [*4*]: i = 0

In [*5*]: l = [i *for* i *in* range(3)]

In [*6*]: i

Out[*6*]: 0

In [*7*]: g = (i *for* i *in* range(3))

In [*8*]: i

Out[*8*]: 0

In [*9*]: list(g)

Out[*9*]: [0, 1, 2]

In [*10*]: i

Out[*10*]: 0

The loop name doesn't "leak" and comprehensions and generator expressions
are the same this regard -- nice.

So what about:

l = [x:=i for i in range(3)]


g = (x:=i for i in range(3))

Is there any way to keep these consistent if the "x" is in the regular
local scope?

Note that this thread is titled "Informal educator feedback on PEP 572".

As an educator -- this is looking harder an harder to explain to newbies...

Though easier if any assignments made in a "comprehension" don't "leak out".

Which does not mean that we'd need a "proper" new local scope (i.e.
locals() returning something new) -- as long as the common usage was

>> I'm not singling out Chris here, but these discussions would be easier
> >> to follow and more illuminating if the answers to such puzzles were
> >> presented when they're posed.

well, I think the point there was that it wasn't obvious without running
the code -- and that point is made regardless of the answer.



Christopher Barker, Ph.D.

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Chris.Barker at noaa.gov
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