[Python-ideas] Inline assignments using "given" clauses
greg.ewing at canterbury.ac.nz
Mon May 14 05:17:03 EDT 2018
Tim Peters wrote:
> Because you never _need_ to use an assignment expression to write a
This whole discussion started because someone wanted a way
to bind a temporary result for use *within* a comprehension.
Those use cases don't require leakage.
> Otherwise it's essentially impossible to explain why:
> total = 0
> sums = [total := total + value for value in data]
> assert sums[-1] == total
> "blows up", despite that its intent is obvious,unless you first
> explain to a user how the listcomp is implemented via an invisible
> synthetic function created by magic, inside of which `total` has
> nothing to do with the `total` they see on the first line.
It's no harder to explain that than it is to explain
x = 42
y = [x * x for x in range(5)]
prints 42 rather than whatever value was last bound to
the x in the comprehension.
Seems to me it would be easier to explain that *all* names
bound within a comprehension are local to the comprehension,
than to have to say that some are and some aren't.
> UnboundLocalError - WTF?
> That's why leaking "is good". It works both directions: the outer
> name leaks _into_ the body too, not just _out_ of it. Things that
> "look like" they should obviously work do work then, and a user can
> remain blissfully unaware of the implementation.. Of course you can
> also find cases in which it's not wanted.
> If it doesn't leak the kind of use shown above can't be done at all
> via listcomps (at least not straightforwardly).
> If it does leak, the subset of cases where leaking is unwanted _of_
> the subset of cases in which a listcomp//genexp uses an assignment
> expression at all are indeed inconvenienced.
> So - surprise! It's a tradeoff, something we've never faced before ;-)
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