[Python-ideas] PEP 572: about the operator precedence of :=

Nick Coghlan ncoghlan at gmail.com
Thu May 10 07:31:41 EDT 2018


On 10 May 2018 at 13:33, Guido van Rossum <guido at python.org> wrote:

> (I vaguely recall this has been brought up before, but I'm too lazy to
> find the subtread. So it goes.)
>
> PEP 572 currently seems to specify that when used in expressions, the
> precedence of `:=` is lower (i.e. it binds more tightly) than all operators
> except for the comma. I derive this from the single example `stuff = [[y :=
> f(x), x/y] for x in range(5)]`.
>
> From this it would follow that `f(a := 1, a)` is equivalent to `a = 1;
> f(1, 1)`, and also that `(a := 1, a)` is equivalent to `a = 1; (1, 1)`.
> (Although M.A.L. objected to this.)
>
> But what should `a := 1, 1` at the top level (as a statement) do? On the
> one hand, analogy with the above suggest that it is equivalent to `a = 1;
> (1, 1)`. But on the other hand, it would be really strange if the following
> two lines had different meanings:
>
>     a = 1, 1   # a = (1, 1)
>     a := 1, 1  # a = 1; (1, 1)
>
> I now think that the best way out is to rule `:=` in the top level
> expression of an expression statement completely (it would still be okay
> inside parentheses, where it would bind tighter than comma).
>

FWIW, this is one of the ambiguities that the generalised postfix
expression form of the given clause would reduce fairly significantly by
separating the result expression from the bound expression:

    a = 1, 1
    a given a = 1, 1 # As above, but also returns a
    a = 1, x, 3 given x = 2

They also compose fairly nicely as an essentially right associative
operator:

    a given a = 1, x, 3 given x = 2
    a given a = (1, x, 3 given x = 2) # Same as previous
    (a given a = 1), x, 3 given x = 2 # Forcing left-associativity

While you do have to repeat the bound name at least once, you gain a much
clearer order of execution (while it's an order of execution that's
right-to-left rather than left-to-right, "rightmost expression first" is
the normal way assignments get evaluated anyway).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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