[Python-Dev] PEP 572: Assignment Expressions (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Apr 21 02:17:42 EDT 2018

On 21 April 2018 at 07:33, Tim Peters <tim.peters at gmail.com> wrote:

I expected that, given that expressions "naturally nest", chained targets could still be specified:

a := b := c:= 5 but since they're all plain names there's no way to tell whether the bindings occur "left to right" or "right to left" short of staring at the generated code.

The fact class namespaces are ordered by default now allow us to demonstrate the order of multiple target assignments and tuple unpacking without staring at generated code:

class AssignmentOrder: ... a = b = c = 0 ... d, e, f = range(3) ... class ReversedAssignmentOrder: ... c = b = a = 0 ... f, e, d = range(3) ... [attr for attr in AssignmentOrder.dict if not attr.startswith("")] ['a', 'b', 'c', 'd', 'e', 'f'] [attr for attr in ReversedAssignmentOrder.dict if not attr.startswith("")] ['c', 'b', 'a', 'f', 'e', 'd']

So that's a situation where "name = alias = value" could end up matching "alias := name := value"

(Even in earlier versions, you can illustrate the same assignment ordering behaviour with the enum module, and there it makes even more of a difference, as it affects which name binding is considered the canonical name, and which are considered aliases).

Cheers, Nick.

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

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