[Python-ideas] Combining test and assignment (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sun Jan 22 04:48:20 CET 2012

This suggestion (i.e. embedded assignment in while and if headers) has been made several times, and always come to a grinding halt on one simple problem: it isn't expressive enough.

The current idiom is this :

x = EXPR
if predicate(x):
    # Do something

Given a value and a predicate, this allows you to execute code conditionally based on whether or not the predicate is true.

Simple embedded assignment, though, only works when the predicate is just "bool" - as soon as the condition differs from the value you want to access, you need to revert to the existing idiom anyway. Given the non-trivial costs of adding new syntax, that gets the idea put into the "not worth the hassle" bucket.

The obvious escalation of the suggestion is to adopt "(EXPR as NAME)" as a general purpose embedded assignment expression. However, this idea has problems of its own:

  1. In with statements and exception handlers, 'as' does not denote ordinary assignment. In the former case, the name is bound to the result of the EXPR.enter() call, in the latter to the exception instance that was actually caught. If assignment expressions were added, these could become a rather subtle trap (especially since many enter() methods just return self)

  2. Generator expressions and the various forms of comprehension create new scopes to avoid leaking iteration variables, so, if embedded assignments were to do anything useful there, additional forms like "(EXPR as nonlocal NAME)" and "(EXPR as global NAME)" would be needed. (And, in the nonlocal case, would still require that the name already be defined in the containing scope)

  3. Adding assignment expressions would mean having two ways to perform assignments at the statement level (either using the existing statement form or using the new expression form)

To date, nobody has been interested enough in the latter idea to put together a formal PEP and reference implementation for python-dev's consideration, and the former idea has been informally rejected several times due to the lack of generality.

Cheers, Nick.

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

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