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

Tim Peters tim.peters at gmail.com
Mon Apr 23 17:41:49 EDT 2018

[Tim]

Surely you're joking. This is math.gcd(), which is expensive for multi-thousand bit integers, and the interpreter knows nothing about it. Adding a cache of any kind (LRU or otherwise) would make it even slower.

[Sven R. Kunze <srkunze at mail.de>]

Alright, if that problem is just about performance,

It's not, but others had already pointed out that it's generally considered Poor Practice (and for good reasons) to textually repeat expressions, so I didn't echo that. Even in purely functional languages, where textually equal snippets are guaranteed to evaluate to the same result every time, "give these expressions these brief names" constructs are heavily used (see, .e.g, "let" and "where" in Haskell).

:then there must be a better way to resolve it rather than inventing a new syntax.

Why? "Give the result of an expression a name" is already heavily used in Python - it's just that the contexts in which it can be done are very limited now.

Btw. storing the result in a local var is also a cache IMHO. And if gcd is immutable, I think Python can do a great job here of optimizing.

After decades, CPython still does nothing of the sort, short of having eventually made, e.g., "None" and "True" and "False" reserved words so at least it can optimize uses of those. It knows nothing at all about which library functions are pure - and there's no code in the implementation currently capable of exploiting such information even if it were known. That remains a fantasy in CPython.

Anyway, your example is the best one I've seen so far.

Guido gave better ones, where binding expressions would allow to collapse arbitrarily deep levels of nesting to just one (if ... elif ... elif ... elif ...). My example only eliminated a single level of artificial indentation. But my example did have the advantage of being taken verbatim from actual, working code ;-)

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