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

Christoph Groth christoph at grothesque.org
Fri Apr 20 12:17:34 EDT 2018

Chris Barker - NOAA Federal wrote:

> Personally, I even slightly prefer > > a := 3 > > to the commonplace > > a = 3 > because it visually expresses the asymmetry of the operation.

Careful here! That’s a fine argument for using := in a new language, but people using := when they don’t need an expression because they like the symbol better is a reason NOT to do this.

Perhaps you are right and it is indeed unrealistic to expect people to (eventually) shift to using := for simple assignments after 28 years of Python...

Then I think it would be also OK to introduce a fully general ":=" but discourage its use in assignment statements. However, it seems strange to forbid the use of one expression (namely ":=") as a statement while all other expressions are allowed. (So there seems no alternative to accepting both = and := in statements, and if I understand you correctly you consider this a problem.)

One way or the other, I'd like to underline a point that I made yesterday: I believe that it's important for sanity that taking any existing assignment statement and replacing all occurrences of "=" by ":=" does not have any effect on the program.

PEP 572 currently proposes to make ":=" a binary operator that is evaluated from right to left.

Christoph

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