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

David Shawley daveshawley at gmail.com
Wed Apr 25 07:03:31 EDT 2018

On Apr 24, 2018, at 2:10 PM, MRAB <python at mrabarnett.plus.com> wrote:

On 2018-04-21 03:15, Tim Peters wrote: [Tim] >> And I'll take this opportunity to repeat the key point for me: I >> tried hard, but never found a single case based on staring at real >> code where allowing fancier (than "plain name") targets would be a >> real improvement. In every case I thought it might help, it turned >> out that it really didn't unless Python also grew an analog to C's >> "comma operator" (take only the last result from a sequence of >> expressions). I'll also note that I asked if anyone else had a >> real-life example, and got no responses.

[MRAB <python at mrabarnett.plus.com>] > Could a semicolon in a parenthesised expression be an equivalent to C's > "comma operator"? I expect it could, but I it's been many years since I tried hacking Python's grammar, and I wouldn't want a comma operator anyway ;-) [snip] Just reading this: https://www.bfilipek.com/2018/04/refactoring-with-c17-stdoptional.html about C++17, and what did I see? An example with a semicolon in parentheses!

A similar pattern shows up in Go's if statement syntax. It is interesting to note that it is part of the grammar specifically for the if statement and not general expression syntax.

IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .

Bindings that occur inside of SimpleStmt are only available within the Expression and blocks that make up the if statement.

https://golang.org/ref/spec#If_statements

This isn't a good reason to parrot the syntax in Python though. IMO, I consider the pattern to be one of the distinguishing features of golang and would be happy leaving it there.

I have often wondered if adding the venerable for loop syntax from C (and many other languages) would solve some of the needs here though. The for loop syntax in golang is interesting in that it serves as both a standard multipart for statement as well as a while statement.

Changing something like this is more of a Python 4 feature and I think that I would be -0 on the concept. I did want to mention the similarities for the posterity though.

ChrisA - we might want to add explicit mentions of golang's if statement and for loop as "considered" syntaxes since they are in a sibling programing language (e.g., similar to async/await in PEP 492).

More information about the Python-Dev mailing list