[Python-Dev] assignment expressions: an alternative proposal (original) (raw)

Chris Angelico rosuav at gmail.com
Tue Apr 24 10:56:31 EDT 2018

On Wed, Apr 25, 2018 at 12:23 AM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:

On Tue, Apr 24, 2018 at 10:07 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

"=" is always an assignment. "==" is always an equality check. That's not the distinction I meant, I meant the difficulty of explaining the discrepancies in this list: a = 1 # Assignment (a = 1) # Also assignment a, b = 1, 2 # Tuple assignment (a, b = 1, 2) # SyntaxError. Why? ... Whereas if binding expressions use a different symbol, the question is far less likely to arise, and if it does come up, then the answer is the same as the one for def statements vs lambda expressions: because one is a statement, and the other is an expression. A lot of other questions arise though. PEP 572 proposes: a = 1 # assignment a := 1 # also assignment (a := 1) # also assignment (a = 1) # error, why?

Your third example is just the same as the second, with parentheses around it. In most of Python, parentheses (if legal) have no effect other than grouping; "a + b * c" is the same thing as "(a + b) * c", just done in the other order. The last one is a clear demonstration that "=" is a statement, not an expression. Are people confused by this sort of thing:

if x > 1: print("x is more than 1") (if x > 1:) print("SyntaxError")

? Yes, the word 'if' does have meaning in an expression context, and yes, it has a similar meaning to the 'if' statement, but people don't parenthesize entire statements. You try that with assignment, you get an error, and bam, it's obvious that you ran into this particular case.

ChrisA

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