Syntax error on '{z} if z is not None else pass'

[none-of-my-names-is-available]

Bug report

Bug description:

My question is in the title.

In the tutorial, I read: Use 'pass' in places, where code is required syntactically, but none is needed.
In order to save indentations, I used the conditional assignement in some segment of code. But I only want to include z in the set x, if it is not None, because later, the None disturbs.

So my thesis is: The code in the title is syntactically correct, but still, I get a syntax error.

Thank you for your kind consideration

CPython versions tested on:

3.10

Operating systems tested on:

Windows

Linked PRs

• gh-129515: Clarify syntax error messages for conditional expressions #129880

[hoodmane]

pass is a statement. lval |= expression is a type of statement. expression if expression else expression is a type of expression. a if b else pass has a statement in a location where the syntax requires an expression so it's a SyntaxError.

>>> a if b else pass
  File "<stdin>", line 1
    a if b else pass
                ^^^^
SyntaxError: invalid syntax

[ericvsmith]

The place you've used pass requires an expression, and pass is a statement, not an expression.

It's possible the documentation could be tweaked to make this clearer. Maybe it should say "where a statement is required", although maybe that's too subtle for the tutorial.

[hoodmane]

Yeah would be fantastic if the SyntaxError here said something like "statement given where expression required".

[tomasr8]

Yeah would be fantastic if the SyntaxError here said something like "statement given where expression required".

It might be possible to add a new invalid_ rule to the parser for this case

[StanFromIreland]

If you still want a working one line solution
z is not None and x.add(z)
as if z is None it will short circuit.

[none-of-my-names-is-available]

Hi Stan, thank you for your solution, but a more obvious one (in my eyes a a beginner in python) is: x |= {z} if z is not None else set () An implicit if without else clause is not so obvious, if it was a real assignment. This is why I think that the comment suggesting a more elaborate error message would be good. But thank you, Günter Becker Am 31.01.2025 um 17:25 schrieb Stan Ulbrych: If you still want a working one line solution z is not None and x.add(z) as if z is None it will short circuit. — Reply to this email directly, view it on GitHub<#129515 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/BC3NZRPVTEPKLGFFAEC7MED2NOPXTAVCNFSM6AAAAABWHRK44CVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDMMRXG42DCOJSHE>. You are receiving this because you authored the thread.Message ID: ***@***.***>

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[terryjreedy]

I fixed the title to focus on the syntactically incorrect expression. An expression must evaluate to a value and pass has no value. The proper null value for a set is set(), not None. {z} if z is not None else set() would be the proper expression in context. Given set x, the statement if z is not None: x.add(z) looks even better to me for adding z if not None.

The pass reference entry says " It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed". The tutorial entry, both now and in 3.10, says essentially the same: "It can be used when a statement is required syntactically but the program requires no action." @none-of-my-names-is-available, you apparently misread statement as code. No doc fix is needed.

I think a better error message would be nice if possible.