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Syntax error on '{z} if z is not None else pass' #129515

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Syntax error on '{z} if z is not None else pass'#129515
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interpreter-core(Objects, Python, Grammar, and Parser dirs)topic-parsertype-featureA feature request or enhancement

Description

@none-of-my-names-is-available
none-of-my-names-is-available
opened on Jan 31, 2025

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

Activity

  1. Imagenone-of-my-names-is-available
    added
    type-bugAn unexpected behavior, bug, or error
    on Jan 31, 2025

  2. hoodmane commented on Jan 31, 2025

    @hoodmane
    hoodmane
    on Jan 31, 2025
    Contributor

    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

  3. ericvsmith commented on Jan 31, 2025

    @ericvsmith
    ericvsmith
    on Jan 31, 2025
    Member

    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.

  4. hoodmane commented on Jan 31, 2025

    @hoodmane
    hoodmane
    on Jan 31, 2025
    Contributor

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

  5. tomasr8 commented on Jan 31, 2025

    @tomasr8
    tomasr8
    on Jan 31, 2025
    Member

    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

  6. StanFromIreland commented on Jan 31, 2025

    @StanFromIreland
    StanFromIreland
    on Jan 31, 2025
    Member

    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.

  7. Imageencukou
    added
    topic-parser
    and removed
    type-bugAn unexpected behavior, bug, or error
    on Jan 31, 2025
  8. Imageterryjreedy
    added
    docsDocumentation in the Doc dir
    on Jan 31, 2025
  9. Imagegithub-project-automation
    added this to docs issueson Jan 31, 2025
  10. Imagegithub-project-automation
    moved this to Todo in docs issueson Jan 31, 2025

  11. none-of-my-names-is-available commented on Jan 31, 2025

    @none-of-my-names-is-available
    none-of-my-names-is-available
    on Jan 31, 2025 via email
    Author
  12. Imageterryjreedy
    changed the title [-]Why syntax error on: x |= {z} if z is not None else pass[/-] [+]Syntax error on '{z} if z is not None else pass'[/+] on Jan 31, 2025
  13. Imageterryjreedy
    removed
    docsDocumentation in the Doc dir
    on Jan 31, 2025

  14. terryjreedy commented on Jan 31, 2025

    @terryjreedy
    terryjreedy
    on Jan 31, 2025
    Member

    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.

  15. 1 remaining item

  16. Imagepicnixz
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    on Jan 31, 2025

  17. sergey-miryanov commented on Feb 8, 2025

    @sergey-miryanov
    sergey-miryanov
    on Feb 8, 2025
    Contributor

    If no one else is doing it, I can take it.

  18. StanFromIreland commented on Feb 8, 2025

    @StanFromIreland
    StanFromIreland
    on Feb 8, 2025
    Member

    @sergey-miryanov Feel free to take on any issue where no one else has mentioned they plan to do so and there is no one assigned.

  19. Imagesergey-miryanov
    added 2 commits that reference this issue on Feb 9, 2025

    pythongh-129515: Clarify syntax error messages for conditional expres…

    4dbba6b

    Merge branch 'main' into pythongh-129515-ifexp-syntax-error

    aa3f17d

  20. sergey-miryanov commented on Feb 10, 2025

    @sergey-miryanov
    sergey-miryanov
    on Feb 10, 2025
    Contributor

    Linked PR added the following rules and messages.

    The following expressions will be handled by a=disjunction 'if' b=disjunction 'else' c=invalid_ifexp_orelse_stmt and generate the message "Syntax Error: statement given where 'orelse' expression required" instead of just "Syntax Error: invalid syntax":

    1 if True else pass
1 if True else return
1 if True else raise Exception('test')
1 if True else del smth
1 if True else yield 2
1 if True else assert False
1 if True else break
1 if True else continue

    The following expressions will be handled by a=invalid_ifexp_body_stmt 'if' b=disjunction 'else' c=expression and generate the message "Syntax Error: statement given where 'body' expression required":

    pass if True else 1
break if True else 1
continue if True else 1

    The followingexpressions will be handled by a=invalid_ifexp_body_stmt 'if' b=disjunction 'else' c=invalid_ifexp_orelse_stmt and generate the message "Syntax Error: statement given where 'body' expression required":

    pass if True else pass
break if True else continue

    Please take a look.

  21. Imagesergey-miryanov
    added 2 commits that reference this issue on Feb 10, 2025

    Merge branch 'pythongh-129515-ifexp-syntax-error' of github.com:serge…

    b43b3a2

    Merge branch 'main' into pythongh-129515-ifexp-syntax-error

    48e4796
  22. Imagepablogsal
    added a commit that references this issue on Feb 10, 2025

    Merge branch 'main' into pythongh-129515-ifexp-syntax-error

    8afe66a
  23. Imagesergey-miryanov
    added 4 commits that reference this issue on Feb 11, 2025

    Merge branch 'main' into pythongh-129515-ifexp-syntax-error

    7e4ed1b

    Merge branch 'main' into pythongh-129515-ifexp-syntax-error

    2eaa968

    Merge branch 'main' into pythongh-129515-ifexp-syntax-error

    63b4400

    Merge branch 'pythongh-129515-ifexp-syntax-error' of github.com:serge…

    c468622
  24. Imagepablogsal
    added a commit that references this issue on Feb 18, 2025

    gh-129515: Clarify syntax error messages for conditional expressions (#…

    bcc9a5d
  25. Imagepablogsal
    closed this as completedon Feb 18, 2025
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