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TypedDict: no way to tell which (if any) keys are optional at runtime #83015

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Zac-HD mannequin opened this issue Nov 18, 2019 · 1 comment
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TypedDict: no way to tell which (if any) keys are optional at runtime #83015

Zac-HD mannequin opened this issue Nov 18, 2019 · 1 comment
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3.8 only security fixes stdlib Python modules in the Lib dir

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@Zac-HD
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Zac-HD mannequin commented Nov 18, 2019

BPO 38834
Nosy @ilevkivskyi, @Zac-HD
PRs
  • bpo-38834: enable runtime inspection of TypeDict classes #17214
  • Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
    closed_at = <Date 2019-11-24.13:06:26.688>
    created_at = <Date 2019-11-18.00:53:54.937>
    labels = ['3.8', 'library']
    title = 'TypedDict: no way to tell which (if any) keys are optional at runtime'
    updated_at = <Date 2019-11-24.13:06:26.687>
    user = 'https://github.com/Zac-HD'

    bugs.python.org fields:

    activity = <Date 2019-11-24.13:06:26.687>
    actor = 'levkivskyi'
    assignee = 'none'
    closed = True
    closed_date = <Date 2019-11-24.13:06:26.688>
    closer = 'levkivskyi'
    components = ['Library (Lib)']
    creation = <Date 2019-11-18.00:53:54.937>
    creator = 'Zac Hatfield-Dodds'
    dependencies = []
    files = []
    hgrepos = []
    issue_num = 38834
    keywords = ['patch']
    message_count = 1.0
    messages = ['356836']
    nosy_count = 2.0
    nosy_names = ['levkivskyi', 'Zac Hatfield-Dodds']
    pr_nums = ['17214']
    priority = 'normal'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = None
    url = 'https://bugs.python.org/issue38834'
    versions = ['Python 3.8']

    @Zac-HD
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    Zac-HD mannequin commented Nov 18, 2019

    Consider the following cases:

    class A(typing.TypedDict):
        a: int  # a is required
    
    class B(A, total=False):
        b: bool  # a is required, b is optional
    
    class C(B):
        c: str  # a is required, b is optional, c is required again

    PEP-589 is clear about the semantics, and this is obvious enough when reading the code. At runtime the __annotations__ attribute of each class gives us the set of allowed keys and the type of each corresponding value, but we have a problem:

    • C has __total__==True, but b is not actually required.
    • B has __total__==False, but a *is* required.
    • I can't see any way to get the parent classes of a TypedDict class!

    The _TypedDictMeta metaclass updates the attributes, but leaves no record of the parent type - at runtime A, B, and C all appear to inherit directly from dict.

    After discussion on the typing-sig mailing list, I propose to add __required_keys__ and __optional_keys__ attributes to TypedDict subclasses, as frozensets of strings.

    This will be very useful for Hypothesis' from_type() strategy, as well as for type-based validation frameworks like pydantic or typeguard.

    @Zac-HD Zac-HD mannequin added 3.8 only security fixes stdlib Python modules in the Lib dir labels Nov 18, 2019
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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