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BoundArguments facility to inject defaults #68378

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pitrou opened this issue May 14, 2015 · 10 comments
Closed

BoundArguments facility to inject defaults #68378

pitrou opened this issue May 14, 2015 · 10 comments
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stdlib Python modules in the Lib dir type-feature A feature request or enhancement

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@pitrou
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pitrou commented May 14, 2015

BPO 24190
Nosy @brettcannon, @pitrou, @larryhastings, @bitdancer, @1st1
Files
  • sig_ba_default.patch
  • 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 = 'https://github.com/1st1'
    closed_at = <Date 2015-05-16.17:45:53.125>
    created_at = <Date 2015-05-14.10:20:00.056>
    labels = ['type-feature', 'library']
    title = 'BoundArguments facility to inject defaults'
    updated_at = <Date 2015-05-16.17:45:53.124>
    user = 'https://github.com/pitrou'

    bugs.python.org fields:

    activity = <Date 2015-05-16.17:45:53.124>
    actor = 'yselivanov'
    assignee = 'yselivanov'
    closed = True
    closed_date = <Date 2015-05-16.17:45:53.125>
    closer = 'yselivanov'
    components = ['Library (Lib)']
    creation = <Date 2015-05-14.10:20:00.056>
    creator = 'pitrou'
    dependencies = []
    files = ['39372']
    hgrepos = []
    issue_num = 24190
    keywords = ['patch']
    message_count = 10.0
    messages = ['243167', '243170', '243177', '243200', '243202', '243204', '243205', '243210', '243341', '243342']
    nosy_count = 6.0
    nosy_names = ['brett.cannon', 'pitrou', 'larry', 'r.david.murray', 'python-dev', 'yselivanov']
    pr_nums = []
    priority = 'normal'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = 'enhancement'
    url = 'https://bugs.python.org/issue24190'
    versions = ['Python 3.5']

    @pitrou
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    pitrou commented May 14, 2015

    The recipe to inject default values in a BoundArguments instance is given in the doc, but it's not trivial. Furthermore, it's actually incomplete: it doesn't handle any star-arguments, e.g.:

    >>> sig = inspect.signature(f)
    >>> ba = sig.bind(2, d=4)
    >>> for param in sig.parameters.values():
    ...     if (param.name not in ba.arguments
    ...             and param.default is not param.empty):
    ...         ba.arguments[param.name] = param.default
    ... 
    >>> ba.arguments
    OrderedDict([('a', 2), ('d', 4), ('b', 5)])

    ba['c'] would ideally contain the empty tuple.

    @pitrou pitrou added stdlib Python modules in the Lib dir type-feature A feature request or enhancement labels May 14, 2015
    @pitrou
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    pitrou commented May 14, 2015

    My example forgets the function declaration, which is:

    >>> def f(a, b=5, *c, d=5): pass
    ...

    @bitdancer
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    bitdancer commented May 14, 2015

    See bpo-22998. The more complete and thus more complex example in the last message makes it look like including this in the library might be a good idea.

    @1st1
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    1st1 commented May 14, 2015

    Well, the docs example only binds explicit defaults in function signature. Implicit defaults for *args and **kwargs (() and {}) aren't usually useful (in my opinion).

    Do you guys have any good use case for such method? I use the Signature API extensively for argument types validation and for serialization of RPC calls, but I never needed this functionality from BoundArguments.

    @pitrou
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    pitrou commented May 14, 2015

    Le 14/05/2015 17:45, Yury Selivanov a écrit :

    Well, the docs example only binds explicit defaults in function
    signature. Implicit defaults for *args and **kwargs (() and {})
    aren't usually useful (in my opinion).

    When the defaults are filled I expect ba.arguments to be "complete",
    that is have a value for every signature parameter. Otherwise I have to
    special-case *args and **kwargs.

    Do you guys have any good use case for such method?

    A use case was given in bpo-22998.
    My use case is JIT-compiling functions and function calls in Numba. We
    reimplement the function calls ourselves, so need a complete mapping of
    arguments to values.

    @1st1
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    1st1 commented May 14, 2015

    > Do you guys have any good use case for such method?

    A use case was given in bpo-22998.
    My use case is JIT-compiling functions and function calls in Numba. We
    reimplement the function calls ourselves, so need a complete mapping of
    arguments to values.

    This is a great use case ;-) Let's add it.

    I propose the following method: BoundArguments.apply_defaults()

    It will iterate through its parent Signature's parameters and assign default values to BoundArguments.arguments (when an arg is missing), including setting '()' for *args, and '{}' for **kwargs.

    If you're OK with this, I can draft a patch.

    @pitrou
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    pitrou commented May 14, 2015

    That sounds good to me, thank you!

    @1st1
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    1st1 commented May 14, 2015

    FWIW it wasn't as easy as I thought it would be :) You were right, docs example is very basic.

    Please take a look at the attached patch.

    @1st1 1st1 self-assigned this May 14, 2015
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    python-dev mannequin commented May 16, 2015

    New changeset ea61d8eb8a28 by Yury Selivanov in branch 'default':
    bpo-24190: Add inspect.BoundArguments.apply_defaults() method.
    https://hg.python.org/cpython/rev/ea61d8eb8a28

    @1st1
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    1st1 commented May 16, 2015

    Thanks for the suggestion, Antoine!

    @1st1 1st1 closed this as completed May 16, 2015
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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