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vars() manipulation encounters problems with Enum #75982

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ethanfurman opened this issue Oct 17, 2017 · 3 comments
Closed

vars() manipulation encounters problems with Enum #75982

ethanfurman opened this issue Oct 17, 2017 · 3 comments
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3.7 type-bug

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@ethanfurman
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@ethanfurman ethanfurman commented Oct 17, 2017

BPO 31801
Nosy @warsaw, @ethanfurman, @eric-wieser
PRs
  • #5237
  • 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/ethanfurman'
    closed_at = <Date 2018-09-10.18:30:27.608>
    created_at = <Date 2017-10-17.00:21:55.188>
    labels = ['type-bug', '3.7']
    title = 'vars() manipulation encounters problems with Enum'
    updated_at = <Date 2018-09-10.18:30:27.607>
    user = 'https://github.com/ethanfurman'

    bugs.python.org fields:

    activity = <Date 2018-09-10.18:30:27.607>
    actor = 'ethan.furman'
    assignee = 'ethan.furman'
    closed = True
    closed_date = <Date 2018-09-10.18:30:27.608>
    closer = 'ethan.furman'
    components = []
    creation = <Date 2017-10-17.00:21:55.188>
    creator = 'ethan.furman'
    dependencies = []
    files = []
    hgrepos = []
    issue_num = 31801
    keywords = ['patch']
    message_count = 3.0
    messages = ['304487', '306257', '310425']
    nosy_count = 4.0
    nosy_names = ['barry', 'eli.bendersky', 'ethan.furman', 'Eric.Wieser']
    pr_nums = ['5237']
    priority = 'normal'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = 'behavior'
    url = 'https://bugs.python.org/issue31801'
    versions = ['Python 3.7']

    @ethanfurman
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    @ethanfurman ethanfurman commented Oct 17, 2017

    The following code should work (from https://stackoverflow.com/a/46780742/208880):

        class BaudRate(Enum):
    
            cls = vars()
            regexp = r"(?:^|,)B(?P<rate>\d+)"
            rates = sorted(map(int, re.findall(regexp, ",".join(dir(termios)))))
            for value in rates:
                cls['B%d' % value] = value
    
            @classmethod
            def valid_rate(cls, value):
                return (any(value == item.value for item in cls))

    This doesn't work because EnumMeta does not allow names to be reassigned nor deleted. aenum gets around this by allowing an _ignore_ attribute which contains the names that should not be tracked (and, in fact, those names are removed from the final class).

    Unless a better idea is put forward, I will add _ignore_ support to Enum.

    @ethanfurman ethanfurman self-assigned this Oct 17, 2017
    @ethanfurman ethanfurman added the type-bug label Oct 17, 2017
    @eric-wieser
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    @eric-wieser eric-wieser mannequin commented Nov 15, 2017

    Not necessarily an argument against this feature, but two workarounds exist for this already:

    1. Use nonlocal to prevent value going into the class namespace:
        value = None
        
        class BaudRate(enum.Enum):    
            nonlocal value
            for value in rates:
                locals()['B%d' % value] = value
        
            @classmethod
            def valid_rate(cls, value):
                return (any(value == item.value for item in cls))
    1. Use types.new_class, which is more suited to dynamic class creation anyway:
        def make_cls(ns):
            for value in rates:
                ns['B%d' % value] = value
    
            @classmethod
            def valid_rate(cls, value):
                return (any(value == item.value for item in cls))
        ns['valid_rate'] = valid_rate
    
    types.new_class('BaudRate', (enum.Enum,), exec_body=make_cls)
    

    @ethanfurman
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    @ethanfurman ethanfurman commented Jan 22, 2018

    New changeset a4b1bb4 by Ethan Furman in branch 'master':
    bpo-31801: Enum: add _ignore_ as class option (bpo-5237)
    a4b1bb4

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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