Type variable substitution in type instances

[skrah]

BPO	25087
Nosy	@gvanrossum, @skrah, @isidentical

^{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 2019-12-29.05:16:12.301>
created_at = <Date 2015-09-13.13:52:18.388>
labels = ['interpreter-core', 'type-bug']
title = 'Type variable substitution in type instances'
updated_at = <Date 2019-12-29.05:16:12.300>
user = 'https://github.com/skrah'

bugs.python.org fields:

activity = <Date 2019-12-29.05:16:12.300>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2019-12-29.05:16:12.301>
closer = 'gvanrossum'
components = ['Interpreter Core']
creation = <Date 2015-09-13.13:52:18.388>
creator = 'skrah'
dependencies = []
files = []
hgrepos = []
issue_num = 25087
keywords = []
message_count = 4.0
messages = ['250565', '250580', '358957', '358979']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'skrah', 'BTaskaya']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25087'
versions = ['Python 3.5']

[skrah]

If a type scheme is instantiated, should the type variables in the
class body be substituted? This is an example (typed by hand on
a locked down Windows machine, may contain errors):

alpha = TypeVar('alpha')
beta = TypeVar('beta')

class ABTuple(Generic[alpha, beta]):
    def __init__(self, a : alpha, b : beta):
        self.value = (a, b)

get_type_hints(ABTuple.__init__)
 ==> {'b': ~beta, 'a': ~alpha}


IntIntTuple = ABTuple[int, int]

IntIntTuple
==> __main__.ABTuple[int, int]

get_type_hints(IntIntTuple.__init__)
{'b': ~beta, 'a': ~alpha}
      ^^^^^^      ^^^^^^

Since the type has been specialized, these should ideally be 'int'.

[gvanrossum]

Good question. Let's discuss this in the type hinting / PEP-484 tracker:

python/typing#156

[isidentical]

It looks like the issue in typing tracker closed, can we also close this?

[gvanrossum]

Yup, see comment in python/typing#308 regarding #156.