Argument Clinic: add ability to specify an existing impl function

[zware]

BPO	22096
Nosy	@loewis, @larryhastings, @zware

^{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 2014-08-01.14:25:28.332>
created_at = <Date 2014-07-28.21:02:31.346>
labels = ['type-feature']
title = 'Argument Clinic: add ability to specify an existing impl function'
updated_at = <Date 2014-08-01.14:25:28.331>
user = 'https://github.com/zware'

bugs.python.org fields:

activity = <Date 2014-08-01.14:25:28.331>
actor = 'zach.ware'
assignee = 'none'
closed = True
closed_date = <Date 2014-08-01.14:25:28.332>
closer = 'zach.ware'
components = ['Demos and Tools']
creation = <Date 2014-07-28.21:02:31.346>
creator = 'zach.ware'
dependencies = []
files = []
hgrepos = []
issue_num = 22096
keywords = []
message_count = 4.0
messages = ['224188', '224218', '224221', '224505']
nosy_count = 3.0
nosy_names = ['loewis', 'larry', 'zach.ware']
pr_nums = []
priority = 'low'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue22096'
versions = ['Python 3.4', 'Python 3.5']

[zware]

It would be nice to be able to specify an existing function as the _impl function, for cases like winreg.OpenKey and OpenKeyEx which are the same function by different names, or _winapi.GetCurrentProcess, whose impl function is "return GetCurrentProcess()".

This is pretty minor, but would allow for some shorter and cleaner code. Unfortunately, I don't have any great ideas for syntax for this, so it may be rejected just on that basis.

[loewis]

I don't think it it is worth the effort. You would need two different entry functions still, to allow error messages to refer to the function name. Since the _impl functions can trivially call each other, the saving in lines of code are minimal, and from a code reading point of view, having aliases actually complicates the code.

[larryhastings]

I'm with Martin. It would be a funny exception, where the reader'd get confused... why is the impl missing?

Also, any halfway-decent optimizer will inline the impl into the parsing function, so this has no runtime cost.

[zware]

Fair enough.