Argument Clinic: broken support for 'O!'

[serhiy-storchaka]

BPO	20141
Nosy	@larryhastings, @serhiy-storchaka
Files	spammodule.c: An example larry.argument.clinic.o-bang.rethink.diff.1.txt

^{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/larryhastings'
closed_at = <Date 2014-01-07.20:18:45.031>
created_at = <Date 2014-01-06.13:12:14.194>
labels = ['type-bug', 'build', 'docs']
title = "Argument Clinic: broken support for 'O!'"
updated_at = <Date 2014-01-07.20:18:45.030>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2014-01-07.20:18:45.030>
actor = 'larry'
assignee = 'larry'
closed = True
closed_date = <Date 2014-01-07.20:18:45.031>
closer = 'larry'
components = ['Build', 'Documentation']
creation = <Date 2014-01-06.13:12:14.194>
creator = 'serhiy.storchaka'
dependencies = []
files = ['33325', '33341']
hgrepos = []
issue_num = 20141
keywords = []
message_count = 9.0
messages = ['207430', '207431', '207432', '207455', '207532', '207538', '207546', '207587', '207588']
nosy_count = 3.0
nosy_names = ['larry', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20141'
versions = ['Python 3.4']

[serhiy-storchaka]

First, the documentation says that syntax for 'O!' conversion is
object(type='name_of_Python_type'), but actually the type argument should be the name of C structure which represents Python type. I.e. object(type='list') is rejected, and object(type='PyList_Type') is accepted.

Second, from this declaration

/*[clinic]
module spam
spam.ham

items: object(type='PyList_Type')
/

[clinic]*/

Argument Clinic generates following parsing code:

 if (!PyArg_ParseTuple(args,
 "O!:ham",
 PyList_Type, &items))
 goto exit;

It is wrong, should be &PyList_Type.

[larryhastings]

So a documentation error and having to manually specify "&" at the front of your string means it's "broken"?

Nevertheless, I'll take a look at it.

[serhiy-storchaka]

You can't manually specify "&" at the front of the name.

[larryhastings]

Actually, the documentation (the "howto") states:

Note that object() must explicitly support each Python type you specify for the type argument. Currently it only supports str. It should be easy to add more, just edit Tools/clinic/clinic.py, search for O! in the text, and add more entries to the dict mapping types to strings just above it.

Maybe this is a bad API. But it's not broken.

[larryhastings]

Attached is a new, simpler approach for supporting O!. The object() converter now takes two arguments:

• type, which is the type you want the parameter declared as
  (e.g. "PyUnicodeObject *")
• subclass_of, which is the PyTypeObject you want to enforce the
  value is an instance of (e.g. "&PyUnicode_Type")
  The old approach was kind of a lovely idea, but was too complicated, and it would have meant registering any new type (like third-party types). This is less convenient, but simple. And it would lend itself well to making a custom converter if you used it a lot.

I'll write some documentation for it now, but I wanted to post the code so I could get a review.

p.s. Why was is marked "crash" and "release blocker"? That's very inaccurate. And assigned to docs? WTH?