Generate correct error check for PyFloat_AsDouble

[rhettinger]

BPO	37942
Nosy	@rhettinger, @larryhastings, @serhiy-storchaka
PRs	bpo-37942: Improve argument clinic float converter #15470 [3.8] bpo-37942: Improve argument clinic float converter (GH-15470) #15480

^{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 2019-08-25.02:45:37.328>
created_at = <Date 2019-08-24.19:09:51.270>
labels = ['type-bug', '3.8', 'expert-argument-clinic', '3.9']
title = 'Generate correct error check for PyFloat_AsDouble'
updated_at = <Date 2019-08-25.09:41:06.925>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2019-08-25.09:41:06.925>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2019-08-25.02:45:37.328>
closer = 'rhettinger'
components = ['Argument Clinic']
creation = <Date 2019-08-24.19:09:51.270>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 37942
keywords = ['patch']
message_count = 5.0
messages = ['350395', '350397', '350419', '350422', '350443']
nosy_count = 3.0
nosy_names = ['rhettinger', 'larry', 'serhiy.storchaka']
pr_nums = ['15470', '15480']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37942'
versions = ['Python 3.8', 'Python 3.9']

[rhettinger]

The API for PyFloat_AsDouble() returns -1.0 to indicate an error. PyErr_Occurred() should only be called if there is a -1.0 return code. This is the normal practice for those calls and it is a bit faster because it avoids unnecessary external call.

[rhettinger]

Note, the argument clinic is already generating code like this for return values in "class double_return_converter". For example,

    _return_value = _statistics__normal_dist_inv_cdf_impl(module, p, mu, sigma);
    if ((_return_value == -1.0) && PyErr_Occurred()) {
	goto exit;
    }

[rhettinger]

New changeset aef9ad8 by Raymond Hettinger in branch 'master':
bpo-37942: Improve argument clinic float converter (GH-15470)
aef9ad8

[rhettinger]

New changeset 21161d7 by Raymond Hettinger in branch '3.8':
[3.8] bpo-37942: Improve argument clinic float converter (GH-15470) (GH-15480)
21161d7

[serhiy-storchaka]

How large is benefit from special casing exact floats?