When function in tracing returns None, tracing continues.

[fabioz]

BPO	34799
Nosy	@fabioz
Files	issue_in_tracing_func.py: Test case showing error.

^{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 = None
created_at = <Date 2018-09-25.15:06:20.105>
labels = ['type-bug', '3.7']
title = 'When function in tracing returns None, tracing continues.'
updated_at = <Date 2018-09-25.15:06:20.105>
user = 'https://github.com/fabioz'

bugs.python.org fields:

activity = <Date 2018-09-25.15:06:20.105>
actor = 'fabioz'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2018-09-25.15:06:20.105>
creator = 'fabioz'
dependencies = []
files = ['47822']
hgrepos = []
issue_num = 34799
keywords = []
message_count = 1.0
messages = ['326360']
nosy_count = 1.0
nosy_names = ['fabioz']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34799'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7']

[fabioz]

https://docs.python.org/3/library/sys.html#sys.settrace explicitly states:

The local trace function should return a reference to itself (or to another function for further tracing in that scope), or None to turn off tracing in that scope.

Yet, it seems this happens only on the return of a 'call'. If None is returned in a 'line' event, apparently the previous tracing function is reused (but if a new function is returned, the new function is used properly).

I'm attaching a test case which shows the issue. I've tested on 2.7, 3.6 and 3.7 and this issue is present on all.

If I set frame.f_trace = None before returning it seems to work though (so, I think that either this behavior should be fixed or the docs should be updated to reflect that).

[brandtbucher]

Fixed the docs in d9246c7.