Clean up some checks in dict implementation

[zhangyangyu]

BPO	28194
Nosy	@vstinner, @methane, @serhiy-storchaka, @zhangyangyu
Files	dict_clean_up.patch dict_clean_up_v2.patch

^{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/serhiy-storchaka'
closed_at = <Date 2016-09-26.18:31:36.753>
created_at = <Date 2016-09-18.09:12:17.100>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'Clean up some checks in dict implementation'
updated_at = <Date 2016-09-26.18:31:36.752>
user = 'https://github.com/zhangyangyu'

bugs.python.org fields:

activity = <Date 2016-09-26.18:31:36.752>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-09-26.18:31:36.753>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2016-09-18.09:12:17.100>
creator = 'xiang.zhang'
dependencies = []
files = ['44734', '44738']
hgrepos = []
issue_num = 28194
keywords = ['patch']
message_count = 4.0
messages = ['276875', '276892', '276894', '277444']
nosy_count = 5.0
nosy_names = ['vstinner', 'methane', 'python-dev', 'serhiy.storchaka', 'xiang.zhang']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28194'
versions = ['Python 3.6', 'Python 3.7']

[zhangyangyu]

The proposed patch cleans up some unnecessary parts in dict implementation especially NULL checks in lookup functions. There are four states a DictKeyEntry can be. Only in unused(empty) and dummy states me_key can be NULL. So NULL checks in used and pending states are not needed.

[serhiy-storchaka]

LGTM if address methane's comment.

[zhangyangyu]

Apply methane's comment, preserving the comment.

[python-dev]

New changeset 1d41d741bb5b by Serhiy Storchaka in branch '3.6':
Issue bpo-28194: Clean up some checks in dict implementation.
https://hg.python.org/cpython/rev/1d41d741bb5b

New changeset b83a70afca39 by Serhiy Storchaka in branch 'default':
Issue bpo-28194: Clean up some checks in dict implementation.
https://hg.python.org/cpython/rev/b83a70afca39