Don't import linecache at warnings toplevel

[pitrou]

BPO	19379
Nosy	@brettcannon, @pitrou, @tiran
Files	warnings_lazy.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 = None
closed_at = <Date 2013-10-24.20:24:49.305>
created_at = <Date 2013-10-24.20:09:10.495>
labels = ['library', 'performance']
title = "Don't import linecache at warnings toplevel"
updated_at = <Date 2013-10-24.20:24:49.303>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2013-10-24.20:24:49.303>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2013-10-24.20:24:49.305>
closer = 'python-dev'
components = ['Library (Lib)']
creation = <Date 2013-10-24.20:09:10.495>
creator = 'pitrou'
dependencies = []
files = ['32340']
hgrepos = []
issue_num = 19379
keywords = ['patch']
message_count = 4.0
messages = ['201174', '201178', '201179', '201182']
nosy_count = 4.0
nosy_names = ['brett.cannon', 'pitrou', 'christian.heimes', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue19379'
versions = ['Python 3.4']

[pitrou]

Importing warnings pulls linecache at the top-level, while it's only necessary when printing warnings. The reason why it wasn't done lazily is obsolete, since Python now has per-module import locks. Attached patch makes the linecache imports lazy.

This is desirable for issue bpo-19375.

[tiran]

LGTM

Good catch! :)

[brettcannon]

LGTM

[python-dev]

New changeset 8939c0196990 by Antoine Pitrou in branch 'default':
Close bpo-19379: Lazily import linecache in the warnings module, to make startup with warnings faster until a warning gets printed.
http://hg.python.org/cpython/rev/8939c0196990