Mercurial > hg
annotate mercurial/policy.py @ 29375:fcaf20175b1b
demandimport: delay loading for "from a import b" with absolute_import
Before this patch, "from a import b" doesn't delay loading module "b",
if absolute_import is enabled, even though "from . import b" does.
For example:
- it is assumed that extension X has "from P import M" for module M
under package P with absolute_import feature
- if importing module M is already delayed before loading extension
X, loading module M in extension X is delayed until actually
referring
util, cmdutil, scmutil or so of Mercurial itself should be
imported by "from . import M" style before loading extension X
- otherwise, module M is loaded immediately at loading extension X,
even if extension X itself isn't used at that "hg" command invocation
Some minor modules (e.g. filemerge or so) of Mercurial itself
aren't imported by "from . import M" style before loading
extension X. And of course, external libraries aren't, too.
This might cause startup performance problem of hg command, because
many bundled extensions already enable absolute_import feature.
To delay loading module for "from a import b" with absolute_import
feature, this patch does below in "from a (or .a) import b" with
absolute_import case:
1. import root module of "name" by system built-in __import__
(referred as _origimport)
2. recurse down the module chain for hierarchical "name"
This logic can be shared with non absolute_import
case. Therefore, this patch also centralizes it into chainmodules().
3. and fall through to process elements in "fromlist" for the leaf
module of "name"
Processing elements in "fromlist" is executed in the code path
after "if _pypy: .... else: ..." clause. Therefore, this patch
replaces "if _pypy:" with "elif _pypy:" to share it.
At 4f1144c3c72b introducing original "work around" for "from a import
b" case, elements in "fromlist" were imported with "level=level". But
"level" might be grater than 1 (e.g. level=2 in "from .. import b"
case) at demandimport() invocation, and importing direct sub-module in
"fromlist" with level grater than 1 causes unexpected result.
IMHO, this seems main reason of "errors for unknown reason" described
in 4f1144c3c72b, and we don't have to worry about it, because this
issue was already fixed by 78d05778907b.
This is reason why this patch removes "errors for unknown reasons"
comment.
author | FUJIWARA Katsunori <foozy@lares.dti.ne.jp> |
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date | Sun, 19 Jun 2016 02:17:33 +0900 |
parents | b3a677c82a35 |
children | b4d117cee636 |
rev | line source |
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1 # policy.py - module policy logic for Mercurial. |
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2 # |
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3 # Copyright 2015 Gregory Szorc <gregory.szorc@gmail.com> |
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4 # |
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5 # This software may be used and distributed according to the terms of the |
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6 # GNU General Public License version 2 or any later version. |
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7 |
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8 from __future__ import absolute_import |
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9 |
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10 import os |
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11 import sys |
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12 |
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13 # Rules for how modules can be loaded. Values are: |
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14 # |
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15 # c - require C extensions |
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16 # allow - allow pure Python implementation when C loading fails |
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17 # py - only load pure Python modules |
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18 # |
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19 # By default, require the C extensions for performance reasons. |
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20 policy = 'c' |
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21 try: |
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22 from . import __modulepolicy__ |
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23 policy = __modulepolicy__.modulepolicy |
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24 except ImportError: |
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25 pass |
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26 |
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27 # PyPy doesn't load C extensions. |
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28 # |
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29 # The canonical way to do this is to test platform.python_implementation(). |
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30 # But we don't import platform and don't bloat for it here. |
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31 if '__pypy__' in sys.builtin_module_names: |
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32 policy = 'py' |
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33 |
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34 # Our C extensions aren't yet compatible with Python 3. So use pure Python |
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35 # on Python 3 for now. |
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36 if sys.version_info[0] >= 3: |
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37 policy = 'py' |
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38 |
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39 # Environment variable can always force settings. |
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40 policy = os.environ.get('HGMODULEPOLICY', policy) |