mercurial/policy.py

demandimport: disable lazy import of __builtin__

Demandimport uses the "try to import __builtin__, else use builtins" trick to
handle Python 3. External libraries and extensions might do something similar.
On Fedora 25 subversion-python-1.9.4-4.fc25.x86_64 will do just that (except
the opposite) ... and it failed all subversion convert tests because
demandimport was hiding that it didn't have builtins but should use
__builtin__.

The builtin module has already been imported when demandimport is loaded so
there is no point in trying to import it on demand. Just always ignore both
variants in demandimport.

# policy.py - module policy logic for Mercurial.
#
# Copyright 2015 Gregory Szorc <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

from __future__ import absolute_import

import os
import sys

# Rules for how modules can be loaded. Values are:
#
#    c - require C extensions
#    allow - allow pure Python implementation when C loading fails
#    cffi - required cffi versions (implemented within pure module)
#    cffi-allow - allow pure Python implementation if cffi version is missing
#    py - only load pure Python modules
#
# By default, require the C extensions for performance reasons.
policy = 'c'
policynoc = ('cffi', 'cffi-allow', 'py')
policynocffi = ('c', 'py')

try:
    from . import __modulepolicy__
    policy = __modulepolicy__.modulepolicy
except ImportError:
    pass

# PyPy doesn't load C extensions.
#
# The canonical way to do this is to test platform.python_implementation().
# But we don't import platform and don't bloat for it here.
if '__pypy__' in sys.builtin_module_names:
    policy = 'cffi'

# Our C extensions aren't yet compatible with Python 3. So use pure Python
# on Python 3 for now.
if sys.version_info[0] >= 3:
    policy = 'py'

# Environment variable can always force settings.
policy = os.environ.get('HGMODULEPOLICY', policy)