# demandimportpy3 - global demand-loading of modules for Mercurial

#

# Copyright 2017 Facebook Inc.

#

# This software may be used and distributed according to the terms of the

# GNU General Public License version 2 or any later version.

"""Lazy loading for Python 3.6 and above.

This uses the new importlib finder/loader functionality available in Python 3.5

and up. The code reuses most of the mechanics implemented inside importlib.util,

but with a few additions:

* Allow excluding certain modules from lazy imports.

* Expose an interface that's substantially the same as demandimport for

  Python 2.

This also has some limitations compared to the Python 2 implementation:

* Much of the logic is per-package, not per-module, so any packages loaded

  before demandimport is enabled will not be lazily imported in the future. In

  practice, we only expect builtins to be loaded before demandimport is

  enabled.

"""

# This line is unnecessary, but it satisfies test-check-py3-compat.t.

from __future__ import absolute_import

import contextlib

import importlib.abc

import importlib.machinery

import importlib.util

import sys

_deactivated = False

class _lazyloaderex(importlib.util.LazyLoader):

    """This is a LazyLoader except it also follows the _deactivated global and

    the ignore list.

    """

    def exec_module(self, module):

        """Make the module load lazily."""

        if _deactivated or module.__name__ in ignore:

            self.loader.exec_module(module)

        else:

            super().exec_module(module)

# This is 3.6+ because with Python 3.5 it isn't possible to lazily load

# extensions. See the discussion in https://bugs.python.org/issue26186 for more.

_extensions_loader = _lazyloaderex.factory(

    importlib.machinery.ExtensionFileLoader)

_bytecode_loader = _lazyloaderex.factory(

    importlib.machinery.SourcelessFileLoader)

_source_loader = _lazyloaderex.factory(importlib.machinery.SourceFileLoader)

def _makefinder(path):

    return importlib.machinery.FileFinder(

        path,

        # This is the order in which loaders are passed in in core Python.

        (_extensions_loader, importlib.machinery.EXTENSION_SUFFIXES),

        (_source_loader, importlib.machinery.SOURCE_SUFFIXES),

        (_bytecode_loader, importlib.machinery.BYTECODE_SUFFIXES),

    )

ignore = []

def init(ignorelist):

    global ignore

    ignore = ignorelist

def isenabled():

    return _makefinder in sys.path_hooks and not _deactivated

def disable():

    try:

        while True:

            sys.path_hooks.remove(_makefinder)

    except ValueError:

        pass

def enable():

    sys.path_hooks.insert(0, _makefinder)

@contextlib.contextmanager

def deactivated():

    # This implementation is a bit different from Python 2's. Python 3

    # maintains a per-package finder cache in sys.path_importer_cache (see

    # PEP 302). This means that we can't just call disable + enable.

    # If we do that, in situations like:

    #

    #   demandimport.enable()

    #   ...

    #   from foo.bar import mod1

    #   with demandimport.deactivated():

    #       from foo.bar import mod2

    #

    # mod2 will be imported lazily. (The converse also holds -- whatever finder

    # first gets cached will be used.)

    #

    # Instead, have a global flag the LazyLoader can use.

    global _deactivated

    demandenabled = isenabled()

    if demandenabled:

        _deactivated = True

    try:

        yield

    finally:

        if demandenabled:

            _deactivated = False