hgdemandimport/demandimportpy3.py

# 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.
"""

import contextlib
import importlib.util
import sys

from . import tracing

_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."""
        with tracing.log('demandimport %s', module):
            if _deactivated or module.__name__ in ignores:
                self.loader.exec_module(module)
            else:
                super().exec_module(module)


class LazyFinder:
    """A wrapper around a ``MetaPathFinder`` that makes loaders lazy.

    ``sys.meta_path`` finders have their ``find_spec()`` called to locate a
    module. This returns a ``ModuleSpec`` if found or ``None``. The
    ``ModuleSpec`` has a ``loader`` attribute, which is called to actually
    load a module.

    Our class wraps an existing finder and overloads its ``find_spec()`` to
    replace the ``loader`` with our lazy loader proxy.

    We have to use __getattribute__ to proxy the instance because some meta
    path finders don't support monkeypatching.
    """

    __slots__ = ("_finder",)

    def __init__(self, finder):
        object.__setattr__(self, "_finder", finder)

    def __repr__(self):
        return "<LazyFinder for %r>" % object.__getattribute__(self, "_finder")

    # __bool__ is canonical Python 3. But check-code insists on __nonzero__ being
    # defined via `def`.
    def __nonzero__(self):
        return bool(object.__getattribute__(self, "_finder"))

    __bool__ = __nonzero__

    def __getattribute__(self, name):
        if name in ("_finder", "find_spec"):
            return object.__getattribute__(self, name)

        return getattr(object.__getattribute__(self, "_finder"), name)

    def __delattr__(self, name):
        return delattr(object.__getattribute__(self, "_finder"))

    def __setattr__(self, name, value):
        return setattr(object.__getattribute__(self, "_finder"), name, value)

    def find_spec(self, fullname, path, target=None):
        finder = object.__getattribute__(self, "_finder")
        try:
            find_spec = finder.find_spec
        except AttributeError:
            loader = finder.find_module(fullname, path)
            if loader is None:
                spec = None
            else:
                spec = importlib.util.spec_from_loader(fullname, loader)
        else:
            spec = find_spec(fullname, path, target)

        # Lazy loader requires exec_module().
        if (
            spec is not None
            and spec.loader is not None
            and getattr(spec.loader, "exec_module", None)
        ):
            spec.loader = _lazyloaderex(spec.loader)

        return spec


ignores = set()


def init(ignoreset):
    global ignores
    ignores = ignoreset


def isenabled():
    return not _deactivated and any(
        isinstance(finder, LazyFinder) for finder in sys.meta_path
    )


def disable():
    new_finders = []
    for finder in sys.meta_path:
        new_finders.append(
            finder._finder if isinstance(finder, LazyFinder) else finder
        )
    sys.meta_path[:] = new_finders


def enable():
    new_finders = []
    for finder in sys.meta_path:
        new_finders.append(
            LazyFinder(finder) if not isinstance(finder, LazyFinder) else finder
        )
    sys.meta_path[:] = new_finders


@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