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Filename : util.py
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"""Utility code for constructing importers, etc."""
from . import abc
from ._bootstrap import module_from_spec
from ._bootstrap import _resolve_name
from ._bootstrap import spec_from_loader
from ._bootstrap import _find_spec
from ._bootstrap_external import MAGIC_NUMBER
from ._bootstrap_external import cache_from_source
from ._bootstrap_external import decode_source
from ._bootstrap_external import source_from_cache
from ._bootstrap_external import spec_from_file_location

from contextlib import contextmanager
import functools
import sys
import types
import warnings


def resolve_name(name, package):
    """Resolve a relative module name to an absolute one."""
    if not name.startswith('.'):
        return name
    elif not package:
        raise ValueError('{!r} is not a relative name '
                         '(no leading dot)'.format(name))
    level = 0
    for character in name:
        if character != '.':
            break
        level += 1
    return _resolve_name(name[level:], package, level)


def _find_spec_from_path(name, path=None):
    """Return the spec for the specified module.

    First, sys.modules is checked to see if the module was already imported. If
    so, then sys.modules[name].__spec__ is returned. If that happens to be
    set to None, then ValueError is raised. If the module is not in
    sys.modules, then sys.meta_path is searched for a suitable spec with the
    value of 'path' given to the finders. None is returned if no spec could
    be found.

    Dotted names do not have their parent packages implicitly imported. You will
    most likely need to explicitly import all parent packages in the proper
    order for a submodule to get the correct spec.

    """
    if name not in sys.modules:
        return _find_spec(name, path)
    else:
        module = sys.modules[name]
        if module is None:
            return None
        try:
            spec = module.__spec__
        except AttributeError:
            raise ValueError('{}.__spec__ is not set'.format(name)) from None
        else:
            if spec is None:
                raise ValueError('{}.__spec__ is None'.format(name))
            return spec


def find_spec(name, package=None):
    """Return the spec for the specified module.

    First, sys.modules is checked to see if the module was already imported. If
    so, then sys.modules[name].__spec__ is returned. If that happens to be
    set to None, then ValueError is raised. If the module is not in
    sys.modules, then sys.meta_path is searched for a suitable spec with the
    value of 'path' given to the finders. None is returned if no spec could
    be found.

    If the name is for submodule (contains a dot), the parent module is
    automatically imported.

    The name and package arguments work the same as importlib.import_module().
    In other words, relative module names (with leading dots) work.

    """
    fullname = resolve_name(name, package) if name.startswith('.') else name
    if fullname not in sys.modules:
        parent_name = fullname.rpartition('.')[0]
        if parent_name:
            # Use builtins.__import__() in case someone replaced it.
            parent = __import__(parent_name, fromlist=['__path__'])
            return _find_spec(fullname, parent.__path__)
        else:
            return _find_spec(fullname, None)
    else:
        module = sys.modules[fullname]
        if module is None:
            return None
        try:
            spec = module.__spec__
        except AttributeError:
            raise ValueError('{}.__spec__ is not set'.format(name)) from None
        else:
            if spec is None:
                raise ValueError('{}.__spec__ is None'.format(name))
            return spec


@contextmanager
def _module_to_load(name):
    is_reload = name in sys.modules

    module = sys.modules.get(name)
    if not is_reload:
        # This must be done before open() is called as the 'io' module
        # implicitly imports 'locale' and would otherwise trigger an
        # infinite loop.
        module = type(sys)(name)
        # This must be done before putting the module in sys.modules
        # (otherwise an optimization shortcut in import.c becomes wrong)
        module.__initializing__ = True
        sys.modules[name] = module
    try:
        yield module
    except Exception:
        if not is_reload:
            try:
                del sys.modules[name]
            except KeyError:
                pass
    finally:
        module.__initializing__ = False


def set_package(fxn):
    """Set __package__ on the returned module.

    This function is deprecated.

    """
    @functools.wraps(fxn)
    def set_package_wrapper(*args, **kwargs):
        warnings.warn('The import system now takes care of this automatically.',
                      DeprecationWarning, stacklevel=2)
        module = fxn(*args, **kwargs)
        if getattr(module, '__package__', None) is None:
            module.__package__ = module.__name__
            if not hasattr(module, '__path__'):
                module.__package__ = module.__package__.rpartition('.')[0]
        return module
    return set_package_wrapper


def set_loader(fxn):
    """Set __loader__ on the returned module.

    This function is deprecated.

    """
    @functools.wraps(fxn)
    def set_loader_wrapper(self, *args, **kwargs):
        warnings.warn('The import system now takes care of this automatically.',
                      DeprecationWarning, stacklevel=2)
        module = fxn(self, *args, **kwargs)
        if getattr(module, '__loader__', None) is None:
            module.__loader__ = self
        return module
    return set_loader_wrapper


def module_for_loader(fxn):
    """Decorator to handle selecting the proper module for loaders.

    The decorated function is passed the module to use instead of the module
    name. The module passed in to the function is either from sys.modules if
    it already exists or is a new module. If the module is new, then __name__
    is set the first argument to the method, __loader__ is set to self, and
    __package__ is set accordingly (if self.is_package() is defined) will be set
    before it is passed to the decorated function (if self.is_package() does
    not work for the module it will be set post-load).

    If an exception is raised and the decorator created the module it is
    subsequently removed from sys.modules.

    The decorator assumes that the decorated function takes the module name as
    the second argument.

    """
    warnings.warn('The import system now takes care of this automatically.',
                  DeprecationWarning, stacklevel=2)
    @functools.wraps(fxn)
    def module_for_loader_wrapper(self, fullname, *args, **kwargs):
        with _module_to_load(fullname) as module:
            module.__loader__ = self
            try:
                is_package = self.is_package(fullname)
            except (ImportError, AttributeError):
                pass
            else:
                if is_package:
                    module.__package__ = fullname
                else:
                    module.__package__ = fullname.rpartition('.')[0]
            # If __package__ was not set above, __import__() will do it later.
            return fxn(self, module, *args, **kwargs)

    return module_for_loader_wrapper


class _Module(types.ModuleType):

    """A subclass of the module type to allow __class__ manipulation."""


class _LazyModule(types.ModuleType):

    """A subclass of the module type which triggers loading upon attribute access."""

    def __getattribute__(self, attr):
        """Trigger the load of the module and return the attribute."""
        # All module metadata must be garnered from __spec__ in order to avoid
        # using mutated values.
        # Stop triggering this method.
        self.__class__ = _Module
        # Get the original name to make sure no object substitution occurred
        # in sys.modules.
        original_name = self.__spec__.name
        # Figure out exactly what attributes were mutated between the creation
        # of the module and now.
        attrs_then = self.__spec__.loader_state
        attrs_now = self.__dict__
        attrs_updated = {}
        for key, value in attrs_now.items():
            # Code that set the attribute may have kept a reference to the
            # assigned object, making identity more important than equality.
            if key not in attrs_then:
                attrs_updated[key] = value
            elif id(attrs_now[key]) != id(attrs_then[key]):
                attrs_updated[key] = value
        self.__spec__.loader.exec_module(self)
        # If exec_module() was used directly there is no guarantee the module
        # object was put into sys.modules.
        if original_name in sys.modules:
            if id(self) != id(sys.modules[original_name]):
                msg = ('module object for {!r} substituted in sys.modules '
                       'during a lazy load')
            raise ValueError(msg.format(original_name))
        # Update after loading since that's what would happen in an eager
        # loading situation.
        self.__dict__.update(attrs_updated)
        return getattr(self, attr)

    def __delattr__(self, attr):
        """Trigger the load and then perform the deletion."""
        # To trigger the load and raise an exception if the attribute
        # doesn't exist.
        self.__getattribute__(attr)
        delattr(self, attr)


class LazyLoader(abc.Loader):

    """A loader that creates a module which defers loading until attribute access."""

    @staticmethod
    def __check_eager_loader(loader):
        if not hasattr(loader, 'exec_module'):
            raise TypeError('loader must define exec_module()')

    @classmethod
    def factory(cls, loader):
        """Construct a callable which returns the eager loader made lazy."""
        cls.__check_eager_loader(loader)
        return lambda *args, **kwargs: cls(loader(*args, **kwargs))

    def __init__(self, loader):
        self.__check_eager_loader(loader)
        self.loader = loader

    def create_module(self, spec):
        """Create a module which can have its __class__ manipulated."""
        return _Module(spec.name)

    def exec_module(self, module):
        """Make the module load lazily."""
        module.__spec__.loader = self.loader
        module.__loader__ = self.loader
        # Don't need to worry about deep-copying as trying to set an attribute
        # on an object would have triggered the load,
        # e.g. ``module.__spec__.loader = None`` would trigger a load from
        # trying to access module.__spec__.
        module.__spec__.loader_state = module.__dict__.copy()
        module.__class__ = _LazyModule