"""Miscellaneous utility functions and classes.

This module is used internally by Tornado.  It is not necessarily expected
that the functions and classes defined here will be useful to other
applications, but they are documented here in case they are.

The one public-facing part of this module is the `Configurable` class
and its `~Configurable.configure` method, which becomes a part of the
interface of its subclasses, including `.AsyncHTTPClient`, `.IOLoop`,
and `.Resolver`.
"""

from __future__ import absolute_import, division, print_function, with_statement

import sys


def import_object(name):
    """Imports an object by name.

    import_object('x') is equivalent to 'import x'.
    import_object('x.y.z') is equivalent to 'from x.y import z'.

    >>> import tornado.escape
    >>> import_object('tornado.escape') is tornado.escape
    True
    >>> import_object('tornado.escape.utf8') is tornado.escape.utf8
    True
    >>> import_object('tornado') is tornado
    True
    >>> import_object('tornado.missing_module')
    Traceback (most recent call last):
        ...
    ImportError: No module named missing_module
    """
    if name.count('.') == 0:
        return __import__(name, None, None)

    parts = name.split('.')
    obj = __import__('.'.join(parts[:-1]), None, None, [parts[-1]], 0)
    try:
        return getattr(obj, parts[-1])
    except AttributeError:
        raise ImportError("No module named %s" % parts[-1])


# Fake unicode literal support:  Python 3.2 doesn't have the u'' marker for
# literal strings, and alternative solutions like "from __future__ import
# unicode_literals" have other problems (see PEP 414).  u() can be applied
# to ascii strings that include \u escapes (but they must not contain
# literal non-ascii characters).
if type('') is not type(b''):
    def u(s):
        return s
    bytes_type = bytes
    unicode_type = str
    basestring_type = str
else:
    def u(s):
        return s.decode('unicode_escape')
    bytes_type = str
    unicode_type = unicode
    basestring_type = basestring


if sys.version_info > (3,):
    exec("""
def raise_exc_info(exc_info):
    raise exc_info[1].with_traceback(exc_info[2])

def exec_in(code, glob, loc=None):
    if isinstance(code, str):
        code = compile(code, '<string>', 'exec', dont_inherit=True)
    exec(code, glob, loc)
""")
else:
    exec("""
def raise_exc_info(exc_info):
    raise exc_info[0], exc_info[1], exc_info[2]

def exec_in(code, glob, loc=None):
    if isinstance(code, basestring):
        # exec(string) inherits the caller's future imports; compile
        # the string first to prevent that.
        code = compile(code, '<string>', 'exec', dont_inherit=True)
    exec code in glob, loc
""")


class Configurable(object):
    """Base class for configurable interfaces.

    A configurable interface is an (abstract) class whose constructor
    acts as a factory function for one of its implementation subclasses.
    The implementation subclass as well as optional keyword arguments to
    its initializer can be set globally at runtime with `configure`.

    By using the constructor as the factory method, the interface
    looks like a normal class, `isinstance` works as usual, etc.  This
    pattern is most useful when the choice of implementation is likely
    to be a global decision (e.g. when `~select.epoll` is available,
    always use it instead of `~select.select`), or when a
    previously-monolithic class has been split into specialized
    subclasses.

    Configurable subclasses must define the class methods
    `configurable_base` and `configurable_default`, and use the instance
    method `initialize` instead of ``__init__``.
    """
    __impl_class = None
    __impl_kwargs = None

    def __new__(cls, **kwargs):
        base = cls.configurable_base()
        args = {}
        if cls is base:
            impl = cls.configured_class()
            if base.__impl_kwargs:
                args.update(base.__impl_kwargs)
        else:
            impl = cls
        args.update(kwargs)
        instance = super(Configurable, cls).__new__(impl)
        # initialize vs __init__ chosen for compatibility with AsyncHTTPClient
        # singleton magic.  If we get rid of that we can switch to __init__
        # here too.
        instance.initialize(**args)
        return instance

    @classmethod
    def configurable_base(cls):
        """Returns the base class of a configurable hierarchy.

        This will normally return the class in which it is defined.
        (which is *not* necessarily the same as the cls classmethod parameter).
        """
        raise NotImplementedError()

    @classmethod
    def configurable_default(cls):
        """Returns the implementation class to be used if none is configured."""
        raise NotImplementedError()

    def initialize(self):
        """Initialize a `Configurable` subclass instance.

        Configurable classes should use `initialize` instead of ``__init__``.
        """

    @classmethod
    def configure(cls, impl, **kwargs):
        """Sets the class to use when the base class is instantiated.

        Keyword arguments will be saved and added to the arguments passed
        to the constructor.  This can be used to set global defaults for
        some parameters.
        """
        base = cls.configurable_base()
        if isinstance(impl, (unicode_type, bytes_type)):
            impl = import_object(impl)
        if impl is not None and not issubclass(impl, cls):
            raise ValueError("Invalid subclass of %s" % cls)
        base.__impl_class = impl
        base.__impl_kwargs = kwargs

    @classmethod
    def configured_class(cls):
        """Returns the currently configured class."""
        base = cls.configurable_base()
        if cls.__impl_class is None:
            base.__impl_class = cls.configurable_default()
        return base.__impl_class

    @classmethod
    def _save_configuration(cls):
        base = cls.configurable_base()
        return (base.__impl_class, base.__impl_kwargs)

    @classmethod
    def _restore_configuration(cls, saved):
        base = cls.configurable_base()
        base.__impl_class = saved[0]
        base.__impl_kwargs = saved[1]

