context.py

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals

import threading


class ContextInfo(object):
    def __init__(self, cls, allow_default, arg_name):
        self.cls = cls
        self.allow_default = allow_default
        self.arg_name = arg_name
        self._local_stack = threading.local()

    @property
    def _stack(self):
        if not hasattr(self._local_stack, 'obj'):
            self._local_stack.obj = []
        return self._local_stack.obj

    def enter(self, value):
        self._stack.append(value)

    def exit(self, value):
        assert len(self._stack) > 0, 'Context %s is empty.' % self.cls
        assert self._stack.pop() == value

    def get_active(self, required=True):
        if len(self._stack) == 0:
            if not required:
                return None
            assert self.allow_default, (
                'Context %s is required but none is active.' % self.cls)
            self.enter(self.cls())
        return self._stack[-1]


class ContextManager(object):
    def __init__(self):
        self._ctxs = {}

    def register(self, ctx_info):
        assert isinstance(ctx_info, ContextInfo)
        assert (ctx_info.cls not in self._ctxs), (
            'Context %s already registered' % ctx_info.cls)
        self._ctxs[ctx_info.cls] = ctx_info

    def get(self, cls):
        assert cls in self._ctxs, 'Context %s not registered.' % cls
        return self._ctxs[cls]


_CONTEXT_MANAGER = ContextManager()


def context_manager():
    global _CONTEXT_MANAGER
    return _CONTEXT_MANAGER


def __enter__(self):
    if self._prev_enter is not None:
        self._prev_enter()
    context_manager().get(self._ctx_class).enter(self)
    return self


def __exit__(self, *args):
    context_manager().get(self._ctx_class).exit(self)
    if self._prev_exit is not None:
        self._prev_exit(*args)


@classmethod
def current(cls, value=None, required=True):
    return get_active_context(cls, value, required)


class define_context(object):
    def __init__(self, arg_name=None, allow_default=False):
        self.arg_name = arg_name
        self.allow_default = allow_default

    def __call__(self, cls):
        assert not hasattr(cls, '_ctx_class'), (
            '%s parent class (%s) already defines context.' % (
                cls, cls._ctx_class))
        context_manager().register(
            ContextInfo(cls, self.allow_default, self.arg_name))
        cls._prev_enter = cls.__enter__ if hasattr(cls, '__enter__') else None
        cls._prev_exit = cls.__exit__ if hasattr(cls, '__exit__') else None
        cls._ctx_class = cls
        cls.__enter__ = __enter__
        cls.__exit__ = __exit__
        cls.current = current
        return cls


def get_active_context(cls, val=None, required=True):
    ctx_info = context_manager().get(cls)
    if val is not None:
        assert isinstance(val, cls), (
            'Wrong context type. Expected: %s, got %s.' % (cls, type(val)))
        return val
    return ctx_info.get_active(required=required)