from django.conf import settings

from django.core.exceptions import ImproperlyConfigured

_standard_context_processors = None

class ContextPopException(Exception):

    "pop() has been called more times than push()"

    pass

class Context(object):

    "A stack container for variable context"

    def __init__(self, dict_=None, autoescape=True):

        dict_ = dict_ or {}

        self.dicts = [dict_]

        self.autoescape = autoescape

    def __repr__(self):

        return repr(self.dicts)

    def __iter__(self):

        for d in self.dicts:

            yield d

    def push(self):

        d = {}

        self.dicts = [d] + self.dicts

        return d

    def pop(self):

        if len(self.dicts) == 1:

            raise ContextPopException

        return self.dicts.pop(0)

    def __setitem__(self, key, value):

        "Set a variable in the current context"

        self.dicts[0][key] = value

    def __getitem__(self, key):

        "Get a variable's value, starting at the current context and going upward"

        for d in self.dicts:

            if key in d:

                return d[key]

        raise KeyError(key)

    def __delitem__(self, key):

        "Delete a variable from the current context"

        del self.dicts[0][key]

    def has_key(self, key):

        for d in self.dicts:

            if key in d:

                return True

        return False

    __contains__ = has_key

    def get(self, key, otherwise=None):

        for d in self.dicts:

            if key in d:

                return d[key]

        return otherwise

    def update(self, other_dict):

        "Like dict.update(). Pushes an entire dictionary's keys and values onto the context."

        if not hasattr(other_dict, '__getitem__'): 

            raise TypeError('other_dict must be a mapping (dictionary-like) object.')

        self.dicts = [other_dict] + self.dicts

        return other_dict

# This is a function rather than module-level procedural code because we only

# want it to execute if somebody uses RequestContext.

def get_standard_processors():

    global _standard_context_processors

    if _standard_context_processors is None:

        processors = []

        for path in settings.TEMPLATE_CONTEXT_PROCESSORS:

            i = path.rfind('.')

            module, attr = path[:i], path[i+1:]

            try:

                mod = __import__(module, {}, {}, [attr])

            except ImportError, e:

                raise ImproperlyConfigured('Error importing request processor module %s: "%s"' % (module, e))

            try:

                func = getattr(mod, attr)

            except AttributeError:

                raise ImproperlyConfigured('Module "%s" does not define a "%s" callable request processor' % (module, attr))

            processors.append(func)

        _standard_context_processors = tuple(processors)

    return _standard_context_processors

class RequestContext(Context):

    """

    This subclass of template.Context automatically populates itself using

    the processors defined in TEMPLATE_CONTEXT_PROCESSORS.

    Additional processors can be specified as a list of callables

    using the "processors" keyword argument.

    """

    def __init__(self, request, dict=None, processors=None):

        Context.__init__(self, dict)

        if processors is None:

            processors = ()

        else:

            processors = tuple(processors)

        for processor in get_standard_processors() + processors:

            self.update(processor(request))