Method _resolve_lookup

django/template/base.py:964–1044

Perform resolution of a real variable (i.e. not a literal) against the given context. As indicated by the method's name, this method is an implementation detail and shouldn't be called by external code. Use Variable.resolve() instead.

(self, context)

        return self.var

    def _resolve_lookup(self, context):
        """
        Perform resolution of a real variable (i.e. not a literal) against the
        given context.

        As indicated by the method's name, this method is an implementation
        detail and shouldn't be called by external code. Use Variable.resolve()
        instead.
        """
        current = context
        try:  # catch-all for silent variable failures
            for bit in self.lookups:
                try:  # dictionary lookup
                    # Only allow if the metaclass implements __getitem__. See
                    # https://docs.python.org/3/reference/datamodel.html#classgetitem-versus-getitem
                    if not hasattr(type(current), "__getitem__"):
                        raise TypeError
                    current = current[bit]
                    # ValueError/IndexError are for numpy.array lookup on
                    # numpy < 1.9 and 1.9+ respectively
                except (TypeError, AttributeError, KeyError, ValueError, IndexError):
                    try:  # attribute lookup
                        # Don't return class attributes if the class is the
                        # context:
                        if isinstance(current, BaseContext) and getattr(
                            type(current), bit
                        ):
                            raise AttributeError
                        current = getattr(current, bit)
                    except (TypeError, AttributeError):
                        # Reraise if the exception was raised by a @property
                        if not isinstance(current, BaseContext) and bit in dir(current):
                            raise
                        try:  # list-index lookup
                            current = current[int(bit)]
                        except (
                            IndexError,  # list index out of range
                            ValueError,  # invalid literal for int()
                            KeyError,  # current is a dict without `int(bit)` key
                            TypeError,
                        ):  # unsubscriptable object
                            raise VariableDoesNotExist(
                                "Failed lookup for key [%s] in %r",
                                (bit, current),
                            )  # missing attribute
                if callable(current):
                    if getattr(current, "do_not_call_in_templates", False):
                        pass
                    elif getattr(current, "alters_data", False):
                        current = context.template.engine.string_if_invalid
                    else:
                        try:  # method call (assuming no args required)
                            current = current()
                        except TypeError:
                            try:
                                current_signature = signature(current)
                            except ValueError:  # No signature found.
                                current = context.template.engine.string_if_invalid