root/django/branches/gis/django/dispatch/robustapply.py

"""Robust apply mechanism

Provides a function "call", which can sort out
what arguments a given callable object can take,
and subset the given arguments to match only
those which are acceptable.
"""

def function( receiver ):
    """Get function-like callable object for given receiver

    returns (function_or_method, codeObject, fromMethod)

    If fromMethod is true, then the callable already
    has its first argument bound
    """
    if hasattr(receiver, '__call__'):
        # receiver is a class instance; assume it is callable.
        # Reassign receiver to the actual method that will be called.
        if hasattr( receiver.__call__, 'im_func') or hasattr( receiver.__call__, 'im_code'):
            receiver = receiver.__call__
    if hasattr( receiver, 'im_func' ):
        # an instance-method...
        return receiver, receiver.im_func.func_code, 1
    elif not hasattr( receiver, 'func_code'):
        raise ValueError('unknown reciever type %s %s'%(receiver, type(receiver)))
    return receiver, receiver.func_code, 0

def robustApply(receiver, *arguments, **named):
    """Call receiver with arguments and an appropriate subset of named
    """
    receiver, codeObject, startIndex = function( receiver )
    acceptable = codeObject.co_varnames[startIndex+len(arguments):codeObject.co_argcount]
    for name in codeObject.co_varnames[startIndex:startIndex+len(arguments)]:
        if named.has_key( name ):
            raise TypeError(
                """Argument %r specified both positionally and as a keyword for calling %r"""% (
                    name, receiver,
                )
            )
    if not (codeObject.co_flags & 8):
        # fc does not have a **kwds type parameter, therefore 
        # remove unacceptable arguments.
        for arg in named.keys():
            if arg not in acceptable:
                del named[arg]
    return receiver(*arguments, **named)