"""
Expression evaluation support.

For the moment this depends on python's eval. In the future it should be 
replaced with a "safe" parser.
"""

from UserDict import DictMixin

class ExpressionContext( object, DictMixin ):
    def __init__( self, dict, parent=None ):
        """
        Create a new expression context that looks for values in the
        container object 'dict', and falls back to 'parent'
        """
        self.dict = dict
        self.parent = parent
    def __getitem__( self, key ):
        if key in self.dict:
            return self.dict[key]
        if self.parent is not None and key in self.parent:
            return self.parent[key]
        raise KeyError( key )
    def __contains__( self, key ):
        if key in self.dict:
            return True
        if self.parent is not None and key in self.parent:
            return True
        return False
    def __str__( self ):
        return str( self.dict )