def __init__(self, expr, env):
self.expr = expr
try:
self.env = {key:env[key] for key in needs(expr)}
except KeyError as e:
raise UndefinedVariable(e.args[0])
def __init__(self, name, value): if name in builtins: raise AssignmentToBuiltin(name) self.name = name self.value = value self.needs = needs(value)
