gistfile1.py

from collections import Counter

class State(dict):
    def __init__(self, parent, counter, committed):
        self.parent = parent
        self.counter = counter    
        self.committed = committed

    def fetch(self,var):
        if var in self:
            return self.get(var)
        if self.parent is None:
            return False
        return self.parent.get(var)

    def do_set(self,var,value):
        if var in self:
            self.counter[self[var]] -= 1
        self[var] = value
        self.counter[value] += 1


    def unset(self,var):
        if self.fetch(var):
            self.counter[self.fetch(var)] -= 1            
            self[var] = None

    def numequalto(self,value):        
        if value in self.counter:
            return self.counter[value]
        if self.parent is None:
            return 0
        return self.parent.numequalto(value)

    def commit(self):        
        if self.committed:
            return False            
        self.committed = True
        return self.parent.commit() or True

    def begin(self):
        return State(self, self.counter.copy(), False)

    def rollback(self):
        return self.parent

    def can_rollback(self):
        return not self.committed

def CLI():
    "Simple CLI without error checking / input sanitizing"
    db = State(None, Counter(), True)
    while True:
        line = input().upper()
        if line == 'END': break
        elif line == 'BEGIN': db = db.begin()
        elif line == 'ROLLBACK':
            if db.can_rollback():
                db = db.rollback()
            else:
                print("NO TRANSACTION")
        elif line == "COMMIT":
            if not db.commit():
                print("NO TRANSACTION")
        else:
            command,args = line.split(' ',1)
            if command == "GET":
                x = db.fetch(args)
                if x:
                    print(x)
                else:
                    print("NULL")
            elif command == "UNSET":
                db.unset(args)
            elif command == "SET":
                var,value = args.split()
                db.do_set(var,int(value))
            elif command == "NUMEQUALTO":
                print(db.numequalto(int(args)))

if __name__ == '__main__':
    CLI()