Very Simple In-Memory Database

simple_mem_db.py

db = dict()
trans_history = [] #function structure
committed_trans_history = [] #outside block commands
trans_block_index = -1 #block index


''' SET [name] [value]: Set a variable [name] to the value [value]. Neither variable names nor values will ever contain spaces'''

def set_db(name, value):
    db[name] = value
    

''' GET [name]: Print out the value stored under the variable [name]. Print NULL if that variable name hasn't been set. '''    

def get_db(name):
    if name in db:
        print db[name]
    else:
        print "NULL"
        

''' UNSET [name]: Unset the variable [name] '''

def unset_db(name):
    try:
        val = db[name]
        del db[name]
        return val

    except:
        print "ENTRY NOT FOUND"



''' NUMEQUALTO [value]: Return the number of variables equal to [value]. If no values are equal, this should output 0. '''

def numequalto_db(value):
    print db.values().count(value)
    

''' COMMIT: Permanently store all of the operations from all presently open transactional blocks.'''

def commit_db():
    global trans_history
    if len(trans_history) == 0:
            print "NO TRANSACTION"
            return 
    del trans_history[:]
    


'''  ROLLBACK: Rollback all of the commands from the most recent transactional block. '''

def rollback_db():
    global trans_history
    global trans_block_index

    if len(trans_history) == 0:
        print "NO TRANSACTION"

    else:
        for command in reversed(trans_history[trans_block_index]):
            ins = command[0]
            if ins == set_db:
                unset_db(str(command[1]))
            elif ins == unset_db:
                set_db(str(command[1]), int(command[2]))

        del trans_history[-1]
        trans_block_index = trans_block_index-1

        for command in committed_trans_history:
            ins = command[0]
            if ins == set_db:
                set_db(str(command[1]), int(command[2]))
            if ins == unset_db:
                unset_db(str(command[1]))
        
        if trans_block_index >= 0:
            for i in range(0, trans_block_index+1):
                for command in trans_history[i]:
                    ins = command[0]
                    if ins == set_db:
                        set_db(str(command[1]), int(command[2]))
                    if ins == unset_db:
                        unset_db(str(command[1]))
    

if __name__ == '__main__':

    while(True):
        cmd = raw_input()
        if cmd == 'END': # END: Exit the program
            exit(0)
            
        if cmd == 'COMMIT':
            commit_db()
                
        if cmd == 'ROLLBACK':
            rollback_db()
                                  
        if cmd == 'BEGIN':
            trans_block_index += 1
            if len(trans_history) <= trans_block_index:
                trans_history.append([])
        else:
            attrs = cmd.split(' ')
            ins = attrs[0]
            if ins == 'GET':
                get_db(str(attrs[1]))
                
            if ins == 'SET':
                set_db(str(attrs[1]), int(attrs[2]))
                if trans_block_index != -1:
                    trans_history[trans_block_index].append((set_db, attrs[1], attrs[2]))
                else:
                    committed_trans_history.append((set_db, attrs[1], attrs[2]))
                    
            if ins == 'UNSET':
                del_val = unset_db(str(attrs[1]))
                if trans_block_index != -1:
                    trans_history[trans_block_index].append((unset_db, attrs[1], del_val))
                else:
                    committed_trans_history.append((unset_db, attrs[1], del_val))
                    
            if ins == 'NUMEQUALTO':
                numequalto_db(int(attrs[1]))