hset() for redis in python

Question

I have next question:

I use the hset() procedure for Redis Server in python. This procedure is described here.

ok, now I couldn't understand the parameter value... From one hand it should be a number 0 or 1, anyway I could use also other numbers for this parameter (and I also find examples in net with other values). For example I define next def:

    def setHashKeyValue(key, value, number):
        assert (key != None), "Please, get a key"
        #assert (number == 0 or number ==1), "Please enter 0 or 1"
        redis_server.hset(key, value, number);

And got right output with the next parameters:

setHashKeyValue('5', 'test ok for key 5', 1)
setHashKeyValue('6', 'test ok for key 6', 1)

On the other hand, I would like to define some loop for time measuring, so I define next def:

def loopKeyValues(number):
    timeUse = []
    for x in range(number):
        start = time.time()
        setHashKeyValue(x, x**2, 1)
        end = time. time()
        timeUse.append(end-start)
    plt.plot(timeUse)
    plt.ylabel("time")
    plt.show()
    return timeUse;

and

print loopKeyValues(1000)

This function return me an error message for HashValue:

redis.exceptions.ResponseError: WRONGTYPE Operation against a key holding the wrong kind of value

When I use set()-def instead of hset(), the program return me a nice time plot. What could be a problem here and what means the parameter value?

Answer 1

The value parameter is just the key of your hash object entry.

Hash objects are like Python dictionaries. They provide associative arrays. When you write:

redis_server.hset(key, value, number)

it means the hash object "key" will be added/set with an entry "value"/"number". In Python, you would write:

key[value] = number

You got an error because in Redis the objects are typed. Once you have stored a string at a given key, Redis cannot consider anymore this object is a hash object: so the operations associated to a hash object will not work for this key.

In other words, if you have:

SET 10 100

then you cannot do:

HSET 10 100 1

However you could do:

DEL 10
HSET 10 100 1

I would suggest to use the redis-cli program to experiment with Redis before using Python scripts.