Generating random string of seedable data

I'm looking for a way to generate a random string of n bytes in Python in a similar way to os.urandom() method except providing a way to seed the data generation.

So far I have:

def genRandData(size):
    buf = chr(random.randint(0,255))
    for i in range(size-1):
        buf = buf + chr(random.randint(0,255))
return str(buf)

However this function is very slow, generating a megabyte of data takes about 1.8 seconds on my machine. Is there any way of improving this (or alternatively a way to seed os.urandom).