performance of script to write very long lines of random chars

[Steven D'Aprano]

On Wed, 10 Apr 2013 18:21:51 -0700, gry wrote:

> Dear pythonistas,
>    I am writing a tiny utility to produce a file consisting of a
> specified number of lines of a given length of random ascii characters. 
> I am hoping to find a more time and memory efficient way, that is still
> fairly simple clear, and _pythonic_.

Here's another option: use string.translate to map random bytes to the 
printable ASCII bytes.


import string

all_bytes = ''.join(map(chr, range(256)))
printable = all_bytes[33:127]
n = 127 + len(printable)
extras = all_bytes[127:n]
_deletions = all_bytes[:33] + all_bytes[n:]
_table = string.maketrans(extras, printable)


def random_chars(length, size=1000):
    # Make local bindings for speed.
    import string, os
    table, deletions = _table, _deletions
    # Generate random strings.
    buffer = []
    while True:
        while len(buffer) < length:
            bytes = string.translate(os.urandom(size), table, deletions)
            buffer.extend(bytes)
        yield ''.join(buffer[:length])
        buffer = buffer[length:]



Then use it like this:

# I want seven lines of twelve char strings.
make = random_chars(12)
for i in range(7):
    print next(make)



One thing to be aware of: urandom may run out of entropy, and then it 
will slow down a lot. If you don't care about cryptographic randomness, 
you could use this instead:

import random
def myrandom(n):
    return [random.randint(0, 255) for i in xrange(n)]


although that will actually be slower unless urandom has run out of 
entropy.



-- 
Steven