Random byte string in Python

from random import randint 
rstr = ''.join( randint(0, 255) for i in range(4) )

Do you want the middle 4 bytes to be set to a random value?

buf = 'x00' + ''.join(chr(random.randint(0,255)) for _ in range(4)) + 'x00'

Simple:

import functools, random, operator
functools.reduce(operator.add, ('%c' % random.randint(0, 255) for i in range(4)))

On POSIX platforms:

open("/dev/urandom","rb").read(4)

Use /dev/random for better randomization.

bytearray(random.getrandbits(8) for _ in xrange(size))

Faster than other solutions but not cryptographically secure.

An alternative way to obtaining a secure random sequence of bytes could be to use the standard library secrets module, available since Python 3.6.

Example, based on the given question:

import secrets
b"x00" + secrets.token_bytes(4) + b"x00"

More information can be found at:
https://docs.python.org/3/library/secrets.html

I like using numpy library for that.

import numpy as np

X_1KB = 1024
X_256KB = 256 * X_1KB
X_1MB = 1024 * 1024
X_4MB = 4 * X_1MB
X_32MB = 32 * X_1MB
X_64MB = 2 * X_32MB
X_128MB = X_1MB * 128


np.random.bytes( X_1MB )

This can be used to generate a string of random bytes (replace n with the desired amount):

import random
random_bytes = bytes([random.randrange(0, 256) for _ in range(0, n)])
-or-
random_bytes = bytes([random.randint(0, 255) for _ in range(0, n)])
-or-
random_bytes = bytes([random.getrandbits(8) for _ in range(0, n)])

The answer to the specific question would then be:

import random
buf = b'x00' + bytes([random.randrange(0, 256) for _ in range(0, 4)]) + b'x00'
-or-
buf = b'x00' + bytes([random.randint(0, 255) for _ in range(0, 4)]) + b'x00'
-or-
buf = b'x00' + bytes([random.getrandbits(8) for _ in range(0, 4)]) + b'x00'

As others pointed out, this should not be used for cryptography, but for everything else it should be perfectly fine.

Python 3.9 adds a new random.randbytes method. This method generates random bytes:

from random import randbytes

randbytes(4)

Output:

b'xf3xf5xf8x98'

Be careful though. It should be used only when you are not dealing with cryptography. As stated in the docs:

This method should not be used for generating security tokens. Use secrets.token_bytes() instead.