Python function to generate a random string (key) of length chars

generate_key.py

"""Function to generate a random string (key) of length chars."""

import binascii
import os


def generate_key(length=40, get_bytes=os.urandom):
    """Return a randomly-generated key of length chars.

    >>> len(generate_key())
    40
    >>> len(generate_key(5))
    5
    >>> keys = set(generate_key() for _ in range(100))
    >>> len(keys)
    100
    >>> all(len(k) == 40 for k in keys)
    True

    >>> def get_bytes(n):
    ...     print('Getting {0} bytes'.format(n))
    ...     return b'ABC123DEF456GHI789J0'[:n]
    >>> generate_key(get_bytes=get_bytes)
    Getting 20 bytes
    '4142433132334445463435364748493738394a30'
    >>> generate_key(10, get_bytes=get_bytes)
    Getting 5 bytes
    '4142433132'
    >>> generate_key(5, get_bytes=get_bytes)
    Getting 3 bytes
    '41424'
    """
    raw_bytes = get_bytes((length + 1) // 2)
    hex_bytes = binascii.b2a_hex(raw_bytes)[:length]
    if not isinstance(hex_bytes, str):
        # On Python 3, b2a_hex returns bytes, convert to str
        hex_bytes = hex_bytes.decode('ascii')
    return hex_bytes


if __name__ == '__main__':
    import doctest
    doctest.testmod()