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"""Generate cryptographically strong pseudo-random numbers suitable for 

managing secrets such as account authentication, tokens, and similar. 

 

See PEP 506 for more information. 

https://www.python.org/dev/peps/pep-0506/ 

 

""" 

 

__all__ = ['choice', 'randbelow', 'randbits', 'SystemRandom', 

'token_bytes', 'token_hex', 'token_urlsafe', 

'compare_digest', 

] 

 

 

import base64 

import binascii 

import os 

 

from hmac import compare_digest 

from random import SystemRandom 

 

_sysrand = SystemRandom() 

 

randbits = _sysrand.getrandbits 

choice = _sysrand.choice 

 

def randbelow(exclusive_upper_bound): 

"""Return a random int in the range [0, n).""" 

if exclusive_upper_bound <= 0: 

raise ValueError("Upper bound must be positive.") 

return _sysrand._randbelow(exclusive_upper_bound) 

 

DEFAULT_ENTROPY = 32 # number of bytes to return by default 

 

def token_bytes(nbytes=None): 

"""Return a random byte string containing *nbytes* bytes. 

 

If *nbytes* is ``None`` or not supplied, a reasonable 

default is used. 

 

>>> token_bytes(16) #doctest:+SKIP 

b'\\xebr\\x17D*t\\xae\\xd4\\xe3S\\xb6\\xe2\\xebP1\\x8b' 

 

""" 

if nbytes is None: 

nbytes = DEFAULT_ENTROPY 

return os.urandom(nbytes) 

 

def token_hex(nbytes=None): 

"""Return a random text string, in hexadecimal. 

 

The string has *nbytes* random bytes, each byte converted to two 

hex digits. If *nbytes* is ``None`` or not supplied, a reasonable 

default is used. 

 

>>> token_hex(16) #doctest:+SKIP 

'f9bf78b9a18ce6d46a0cd2b0b86df9da' 

 

""" 

return binascii.hexlify(token_bytes(nbytes)).decode('ascii') 

 

def token_urlsafe(nbytes=None): 

"""Return a random URL-safe text string, in Base64 encoding. 

 

The string has *nbytes* random bytes. If *nbytes* is ``None`` 

or not supplied, a reasonable default is used. 

 

>>> token_urlsafe(16) #doctest:+SKIP 

'Drmhze6EPcv0fN_81Bj-nA' 

 

""" 

tok = token_bytes(nbytes) 

return base64.urlsafe_b64encode(tok).rstrip(b'=').decode('ascii')