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

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

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See PEP 506 for more information.

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

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"""

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

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

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'compare_digest',

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]

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import base64

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import binascii

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from hmac import compare_digest

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from random import SystemRandom

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_sysrand = SystemRandom()

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randbits = _sysrand.getrandbits

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choice = _sysrand.choice

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def randbelow(exclusive_upper_bound):

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

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if exclusive_upper_bound <= 0:

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

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return _sysrand._randbelow(exclusive_upper_bound)

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DEFAULT_ENTROPY = 32 # number of bytes to return by default

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def token_bytes(nbytes=None):

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

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

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default is used.

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>>> token_bytes(16) #doctest:+SKIP

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

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"""

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if nbytes is None:

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nbytes = DEFAULT_ENTROPY

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return _sysrand.randbytes(nbytes)

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def token_hex(nbytes=None):

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

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

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

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default is used.

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>>> token_hex(16) #doctest:+SKIP

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'f9bf78b9a18ce6d46a0cd2b0b86df9da'

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"""

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

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def token_urlsafe(nbytes=None):

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

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

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or not supplied, a reasonable default is used.

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>>> token_urlsafe(16) #doctest:+SKIP

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'Drmhze6EPcv0fN_81Bj-nA'

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"""

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tok = token_bytes(nbytes)

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

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