18.12. "base64" — RFC 3548: Base16, Base32, Base64 Data Encodings
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This module provides data encoding and decoding as specified in **RFC
3548**. This standard defines the Base16, Base32, and Base64
algorithms for encoding and decoding arbitrary binary strings into
text strings that can be safely sent by email, used as parts of URLs,
or included as part of an HTTP POST request.  The encoding algorithm
is not the same as the **uuencode** program.

There are two interfaces provided by this module.  The modern
interface supports encoding and decoding string objects using both
base-64 alphabets defined in **RFC 3548** (normal, and URL- and
filesystem-safe).  The legacy interface provides for encoding and
decoding to and from file-like objects as well as strings, but only
using the Base64 standard alphabet.

The modern interface, which was introduced in Python 2.4, provides:

base64.b64encode(s[, altchars])

   Encode a string using Base64.

   *s* is the string to encode.  Optional *altchars* must be a string
   of at least length 2 (additional characters are ignored) which
   specifies an alternative alphabet for the "+" and "/" characters.
   This allows an application to e.g. generate URL or filesystem safe
   Base64 strings.  The default is "None", for which the standard
   Base64 alphabet is used.

   The encoded string is returned.

base64.b64decode(s[, altchars])

   Decode a Base64 encoded string.

   *s* is the string to decode.  Optional *altchars* must be a string
   of at least length 2 (additional characters are ignored) which
   specifies the alternative alphabet used instead of the "+" and "/"
   characters.

   The decoded string is returned.  A "TypeError" is raised if *s* is
   incorrectly padded.  Characters that are neither in the normal
   base-64 alphabet nor the alternative alphabet are discarded prior
   to the padding check.

base64.standard_b64encode(s)

   Encode string *s* using the standard Base64 alphabet.

base64.standard_b64decode(s)

   Decode string *s* using the standard Base64 alphabet.

base64.urlsafe_b64encode(s)

   Encode string *s* using the URL- and filesystem-safe alphabet,
   which substitutes "-" instead of "+" and "_" instead of "/" in the
   standard Base64 alphabet.  The result can still contain "=".

base64.urlsafe_b64decode(s)

   Decode string *s* using the URL- and filesystem-safe alphabet,
   which substitutes "-" instead of "+" and "_" instead of "/" in the
   standard Base64 alphabet.

base64.b32encode(s)

   Encode a string using Base32.  *s* is the string to encode.  The
   encoded string is returned.

base64.b32decode(s[, casefold[, map01]])

   Decode a Base32 encoded string.

   *s* is the string to decode.  Optional *casefold* is a flag
   specifying whether a lowercase alphabet is acceptable as input.
   For security purposes, the default is "False".

   **RFC 3548** allows for optional mapping of the digit 0 (zero) to
   the letter O (oh), and for optional mapping of the digit 1 (one) to
   either the letter I (eye) or letter L (el).  The optional argument
   *map01* when not "None", specifies which letter the digit 1 should
   be mapped to (when *map01* is not "None", the digit 0 is always
   mapped to the letter O).  For security purposes the default is
   "None", so that 0 and 1 are not allowed in the input.

   The decoded string is returned.  A "TypeError" is raised if *s* is
   incorrectly padded or if there are non-alphabet characters present
   in the string.

base64.b16encode(s)

   Encode a string using Base16.

   *s* is the string to encode.  The encoded string is returned.

base64.b16decode(s[, casefold])

   Decode a Base16 encoded string.

   *s* is the string to decode.  Optional *casefold* is a flag
   specifying whether a lowercase alphabet is acceptable as input.
   For security purposes, the default is "False".

   The decoded string is returned.  A "TypeError" is raised if *s*
   were incorrectly padded or if there are non-alphabet characters
   present in the string.

The legacy interface:

base64.decode(input, output)

   Decode the contents of the *input* file and write the resulting
   binary data to the *output* file. *input* and *output* must either
   be file objects or objects that mimic the file object interface.
   *input* will be read until "input.read()" returns an empty string.

base64.decodestring(s)

   Decode the string *s*, which must contain one or more lines of
   base64 encoded data, and return a string containing the resulting
   binary data.

base64.encode(input, output)

   Encode the contents of the *input* file and write the resulting
   base64 encoded data to the *output* file. *input* and *output* must
   either be file objects or objects that mimic the file object
   interface. *input* will be read until "input.read()" returns an
   empty string.  "encode()" returns the encoded data plus a trailing
   newline character ("'\n'").

base64.encodestring(s)

   Encode the string *s*, which can contain arbitrary binary data, and
   return a string containing one or more lines of base64-encoded
   data. "encodestring()" returns a string containing one or more
   lines of base64-encoded data always including an extra trailing
   newline ("'\n'").

An example usage of the module:

>>> import base64
>>> encoded = base64.b64encode('data to be encoded')
>>> encoded
'ZGF0YSB0byBiZSBlbmNvZGVk'
>>> data = base64.b64decode(encoded)
>>> data
'data to be encoded'

See also:

  Module "binascii"
     Support module containing ASCII-to-binary and binary-to-ASCII
     conversions.

  **RFC 1521** - MIME (Multipurpose Internet Mail Extensions) Part
  One: Mechanisms for Specifying and Describing the Format of Internet
  Message Bodies
     Section 5.2, “Base64 Content-Transfer-Encoding,” provides the
     definition of the base64 encoding.
