:mod:`base64` --- RFC 3548: Base16, Base32, Base64 Data Encodings

=================================================================

.. module:: base64

   :synopsis: RFC 3548: Base16, Base32, Base64 Data Encodings

.. index::

   pair: base64; encoding

   single: MIME; base64 encoding

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 :program:`uuencode` program.

There are two interfaces provided by this module.  The modern interface supports

encoding and decoding string objects using all three alphabets.  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:

.. function:: b64encode(s[, altchars])

   Encode a string use 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.

.. function:: 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 :exc:`TypeError` is raised if *s* were

   incorrectly padded or if there are non-alphabet characters present in the

   string.

.. function:: standard_b64encode(s)

   Encode string *s* using the standard Base64 alphabet.

.. function:: standard_b64decode(s)

   Decode string *s* using the standard Base64 alphabet.

.. function:: urlsafe_b64encode(s)

   Encode string *s* using a URL-safe alphabet, which substitutes ``-`` instead of

   ``+`` and ``_`` instead of ``/`` in the standard Base64 alphabet.

.. function:: urlsafe_b64decode(s)

   Decode string *s* using a URL-safe alphabet, which substitutes ``-`` instead of

   ``+`` and ``_`` instead of ``/`` in the standard Base64 alphabet.

.. function:: b32encode(s)

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

   is returned.

.. function:: 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 :exc:`TypeError` is raised if *s* were

   incorrectly padded or if there are non-alphabet characters present in the

   string.

.. function:: b16encode(s)

   Encode a string using Base16.

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

.. function:: 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 :exc:`TypeError` is raised if *s* were

   incorrectly padded or if there are non-alphabet characters present in the

   string.

The legacy interface:

.. function:: 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.

.. function:: 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.

.. function:: 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.  :func:`encode` returns the encoded

   data plus a trailing newline character (``'\n'``).

.. function:: 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.

   :func:`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'

.. seealso::

   Module :mod:`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.