19.6. base64 — RFC 3548: Base16, Base32, Base64 Data Encodings

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 ASCII-only byte 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 ASCII byte string objects using all three alphabets. Additionally, the decoding functions of the modern interface also accept Unicode strings containing only ASCII characters. The legacy interface provides for encoding and decoding to and from file-like objects as well as byte strings, but only using the Base64 standard alphabet.

Changed in version 3.3: ASCII-only Unicode strings are now accepted by the decoding functions of the modern interface.

Changed in version 3.4: Any bytes-like objects are now accepted by all encoding and decoding functions in this module.

The modern interface provides:

base64.b64encode(s, altchars=None)

Encode a byte 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 byte string is returned.

base64.b64decode(s, altchars=None, validate=False)

Decode a Base64 encoded byte string.

s is the byte 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 binascii.Error exception is raised if s is incorrectly padded.

If validate is False (the default), non-base64-alphabet characters are discarded prior to the padding check. If validate is True, non-base64-alphabet characters in the input result in a binascii.Error.

base64.standard_b64encode(s)

Encode byte string s using the standard Base64 alphabet.

base64.standard_b64decode(s)

Decode byte string s using the standard Base64 alphabet.

base64.urlsafe_b64encode(s)

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

base64.urlsafe_b64decode(s)

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

base64.b32encode(s)

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

base64.b32decode(s, casefold=False, map01=None)

Decode a Base32 encoded byte string.

s is the byte 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 byte string is returned. A binascii.Error is raised if s were incorrectly padded or if there are non-alphabet characters present in the string.

base64.b16encode(s)

Encode a byte string using Base16.

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

base64.b16decode(s, casefold=False)

Decode a Base16 encoded byte 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 byte string is returned. A TypeError is raised if s were incorrectly padded or if there are non-alphabet characters present in the string.

base64.a85encode(s, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)

Encode a byte string using Ascii85.

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

foldspaces is an optional flag that uses the special short sequence ‘y’ instead of 4 consecutive spaces (ASCII 0x20) as supported by ‘btoa’. This feature is not supported by the “standard” Ascii85 encoding.

wrapcol controls whether the output should have newline (‘n’) characters added to it. If this is non-zero, each output line will be at most this many characters long.

pad controls whether the input string is padded to a multiple of 4 before encoding. Note that the btoa implementation always pads.

adobe controls whether the encoded byte sequence is framed with <~ and ~>, which is used by the Adobe implementation.

New in version 3.4.

base64.a85decode(s, *, foldspaces=False, adobe=False, ignorechars=b' tnrv')

Decode an Ascii85 encoded byte string.

s is the byte string to decode.

foldspaces is a flag that specifies whether the ‘y’ short sequence should be accepted as shorthand for 4 consecutive spaces (ASCII 0x20). This feature is not supported by the “standard” Ascii85 encoding.

adobe controls whether the input sequence is in Adobe Ascii85 format (i.e. is framed with <~ and ~>).

ignorechars should be a byte string containing characters to ignore from the input. This should only contain whitespace characters, and by default contains all whitespace characters in ASCII.

New in version 3.4.

base64.b85encode(s, pad=False)

Encode a byte string using base85, as used in e.g. git-style binary diffs.

If pad is true, the input is padded with “\0” so its length is a multiple of 4 characters before encoding.

New in version 3.4.

base64.b85decode(b)

Decode base85-encoded byte string. Padding is implicitly removed, if necessary.

New in version 3.4.

Note

Both Base85 and Ascii85 have an expansion factor of 5 to 4 (5 Base85 or Ascii85 characters can encode 4 binary bytes), while the better-known Base64 has an expansion factor of 6 to 4. They are therefore more efficient when space expensive. They differ by details such as the character map used for encoding.

The legacy interface:

base64.decode(input, output)

Decode the contents of the binary input file and write the resulting binary data to the output file. input and output must be file objects. input will be read until input.read() returns an empty bytes object.

base64.decodebytes(s)
base64.decodestring(s)

Decode the byte string s, which must contain one or more lines of base64 encoded data, and return a byte string containing the resulting binary data. decodestring is a deprecated alias.

New in version 3.1.

base64.encode(input, output)

Encode the contents of the binary input file and write the resulting base64 encoded data to the output file. input and output must be file objects. input will be read until input.read() returns an empty bytes object. encode() returns the encoded data plus a trailing newline character (b'\n').

base64.encodebytes(s)
base64.encodestring(s)

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

An example usage of the module:

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

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