19.6. base64 — Base16, Base32, Base64, Base85 Data Encodings

Source code: Lib/base64.py

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This module provides functions for encoding binary data to printable ASCII characters and decoding such encodings back to binary data. It provides encoding and decoding functions for the encodings specified in RFC 3548, which defines the Base16, Base32, and Base64 algorithms, and for the de-facto standard Ascii85 and Base85 encodings.

The RFC 3548 encodings are suitable for encoding binary data so that it can 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 bytes-like objects to ASCII bytes, and decoding bytes-like objects or strings containing ASCII to bytes. Both base-64 alphabets defined in RFC 3548 (normal, and URL- and filesystem-safe) are supported.

The legacy interface does not support decoding from strings, but it does provide functions for encoding and decoding to and from file objects. It only supports the Base64 standard alphabet, and it adds newlines every 76 characters as per RFC 2045. Note that if you are looking for RFC 2045 support you probably want to be looking at the email package instead.

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. Ascii85/Base85 support added.

The modern interface provides:

base64.b64encode(s, altchars=None)

Encode the bytes-like object s using Base64 and return the encoded bytes.

Optional altchars must be a bytes-like object 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.

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

Decode the Base64 encoded bytes-like object or ASCII string s and return the decoded bytes.

Optional altchars must be a bytes-like object or ASCII string of at least length 2 (additional characters are ignored) which specifies the alternative alphabet used instead of the + and / characters.

A binascii.Error exception is raised if s is incorrectly padded.

If validate is False (the default), characters that are neither in the normal base-64 alphabet nor the alternative alphabet are discarded prior to the padding check. If validate is True, these non-alphabet characters in the input result in a binascii.Error.

base64.standard_b64encode(s)

Encode bytes-like object s using the standard Base64 alphabet and return the encoded bytes.

base64.standard_b64decode(s)

Decode bytes-like object or ASCII string s using the standard Base64 alphabet and return the decoded bytes.

base64.urlsafe_b64encode(s)

Encode bytes-like object s using the URL- and filesystem-safe alphabet, which substitutes - instead of + and _ instead of / in the standard Base64 alphabet, and return the encoded bytes. The result can still contain =.

base64.urlsafe_b64decode(s)

Decode bytes-like object or ASCII string s using the URL- and filesystem-safe alphabet, which substitutes - instead of + and _ instead of / in the standard Base64 alphabet, and return the decoded bytes.

base64.b32encode(s)

Encode the bytes-like object s using Base32 and return the encoded bytes.

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

Decode the Base32 encoded bytes-like object or ASCII string s and return the decoded bytes.

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.

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

base64.b16encode(s)

Encode the bytes-like object s using Base16 and return the encoded bytes.

base64.b16decode(s, casefold=False)

Decode the Base16 encoded bytes-like object or ASCII string s and return the decoded bytes.

Optional casefold is a flag specifying whether a lowercase alphabet is acceptable as input. For security purposes, the default is False.

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

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

Encode the bytes-like object b using Ascii85 and return the encoded bytes.

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 (b'\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 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(b, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\v')

Decode the Ascii85 encoded bytes-like object or ASCII string b and return the decoded bytes.

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 bytes-like object or ASCII 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(b, pad=False)

Encode the bytes-like object b using base85 (as used in e.g. git-style binary diffs) and return the encoded bytes.

If pad is true, the input is padded with b'\0' so its length is a multiple of 4 bytes before encoding.

New in version 3.4.

base64.b85decode(b)

Decode the base85-encoded bytes-like object or ASCII string b and return the decoded bytes. Padding is implicitly removed, if necessary.

New in version 3.4.

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.readline() returns an empty bytes object.

base64.decodebytes(s)

Decode the bytes-like object s, which must contain one or more lines of base64 encoded data, and return the decoded bytes.

New in version 3.1.

base64.decodestring(s)

Deprecated alias of decodebytes().

Deprecated since 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() inserts a newline character (b'\n') after every 76 bytes of the output, as well as ensuring that the output always ends with a newline, as per RFC 2045 (MIME).

base64.encodebytes(s)

Encode the bytes-like object s, which can contain arbitrary binary data, and return bytes containing the base64-encoded data, with newlines (b'\n') inserted after every 76 bytes of output, and ensuring that there is a trailing newline, as per RFC 2045 (MIME).

New in version 3.1.

base64.encodestring(s)

Deprecated alias of encodebytes().

Deprecated since version 3.1.

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.