"base64" —— Base16、Base32、Base64、Base85 資料編碼
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**Source code:** Lib/base64.py

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這個模組提供將二進位資料編碼成可顯示 ASCII 字元以及解碼回原始資料的功
能，包括了**RFC 3548**中的 Base16、Base32、Base64 等編碼方式，以及標準
Ascii85、Base85 編碼等。

**RFC 3548**標準編碼可以使電子郵件、URL，或是 HTTP POST 內容等傳輸管道
安全地傳遞資料。這些編碼演算法與**uuencode** 並不相同。

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.

3.3 版更變: ASCII-only Unicode strings are now accepted by the
decoding functions of the modern interface.

3.4 版更變: 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.

   3.4 版新加入.

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.

   3.4 版新加入.

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.

   3.4 版新加入.

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.

   3.4 版新加入.

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".

   3.1 版新加入.

base64.decodestring(s)

   Deprecated alias of "decodebytes()".

   3.1 版後已棄用.

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).

   3.1 版新加入.

base64.encodestring(s)

   Deprecated alias of "encodebytes()".

   3.1 版後已棄用.

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'

也參考:

  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.
