19.8. "binascii" --- Convert between binary and ASCII
*****************************************************

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

The "binascii" module contains a number of methods to convert between
binary and various ASCII-encoded binary representations. Normally, you
will not use these functions directly but use wrapper modules like
"uu", "base64", or "binhex" instead. The "binascii" module contains
low-level functions written in C for greater speed that are used by
the higher-level modules.

備註: "a2b_*" functions accept Unicode strings containing only ASCII
  characters. Other functions only accept *bytes-like objects* (such
  as "bytes", "bytearray" and other objects that support the buffer
  protocol).

  3.3 版更變: ASCII-only unicode strings are now accepted by the
  "a2b_*" functions.

The "binascii" module defines the following functions:

binascii.a2b_uu(string)

   Convert a single line of uuencoded data back to binary and return
   the binary data. Lines normally contain 45 (binary) bytes, except
   for the last line. Line data may be followed by whitespace.

binascii.b2a_uu(data)

   Convert binary data to a line of ASCII characters, the return value
   is the converted line, including a newline char. The length of
   *data* should be at most 45.

binascii.a2b_base64(string)

   Convert a block of base64 data back to binary and return the binary
   data. More than one line may be passed at a time.

binascii.b2a_base64(data, *, newline=True)

   Convert binary data to a line of ASCII characters in base64 coding.
   The return value is the converted line, including a newline char if
   *newline* is true.  The output of this function conforms to **RFC
   3548**.

   3.6 版更變: Added the *newline* parameter.

binascii.a2b_qp(data, header=False)

   Convert a block of quoted-printable data back to binary and return
   the binary data. More than one line may be passed at a time. If the
   optional argument *header* is present and true, underscores will be
   decoded as spaces.

binascii.b2a_qp(data, quotetabs=False, istext=True, header=False)

   Convert binary data to a line(s) of ASCII characters in quoted-
   printable encoding.  The return value is the converted line(s). If
   the optional argument *quotetabs* is present and true, all tabs and
   spaces will be encoded.   If the optional argument *istext* is
   present and true, newlines are not encoded but trailing whitespace
   will be encoded. If the optional argument *header* is present and
   true, spaces will be encoded as underscores per **RFC 1522**. If
   the optional argument *header* is present and false, newline
   characters will be encoded as well; otherwise linefeed conversion
   might corrupt the binary data stream.

binascii.a2b_hqx(string)

   Convert binhex4 formatted ASCII data to binary, without doing RLE-
   decompression. The string should contain a complete number of
   binary bytes, or (in case of the last portion of the binhex4 data)
   have the remaining bits zero.

binascii.rledecode_hqx(data)

   Perform RLE-decompression on the data, as per the binhex4 standard.
   The algorithm uses "0x90" after a byte as a repeat indicator,
   followed by a count. A count of "0" specifies a byte value of
   "0x90". The routine returns the decompressed data, unless data
   input data ends in an orphaned repeat indicator, in which case the
   "Incomplete" exception is raised.

   3.2 版更變: Accept only bytestring or bytearray objects as input.

binascii.rlecode_hqx(data)

   Perform binhex4 style RLE-compression on *data* and return the
   result.

binascii.b2a_hqx(data)

   Perform hexbin4 binary-to-ASCII translation and return the
   resulting string. The argument should already be RLE-coded, and
   have a length divisible by 3 (except possibly the last fragment).

binascii.crc_hqx(data, value)

   Compute a 16-bit CRC value of *data*, starting with *value* as the
   initial CRC, and return the result.  This uses the CRC-CCITT
   polynomial *x*^16 + *x*^12 + *x*^5 + 1, often represented as
   0x1021.  This CRC is used in the binhex4 format.

binascii.crc32(data[, value])

   Compute CRC-32, the 32-bit checksum of *data*, starting with an
   initial CRC of *value*.  The default initial CRC is zero.  The
   algorithm is consistent with the ZIP file checksum.  Since the
   algorithm is designed for use as a checksum algorithm, it is not
   suitable for use as a general hash algorithm.  Use as follows:

      print(binascii.crc32(b"hello world"))
      # Or, in two pieces:
      crc = binascii.crc32(b"hello")
      crc = binascii.crc32(b" world", crc)
      print('crc32 = {:#010x}'.format(crc))

   3.0 版更變: The result is always unsigned. To generate the same
   numeric value across all Python versions and platforms, use
   "crc32(data) & 0xffffffff".

binascii.b2a_hex(data)
binascii.hexlify(data)

   Return the hexadecimal representation of the binary *data*.  Every
   byte of *data* is converted into the corresponding 2-digit hex
   representation.  The returned bytes object is therefore twice as
   long as the length of *data*.

binascii.a2b_hex(hexstr)
binascii.unhexlify(hexstr)

   Return the binary data represented by the hexadecimal string
   *hexstr*.  This function is the inverse of "b2a_hex()". *hexstr*
   must contain an even number of hexadecimal digits (which can be
   upper or lower case), otherwise an "Error" exception is raised.

exception binascii.Error

   Exception raised on errors. These are usually programming errors.

exception binascii.Incomplete

   Exception raised on incomplete data. These are usually not
   programming errors, but may be handled by reading a little more
   data and trying again.

也參考:

  Module "base64"
     Support for RFC compliant base64-style encoding in base 16, 32,
     64, and 85.

  Module "binhex"
     Support for the binhex format used on the Macintosh.

  Module "uu"
     Support for UU encoding used on Unix.

  Module "quopri"
     Support for quoted-printable encoding used in MIME email
     messages.
