18.14. "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.

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)

   Convert binary data to a line of ASCII characters in base64 coding.
   The return value is the converted line, including a newline char.
   The newline is added because the original use case for this
   function was to feed it a series of 57 byte input lines to get
   output lines that conform to the MIME-base64 standard.  Otherwise
   the output conforms to **RFC 3548**.

binascii.a2b_qp(string[, header])

   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, istext, header])

   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 RFC1522. 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.

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, crc)

   Compute a 16-bit CRC value of *data*, starting with an initial
   *crc* and returning 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[, crc])

   Compute CRC-32, the 32-bit checksum of data, starting with an
   initial crc.  This 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("hello world")
      # Or, in two pieces:
      crc = binascii.crc32("hello")
      crc = binascii.crc32(" world", crc) & 0xffffffff
      print 'crc32 = 0x%08x' % crc

Note: To generate the same numeric value across all Python versions
  and platforms use crc32(data) & 0xffffffff.  If you are only using
  the checksum in packed binary format this is not necessary as the
  return value is the correct 32bit binary representation regardless
  of sign.

Changed in version 2.6: The return value is in the range [-2**31,
2**31-1] regardless of platform.  In the past the value would be
signed on some platforms and unsigned on others.  Use & 0xffffffff on
the value if you want it to match Python 3 behavior.

Changed in version 3.0: The return value is unsigned and in the range
[0, 2**32-1] regardless of platform.

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 resulting string 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 a "TypeError" 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.

See also:

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

  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.
