
:mod:`binascii` --- Convert between binary and ASCII
====================================================

.. module:: binascii
   :synopsis: Tools for converting between binary and various ASCII-encoded binary
              representations.


.. index::
   module: uu
   module: base64
   module: binhex

The :mod:`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 :mod:`uu`,
:mod:`base64`, or :mod:`binhex` instead. The :mod:`binascii` module contains
low-level functions written in C for greater speed that are used by the
higher-level modules.

The :mod:`binascii` module defines the following functions:


.. function:: 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.


.. function:: 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.


.. function:: 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.


.. function:: 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`.


.. function:: 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.


.. function:: 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.


.. function:: 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.


.. function:: 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 :exc:`Incomplete` exception is raised.


.. function:: rlecode_hqx(data)

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


.. function:: 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).


.. function:: 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*:sup:`16` + *x*:sup:`12` + *x*:sup:`5` + 1, often represented as
   0x1021.  This CRC is used in the binhex4 format.


.. function:: 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.

.. versionchanged:: 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.

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


.. function:: b2a_hex(data)
              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*.


.. function:: a2b_hex(hexstr)
              unhexlify(hexstr)

   Return the binary data represented by the hexadecimal string *hexstr*.  This
   function is the inverse of :func:`b2a_hex`. *hexstr* must contain an even number
   of hexadecimal digits (which can be upper or lower case), otherwise a
   :exc:`TypeError` is raised.


.. exception:: Error

   Exception raised on errors. These are usually programming errors.


.. exception:: 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.


.. seealso::

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

   Module :mod:`binhex`
      Support for the binhex format used on the Macintosh.

   Module :mod:`uu`
      Support for UU encoding used on Unix.

   Module :mod:`quopri`
      Support for quoted-printable encoding used in MIME email messages.

