Field Types
===========

Supported Field Types
---------------------

==  =============  ========================================================
:   Field type     Converted to
==  =============  ========================================================
\+  autoincrement  int
@   time           datetime.datetime
0   flags          byte string (int before 2.0)
B   double         float (Visual FoxPro)
B   binary memo    byte string (other versions)
C   text           unicode string
D   date           datetime.date or None
F   float          float
G   OLE object     byte string
I   integer        int
L   logical        True, False or None
M   memo           unicode string (memo), byte string (picture or object)
                   or None
N   numeric        int, float or None
O   double         float (floats are doubles in Python)
P   picture        byte string
T   time           datetime.datetime
V   varchar        unicode string
Y   currency       decimal.Decimal
==  =============  ========================================================

Text values ('C') can be up to 65535 bytes long. DBF was originally
limited to 255 bytes but some vendors have reused the
``decimal_count`` field to get another byte for field length.

The 'B' field type is used to store double precision (64 bit) floats
in Visual FoxPro databases and binary memos in other
versions. ``dbfread`` will look at the database version to parse and
return the correct data type.

The '0' field type is used for '_NullFlags' in Visual FoxPro.  It was
mistakenly though to always be one byte long and was interpreted as an
integer. From 2.0.1 on it is returned as a byte string.

The 'V' field is an alternative character field used by Visual
FoxPro. The binary version of this field is not yet supported. (See
https://msdn.microsoft.com/en-us/library/st4a0s68%28VS.80%29.aspx for
more.)


Adding Custom Field Types
-------------------------

You can add new field types by subclassing
:py:class:`FieldParser`. For example:

.. literalinclude:: ../examples/custom_field_type.py

The ``FieldParser`` object has the following attributes:

self.table
  A reference to the ``DBF`` objects. This can be used to get the headers
  to find ``dbversion`` and other things.

self.encoding
  The character encoding. (A a shortcut for ``self.table.encoding`` to
  speed things up a bit.)

self.char_decode_errors
  Error handling scheme to use while decoding. (A shortcut for ``self.table.char_decode_errors``.)

self.dbversion
  The database version as an integer. (A shortcut for
  ``self.table.header.dbversion``.)

self.get_memo(index)
  Returns a memo from the memo file using the index stored in the field data.

  This returns a byte string (``bytes``) which you can then
  decode.

  For Visual FoxPro (``.FPT``) files it will return ``TextMemo``,
  ``PictureMemo`` and ``ObjectMemo`` objects depending on the type of
  memo. These are all subclasses of ``bytes`` so the type is only used
  to annotate the memo type without breaking code elsewhere. The full
  class tree::

      bytes
        VFPMemo
          TextMemo
          BinaryMemo
            PictureMemo
            ObjectMemo

  These are all found in ``dbfread.memo``.

self.decode_text(text)

  This will decode the text using the correct encoding and the user
  supplied ``char_decode_errors`` option.


Special Characters in Field Type Names
--------------------------------------

For a field type like '+' (autoincrement) the method would be named
``parse+()``.  Since this is not allowed in Python you can instead use
its ASCII value in hexadecimal. For example, the '+' parser is called
``parse3F()``.

You can name your method with::

    >>> 'parse' + format(ord('?'), 'x').upper()
    'parse3F'

Just replace ``'?'`` with your field type.


InvalidValue
------------

The field parser will normally raise ``ValueError`` when invalid
values are encountered. If instead you want them returned as raw data
you can do this:

.. literalinclude:: ../examples/print_invalid_values.py

``InvalidValue`` is a subclass of ``bytes``, and allows you to tell
invalid data apart from valid data that happens to be byte
strings. You can test for this with::

    isinstance(value, InvalidData)

You can also tell from the ``repr()`` string::

    >>> value
    InvalidData(b'not a number')
