===========
recordtype
===========

recordtype provides a factory function, named
recordtype.recordtype. It is similar to collections.namedtuple, with
the following differences:

* recordtype instances are mutable.

* recordtype supports per-field default values.

* recordtype supports an optional "default default", to be used by all
  fields do not have an explicit default value.

Typical usage::

    from recordtype import recordtype

    Point = recordtype('Point', [('x', 0), ('y', 100)])
    p = Point()
    assert p.x == 0
    assert p.y == 100

In addition to per-field defaults, you can also specify a "default
default" that will be used if a per-field default is not given::

    Point = recordtype('Point', 'x y z', default_default=0)
    p = Point(y=3)
    assert p.x == 0
    assert p.y == 3
    assert p.z == 0

Creating types
==============

Specifying Fields
-----------------

Fields can be specified as in namedtuple: as either a string specifing
the field names, or as a list of field names. These two uses are
equivalent::

    Point = recordtype('Point', 'x y')
    Point = recordtype('Point', ['x', 'y'])

If using a string, commas are first converted to spaces. So these are
equivalent::

    Point = recordtype('Point', 'x y')
    Point = recordtype('Point', 'x,y')


Specifying Defaults
-------------------

Per-field defaults can be specified by supplying a 2-tuple (name,
default_value) instead of just a string for the field name. This is
only supported when you specify a list of field names::

    Point = recordtype('Point', [('x', 0), ('y', 0)])
    p = Point(3)
    assert p.x == 3
    assert p.y == 0

In addition to, or instead of, these per-field defaults, you can also
specify a "default default" which is used when no other default value
is specified for a field::

    Point = recordtype('Point', 'x y z', default_default=0)
    p = Point(y=3)
    assert p.x == 0
    assert p.y == 3
    assert p.z == 0

    Point = recordtype('Point', [('x', 0), 'y', ('z', 0)], default_default=4)
    p = Point(z=2)
    assert p.x == 0
    assert p.y == 4
    assert p.z == 2


Writing to values
-----------------

The objects retured by the factory function are fully writable, unlike
the tuple-derived classes returned by namedtuple::

    Point = recordtype('Point', 'x y')
    p = Point(1, 2)
    p.y = 4
    assert p.x == 1
    assert p.y == 4


Specifying __slots__
--------------------

By default, the returned class sets __slots__, which initialized to
the field names. While this decreases memory usage by eliminating the
instance dict, it also means that you cannot create new instance
members.

To change this behavior, specify use_slots=False when creating the
recordtype::

    Point = recordtype('Point', 'x y', use_slots=False)
    p = Point(0, 1)
    p.z = 2
    assert p.x == 0
    assert p.y == 1
    assert p.z == 2


Additional class members
------------------------

recordtype classes contain these members:

* _asdict(): Returns a dict which maps field names to their
  corresponding values.

* _source: A string with the pure Python source code used to create
  the recordtype class. The source makes the recordtype
  self-documenting. It can be printed, executed using exec(), or saved
  to a file and imported.

* ._fields: Tuple of strings listing the field names. Useful for introspection.


Renaming invalid field names
----------------------------

This functionality is identical to namedtuple. If you specify
rename=True, then any invalid field names are changed to _0, _1,
etc. Reasons for a field name to be invalid are:

* Zero length strings.

* Containing characters other than alphanumerics and underscores.

* A conflict with a Python reserved identifier.

* Beginning with a digit.

* Beginning with an underscore.

* Using the same field name more than once.

For example::

    Point = recordtype('Point', 'x x for', rename=True)
    assert Point._fields == ('x', '_1', '_2')


Creating and using instances
============================

Because the type returned by recordtype is a normal Python class, you
create instances as you would with any Python class.

