recordtype 1.3

Specifying Fields

Fields can be specified as in namedtuple: as either a string specifing the field names, or as a iterable 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 value which is used when no per-field default value is specified:

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

In addition to supplying the field names as an iterable of 2-tuples, you can also specify a mapping. The keys will be the field names, and the values will be the per-field default values. This is most useful with an OrderedDict, as the order of the fields will then be deterministic. The module variable NO_DEFAULT can be specified if you want a field to use the per-type default value instead of specifying it with a field:

>>> Point = recordtype('Point', OrderedDict((('x', 0),
...                                          ('y', NO_DEFAULT),
...                                          ('z', 0),
...                                          )),
...                             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 is 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')

Mutable default values

Be aware of creating mutable default values. Due to the way Python handles default values, each instance of a recordtype will share the default. This is especially problematic with default values that are lists. For example:

>>> A = recordtype('A', [('x', [])])
>>> a = A()
>>> a.x.append(4)
>>> b = A()
>>> assert b.x == [4]

This is probably not the desired behavior.

1.3 2018-08-03 Eric V. Smith

• Python 3 support (thanks Jakob Stasiak).

• Reformat with black.

1.2 <unreleased> Eric V. Smith

• Switch README.txt to support doctest.

• Add tests for mutable default values.

• Add warning for mutable default values.

1.1 2011-11-14 Eric V. Smith

• No API or code changes.

• Fixed project URL in setup.py.

• Fixed license description in setup.py.

1.0 2011-10-23 Eric V. Smith

• Stabilize API, move to version 1.0.

• Added “python setup.py test” support to run unittests.

• Improve documentation examples.

• Renamed “default_default” to just “default”.

• Expose “NO_DEFAULT”.

• Support mapping objects in addition to strings and lists for field_names.

• Add tests for iterables, not just lists, for field_names.

0.2 2011-10-13 Eric V. Smith

• Fix a typo in the documentation, no code changes.

0.1 2011-10-12 Eric V. Smith

• Initial release.