itypes

Basic immutable container types for Python.

A simple implementation that's designed for simplicity over performance.

Use these in circumstances where it may result in more comprehensible code, or when you want to create custom types with restricted, immutable interfaces.

For an alternative implementation designed for performance, please see pyrsistent.

Installation

Install using pip:

pip install itypes

Instantiating dictionaries and lists.

>>> import itypes
>>> d = itypes.Dict({'a': 1, 'b': 2, 'c': 3})
>>> l = itypes.List(['a', 'b', 'c'])

On instantiation, nested types are coerced to immutables.

>>> d = itypes.Dict({'a': 123, 'b': ['a', 'b', 'c']})
>>> d['b']
List(['a', 'b', 'c'])

Assignments and deletions return new copies.

Methods: set(key, value), delete(key)

>>> d2 = d.set('c', 456)
>>> d2
Dict({'a': 123, 'b': ['a', 'b', 'c'], 'c': 456})
>>> d3 = d2.delete('a')
>>> d3
Dict({'b': ['a', 'b', 'c'], 'c': 456})

Standard assignments and deletions fail.

>>> d['z'] = 123
TypeError: 'Dict' object doesn't support item assignment
>>> del(d['c'])
TypeError: 'Dict' object doesn't support item deletion

Nested lookups.

Method: get_in(keys, default=None)

>>> d['b'][-1]
'c'
>>> d['b'][5]
IndexError: list index out of range
>>> d.get_in(['b', -1])
'c'
>>> d.get_in(['b', 5])
None

Nested assignments and deletions.

Methods: set_in(keys, value), delete_in(keys)

>>> d2 = d.set_in(['b', 1], 'xxx')
>>> d2
Dict({'a': 123, 'b': ['a', 'xxx', 'c']})
>>> d3 = d2.delete_in(['b', 0])
>>> d3
Dict({'a': 123, 'b': ['xxx', 'c']})

Equality works against standard types.

>>> d = itypes.Dict({'a': 1, 'b': 2, 'c': 3})
>>> d == {'a': 1, 'b': 2, 'c': 3}
True

Objects are hashable.

>>> hash(d)
277752239

Shortcuts for switching between mutable and immutable types.

Functions: to_mutable(instance), to_immutable(value)

>>> value = itypes.to_mutable(d)
>>> value
{'a': 123, 'b': ['a', 'b', 'c']}
>>> itypes.to_immutable(value)
Dict({'a': 123, 'b': ['a', 'b', 'c']})

Subclassing.

Only private attribute names may be set on instances. Use @property for attribute access.

Define a .clone(self, data) method if objects have additional state.

Example:

class Configuration(itypes.Dict):
    def __init__(self, title, *args, **kwargs):
        self._title = title
        super(Configuration, self).__init__(*args, **kwargs)

    @property
    def title(self):
        return self._title

    def clone(self, data):
        return Configuration(self._title, data)

Using the custom class:

>>> config = Configuration('worker-process', {'hostname': 'example.com', 'dynos': 4})
>>> config.title
'worker-process'
>>> new = config.set('dynos', 2)
>>> new
Configuration({'dynos': 2, 'hostname': 'example.com'})
>>> new.title
'worker-process'

Custom immutable objects.

Subclass itypes.Object for an object that prevents setting public attributes.

>>> class Custom(itypes.Object):
...     pass

Only private attribute names may be set on instances. Use @property for attribute access.

>>> class Document(itypes.Object):
... def __init__(self, title, content):
...     self._title = title
...     self._content = title
... @property
... def title(self):
...     return self._title
... @property
... def content(self):
...     return self._content

Using immutable objects:

>>> doc = Document(title='Immutability', content='For simplicity')
>>> doc.title
'Immutability'
>>> doc.title = 'Changed'
TypeError: 'Document' object doesn't support property assignment.