Other bidict Types

Now that we’ve covered Basic Usage with the bidict.bidict type, let’s look at some other bidirectional mapping types.

Bidict Types Diagram

bidict types diagram

All bidirectional mapping types that bidict provides are subclasses of bidict.BidirectionalMapping. This abstract base class extends collections.abc.Mapping by adding the “invabstractproperty. [1]

[1]In fact, any collections.abc.Mapping that provides an inv attribute will be considered a virtual subclass of bidict.BidirectionalMapping automatically, enabling interoperability with external implementations.

As you may have noticed, bidict.bidict is also a collections.abc.MutableMapping. But bidict provides immutable bidirectional mapping types as well.

frozenbidict

frozenbidict is an immutable, hashable bidirectional mapping type.

As you would expect, attempting to mutate a frozenbidict causes an error:

>>> from bidict import frozenbidict
>>> f = frozenbidict({'H': 'hydrogen'})
>>> f['C'] = 'carbon'
Traceback (most recent call last):
    ...
TypeError: ...

frozenbidict also implements collections.abc.Hashable, so it’s suitable for insertion into sets or other mappings:

>>> my_set = {f}      # not an error
>>> my_dict = {f: 1}  # also not an error

See the frozenbidict API documentation for more information.

OrderedBidict

bidict.OrderedBidict is a mutable BidirectionalMapping that preserves the order in which its items are inserted. It’s like a bidirectional version of collections.OrderedDict.

>>> from bidict import OrderedBidict
>>> element_by_symbol = OrderedBidict([
...     ('H', 'hydrogen'), ('He', 'helium'), ('Li', 'lithium')])

>>> element_by_symbol.inv
OrderedBidict([('hydrogen', 'H'), ('helium', 'He'), ('lithium', 'Li')])

>>> first, second, third = element_by_symbol.values()
>>> first, second, third
('hydrogen', 'helium', 'lithium')

>>> # Insert an additional item and verify it now comes last:
>>> element_by_symbol['Be'] = 'beryllium'
>>> last_item = list(element_by_symbol.items())[-1]
>>> last_item
('Be', 'beryllium')

Additional functionality modeled after OrderedDict is provided as well:

>>> element_by_symbol.popitem(last=True)   # Remove the last item
('Be', 'beryllium')
>>> element_by_symbol.popitem(last=False)  # Remove the first item
('H', 'hydrogen')

>>> # Re-adding hydrogen after it's been removed moves it to the end:
>>> element_by_symbol['H'] = 'hydrogen'
>>> element_by_symbol
OrderedBidict([('He', 'helium'), ('Li', 'lithium'), ('H', 'hydrogen')])

>>> # But there's also a `move_to_end` method just for this purpose:
>>> element_by_symbol.move_to_end('Li')
>>> element_by_symbol
OrderedBidict([('He', 'helium'), ('H', 'hydrogen'), ('Li', 'lithium')])

>>> element_by_symbol.move_to_end('H', last=False)  # move to front
>>> element_by_symbol
OrderedBidict([('H', 'hydrogen'), ('He', 'helium'), ('Li', 'lithium')])

As with OrderedDict, updating an existing item preserves its position in the order:

>>> element_by_symbol['He'] = 'updated in place!'
>>> element_by_symbol
OrderedBidict([('H', 'hydrogen'), ('He', 'updated in place!'), ('Li', 'lithium')])

Collapsing overwrites

When setting an item in an ordered bidict whose key duplicates that of an existing item, and whose value duplicates that of a different existing item, the existing item whose value is duplicated will be dropped, and the existing item whose key is duplicated will have its value overwritten in place:

>>> o = OrderedBidict([(1, 2), (3, 4), (5, 6), (7, 8)])
>>> o.forceput(3, 8)  # item with duplicated value (7, 8) is dropped...
>>> o  # and the item with duplicated key (3, 4) is updated in place:
OrderedBidict([(1, 2), (3, 8), (5, 6)])
>>> # (3, 8) took the place of (3, 4), not (7, 8)

>>> o = OrderedBidict([(1, 2), (3, 4), (5, 6), (7, 8)])  # as before
>>> o.forceput(5, 2)  # another example
>>> o
OrderedBidict([(3, 4), (5, 2), (7, 8)])
>>> # (5, 2) took the place of (5, 6), not (1, 2)

__eq__() is order-insensitive

To ensure that equality of bidicts is transitive (and to uphold the Liskov substitution principle), equality tests between an ordered bidict and other mappings are always order-insensitive:

>>> b = bidict([('one', 1), ('two', 2)])
>>> o1 = OrderedBidict([('one', 1), ('two', 2)])
>>> o2 = OrderedBidict([('two', 2), ('one', 1)])
>>> b == o1
True
>>> b == o2
True
>>> o1 == o2
True

For order-sensitive equality tests, use equals_order_sensitive():

>>> o1.equals_order_sensitive(o2)
False
>>> from collections import OrderedDict
>>> od = OrderedDict(o2)
>>> o1.equals_order_sensitive(od)
False

Note that this differs from the behavior of collections.OrderedDict’s __eq__(), by recommendation of Raymond Hettinger (the author) himself. He later said that making OrderedDict’s __eq__() intransitive was a mistake.

What if my Python version has order-preserving dicts?

In PyPy as well as CPython ≥ 3.6, dict preserves insertion order. If you are using one of these versions of Python, you may wonder whether you can get away with using a regular bidict.bidict in places where you need an insertion order-preserving bidirectional mapping.

In general the answer is no, particularly if you need to be able to change existing associations in the bidirectional mapping while preserving order correctly.

Consider this example using a regular bidict with an order-preserving dict version of Python:

 >>> b = bidict([(1, -1), (2, -2), (3, -3)])
 >>> b[2] = 'UPDATED'
 >>> b
 bidict({1: -1, 2: 'UPDATED', 3: -3})
 >>> b.inv  # oops:
 bidict({-1: 1, -3: 3, 'UPDATED': 2})

When the value associated with the key 2 was changed, the corresponding item stays in place in the forward mapping, but moves to the end of the inverse mapping. Since regular bidicts provide no guarantees about order preservation (which allows for a more efficient implementation), non-order-preserving behavior (as in the example above) is exactly what you get.

If you never mutate a bidict (or are even using a frozenbidict) and you’re running a version of Python with order-preserving dicts, then you’ll find that the order of the items in your bidict and its inverse happens to be preserved. However, you won’t get the additional order-specific APIs (such as move_to_end(), equals_order_sensitive(), and __reversed__() – indeed the lack of a dict.__reversed__ API is what stops us from making FrozenOrderedBidict an alias of frozenbidict on dict-order-preserving Pythons, as this would mean FrozenOrderedBidict.__reversed__() would have to be O(n) in space complexity).

If you need order-preserving behavior guaranteed, then OrderedBidict is your best choice.

FrozenOrderedBidict

FrozenOrderedBidict is an immutable ordered bidict type. It’s like an OrderedBidict without the mutating APIs, or equivalently like an order-preserving frozenbidict.

namedbidict()

bidict.namedbidict(), inspired by collections.namedtuple(), allows you to easily generate a new bidirectional mapping type with custom attribute-based access to forward and inverse mappings:

>>> from bidict import namedbidict
>>> ElementMap = namedbidict('ElementMap', 'symbol', 'name')
>>> noble_gases = ElementMap(He='helium')
>>> noble_gases.name_for['He']
'helium'
>>> noble_gases.symbol_for['helium']
'He'
>>> noble_gases.name_for['Ne'] = 'neon'
>>> del noble_gases.symbol_for['helium']
>>> noble_gases
ElementMap({'Ne': 'neon'})

Using the base_type keyword arg – whose default value is bidict.bidict – you can override the bidict type used as the base class, allowing the creation of e.g. a named frozenbidict type:

>>> ElMap = namedbidict('ElMap', 'symbol', 'name', base_type=frozenbidict)
>>> noble = ElMap(He='helium')
>>> noble.symbol_for['helium']
'He'
>>> hash(noble) is not 'an error'
True
>>> noble['C'] = 'carbon'  # mutation fails
Traceback (most recent call last):
...
TypeError: ...

Polymorphism

(Or: ABCs ftw!)

You may be tempted to write something like isinstance(obj, dict) to check whether obj is a Mapping. However, this check is too specific, and will fail for many types that implement the Mapping interface:

>>> from collections import ChainMap
>>> issubclass(ChainMap, dict)
False

The same is true for all the bidict types:

>>> issubclass(bidict, dict)
False

The proper way to check whether an object is a Mapping is to use the abstract base classes (ABCs) from the collections module that are provided for this purpose:

>>> issubclass(ChainMap, Mapping)
True
>>> isinstance(bidict(), Mapping)
True

Also note that the proper way to check whether an object is an (im)mutable mapping is to use the MutableMapping ABC:

>>> from bidict import BidirectionalMapping

>>> def is_immutable_bimap(obj):
...     return (isinstance(obj, BidirectionalMapping)
...             and not isinstance(obj, MutableMapping))

>>> is_immutable_bimap(bidict())
False

>>> is_immutable_bimap(frozenbidict())
True

Checking for isinstance(obj, frozenbidict) is too specific and could fail in some cases. For example, FrozenOrderedBidict is an immutable mapping but it does not subclass frozenbidict:

>>> from bidict import FrozenOrderedBidict
>>> obj = FrozenOrderedBidict()
>>> is_immutable_bimap(obj)
True
>>> isinstance(obj, frozenbidict)
False

Besides the above, there are several other collections ABCs whose interfaces are implemented by various bidict types. Have a look through the collections.abc documentation if you’re interested.

One thing you might notice is that there is no Ordered or OrderedMapping ABC. However, Python 3.6 introduced the collections.abc.Reversible ABC. Since being reversible implies having an ordering, you could check for reversibility instead. For example:

>>> from collections.abc import Reversible

>>> def is_reversible_mapping(cls):
...     return issubclass(cls, Reversible) and issubclass(cls, Mapping)
...

>>> is_reversible_mapping(OrderedBidict)
True

>>> is_reversible_mapping(OrderedDict)
True

For more you can do with bidict, check out Extending bidict next.