Basic Usage

Let’s return to the example from the Introduction:

>>> element_by_symbol = bidict(H='hydrogen')

As we saw, this behaves just like a dict, but maintains a special inverse attribute giving access to inverse items:

>>> element_by_symbol.inverse['helium'] = 'He'
>>> del element_by_symbol.inverse['hydrogen']
>>> element_by_symbol
bidict({'He': 'helium'})

bidict.bidict supports the rest of the collections.abc.MutableMapping interface as well:

>>> 'C' in element_by_symbol
False
>>> element_by_symbol.get('C', 'carbon')
'carbon'
>>> element_by_symbol.pop('He')
'helium'
>>> element_by_symbol
bidict()
>>> element_by_symbol.update(Hg='mercury')
>>> element_by_symbol
bidict({'Hg': 'mercury'})
>>> 'mercury' in element_by_symbol.inverse
True
>>> element_by_symbol.inverse.pop('mercury')
'Hg'

Because inverse items are maintained alongside forward items, referencing a bidict’s inverse is always a constant-time operation.

Values Must Be Hashable

Because you must be able to look up keys by value as well as values by key, values must also be hashable.

Attempting to insert an unhashable value will result in an error:

>>> anagrams_by_alphagram = dict(opt=['opt', 'pot', 'top'])
>>> bidict(anagrams_by_alphagram)
Traceback (most recent call last):
...
TypeError: ...

So in this example, using a tuple or a frozenset instead of a list would do the trick:

>>> bidict(opt=('opt', 'pot', 'top'))
bidict({'opt': ('opt', 'pot', 'top')})

Values Must Be Unique

As we know, in a bidirectional map, not only must keys be unique, but values must be unique as well. This has immediate implications for bidict’s API.

Consider the following:

>>> b = bidict({'one': 1})
>>> b['two'] = 1  

What should happen next?

If the bidict allowed this to succeed, because of the uniqueness-of-values constraint, it would silently clobber the existing item, resulting in:

>>> b  
bidict({'two': 1})

This could result in surprises or problems down the line.

Instead, bidict raises a ValueDuplicationError so you have an opportunity to catch this early and resolve the conflict before it causes problems later on:

>>> b['two'] = 1
Traceback (most recent call last):
    ...
bidict.ValueDuplicationError: 1

The purpose of this is to be more in line with the Zen of Python, which advises,

Errors should never pass silently.

Unless explicitly silenced.

So if you really just want to clobber any existing items, all you have to do is say so explicitly:

>>> b.forceput('two', 1)
>>> b
bidict({'two': 1})

Similarly, initializations and update() calls that would overwrite the key of an existing value raise an exception too:

>>> bidict({'one': 1, 'uno': 1})
Traceback (most recent call last):
    ...
bidict.ValueDuplicationError: 1

>>> b = bidict({'one': 1})
>>> b.update([('uno', 1)])
Traceback (most recent call last):
    ...
bidict.ValueDuplicationError: 1

>>> b
bidict({'one': 1})

Setting an existing key to a new value does not cause an error, and is considered an intentional overwrite of the value associated with the existing key, in keeping with dict’s behavior:

>>> b = bidict({'one': 1})
>>> b['one'] = 2  # succeeds
>>> b
bidict({'one': 2})
>>> b.update([('one', 3), ('one', 4), ('one', 5)])
>>> b
bidict({'one': 5})
>>> bidict([('one', 1), ('one', 2)])
bidict({'one': 2})

In summary, when attempting to insert an item whose key duplicates an existing item’s, bidict’s default behavior is to allow the insertion, overwriting the existing item with the new one. When attempting to insert an item whose value duplicates an existing item’s, bidict’s default behavior is to raise. This design naturally falls out of the behavior of Python’s built-in dict, and protects against unexpected data loss.

One set of alternatives to this behavior is provided by forceput() (mentioned above) and forceupdate(), which allow you to explicitly overwrite existing keys and values:

>>> b = bidict({'one': 1})
>>> b.forceput('two', 1)
>>> b
bidict({'two': 1})

>>> b.forceupdate([('three', 1), ('four', 1)])
>>> b
bidict({'four': 1})

For even more control, you can use put() and putall(). These variants allow you to pass an OnDup instance to specify custom OnDupActions for each type of duplication that can occur.

>>> b = bidict({1: 'one'})
>>> b.put(1, 'uno', OnDup(key=RAISE))
Traceback (most recent call last):
    ...
bidict.KeyDuplicationError: 1
>>> b
bidict({1: 'one'})

bidict provides the ON_DUP_DEFAULT, ON_DUP_RAISE, and ON_DUP_DROP_OLD OnDup instances for convenience.

If no on_dup argument is passed, put() and putall() will use ON_DUP_RAISE, providing stricter-by-default alternatives to __setitem__() and update(). (These defaults complement the looser alternatives provided by forceput() and forceupdate().)

Key and Value Duplication

Note that it’s possible for a given item to duplicate the key of one existing item, and the value of another existing item. In the following example, the key of the third item duplicates the first item’s key, and the value of the third item dulicates the second item’s value:

b.putall([(1, 2), (3, 4), (1, 4)], OnDup(key=...))

What should happen next?

Keep in mind, the active OnDup may specify one OnDupAction for key duplication and a different OnDupAction for value duplication.

To account for this, OnDup allows you to use its kv field to indicate how you want to handle this case without ambiguity:

>>> from bidict import DROP_OLD
>>> on_dup = OnDup(key=DROP_OLD, val=RAISE, kv=RAISE)
>>> b.putall([(1, 2), (3, 4), (1, 4)], on_dup)
Traceback (most recent call last):
    ...
bidict.KeyAndValueDuplicationError: (1, 4)

If not specified, kv defaults to whatever was provided for val.

Note that repeated insertions of the same item are construed as a no-op and will not raise, no matter what the active OnDup is:

>>> b = bidict({1: 'one'})
>>> b.put(1, 'one')  # no-op, not a DuplicationError
>>> b.putall([(2, 'two'), (2, 'two')])  # The repeat (2, 'two') is also a no-op.
>>> sorted(b.items())
[(1, 'one'), (2, 'two')]

See the YoloBidict Recipe for another way to customize this behavior.

Updates Fail Clean

If an update to a bidict fails, you can be sure that it fails clean. In other words, a bidict will never apply only part of an update that ultimately fails, without restoring itself to the state it was in before processing the update:

>>> b = bidict({1: 'one', 2: 'two'})
>>> b.putall([(3, 'three'), (1, 'uno')])
Traceback (most recent call last):
    ...
bidict.KeyDuplicationError: 1

>>> # (1, 'uno') was the problem...
>>> b  # ...but (3, 'three') was not added either:
bidict({1: 'one', 2: 'two'})

Order Matters

Performing a bulk insert operation – i.e. passing multiple items to __init__(), update(), forceupdate(), or putall() – is like inserting each of those items individually in sequence. [1]

Therefore, the order of the items provided to the bulk insert operation is significant to the result:

>>> b = bidict({0: 0, 1: 2})
>>> b.forceupdate([(2, 0), (0, 1), (0, 0)])

>>> # 1. (2, 0) overwrites (0, 0)             -> bidict({2: 0, 1: 2})
>>> # 2. (0, 1) is added                      -> bidict({2: 0, 1: 2, 0: 1})
>>> # 3. (0, 0) overwrites (0, 1) and (2, 0)  -> bidict({0: 0, 1: 2})

>>> sorted(b.items())
[(0, 0), (1, 2)]

>>> b = bidict({0: 0, 1: 2})  # as before
>>> # Give the same items to forceupdate() but in a different order:
>>> b.forceupdate([(0, 1), (0, 0), (2, 0)])

>>> # 1. (0, 1) overwrites (0, 0)             -> bidict({0: 1, 1: 2})
>>> # 2. (0, 0) overwrites (0, 1)             -> bidict({0: 0, 1: 2})
>>> # 3. (2, 0) overwrites (0, 0)             -> bidict({1: 2, 2: 0})

>>> sorted(b.items())  # different items!
[(1, 2), (2, 0)]

[1]

Albeit with the extremely important advantage of failing clean.

Interop

bidicts interoperate well with other types of mappings. For example, they support (efficient) polymorphic equality testing:

>>> bidict(a=1) == dict(a=1)
True

And converting back and forth works as expected (assuming no value duplication):

>>> dict(bidict(a=1))
{'a': 1}
>>> bidict(dict(a=1))
bidict({'a': 1})

See the Polymorphism section for more interoperability documentation.

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Hopefully bidict feels right at home among the Python built-ins you already know. Proceed to Other bidict Types for documentation on the remaining bidict variants.