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c108.collections

Bidirectional mapping utilities providing a generic BiDirectionalMap with forward and reverse lookups.

BiDirectionalMap

Bases: Mapping[K, V], Generic[K, V]

A bidirectional mapping that maintains one-to-one correspondence between keys and values.

Implements the standardMapping protocol for forward lookups (key -> value) while providing efficient reverse lookups (value -> key). Both keys and values must be hashable and unique.

Examples:

>>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
>>> bimap['a']  # Forward lookup
1
>>> bimap.get_key(1)  # Reverse lookup
'a'
>>> 'a' in bimap  # Key membership
True
>>> bimap.has_value(1)  # Value membership
True
>>> bimap.to_dict()  # Extract as standarddict
{'a': 1, 'b': 2}
>>> bimap.reversed()  # Get reversed mapping
BiDirectionalMap({1: 'a', 2: 'b'})

Forward direction (key -> value): - Implements full Mapping protocol: getitem, iter, len, keys(), values(), items(), get() - Membership testing (x in bimap) applies to keys only, like dict

Reverse direction (value -> key): - get_key(value): lookup key by value (raises KeyError if not found) - has_value(value): test if value exists

Conversion and views
  • to_dict(): extract as standarddict (key -> value)
  • reversed(): create new BiDirectionalMap with swapped keys/values
Mutation operations maintain bidirectional consistency
  • add(key, value): add new mapping, both must be unique
  • set(key, value): set/update mapping for key, value must be unique
  • pop(key, default=None): remove and return value for key
  • delete(key): remove mapping for key
  • update(mapping): bulk update with uniqueness validation
  • clear(): remove all mappings

Raises:

Type Description
ValueError

When attempting to add duplicate keys or values
KeyError

When accessing non-existent keys or values
TypeError

When keys or values are not hashable
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class BiDirectionalMap(Mapping[K, V], Generic[K, V]):
    """
    A bidirectional mapping that maintains one-to-one correspondence between keys and values.

    Implements the standardMapping protocol for forward lookups (key -> value) while providing
    efficient reverse lookups (value -> key). Both keys and values must be hashable and unique.

    Examples:
        >>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
        >>> bimap['a']  # Forward lookup
        1
        >>> bimap.get_key(1)  # Reverse lookup
        'a'
        >>> 'a' in bimap  # Key membership
        True
        >>> bimap.has_value(1)  # Value membership
        True
        >>> bimap.to_dict()  # Extract as standarddict
        {'a': 1, 'b': 2}
        >>> bimap.reversed()  # Get reversed mapping
        BiDirectionalMap({1: 'a', 2: 'b'})

    Forward direction (key -> value):
        - Implements full Mapping protocol: __getitem__, __iter__, __len__, keys(), values(), items(), get()
        - Membership testing (x in bimap) applies to keys only, like dict

    Reverse direction (value -> key):
        - get_key(value): lookup key by value (raises KeyError if not found)
        - has_value(value): test if value exists

    Conversion and views:
        - to_dict(): extract as standarddict (key -> value)
        - reversed(): create new BiDirectionalMap with swapped keys/values

    Mutation operations maintain bidirectional consistency:
        - add(key, value): add new mapping, both must be unique
        - set(key, value): set/update mapping for key, value must be unique
        - pop(key, default=None): remove and return value for key
        - delete(key): remove mapping for key
        - update(mapping): bulk update with uniqueness validation
        - clear(): remove all mappings

    Raises:
        ValueError: When attempting to add duplicate keys or values
        KeyError: When accessing non-existent keys or values
        TypeError: When keys or values are not hashable
    """

    def __init__(self, initial: Mapping[K, V] | Iterable[tuple[K, V]] | None = None) -> None:
        """
        Initialize bidirectional map.

        Args:
            initial: Optional initial data as mapping or iterable of (key, value) pairs

        Raises:
            ValueError: If initial data contains duplicate keys or values
            TypeError: If keys or values are not hashable
        """
        self._forward_map: dict[K, V] = {}
        self._backward_map: dict[V, K] = {}
        if initial:
            self._init_from_data(initial)

    def _init_from_data(self, data: Mapping[K, V] | Iterable[tuple[K, V]]) -> None:
        """
        Initialize from data with strict duplicate validation.

        If the input `data` is another `BiDirectionalMap` instance, its internal maps are
        directly copied for efficiency, bypassing re-validation. Otherwise, it iterates
        through the data, validating uniqueness of keys and values.

        Args:
            data: Mapping or iterable of (key, value) pairs

        Raises:
            ValueError: If duplicate keys or values are found
        """
        if isinstance(data, BiDirectionalMap):
            # If initializing from another BiDirectionalMap, we can directly copy
            # its internal maps as uniqueness is already guaranteed.
            self._forward_map.update(data._forward_map)
            self._backward_map.update(data._backward_map)
        else:
            iterable = data.items() if isinstance(data, abc.Mapping) else data
            seen_keys: set[K] = set()
            seen_values: set[V] = set()

            for k, v in iterable:
                if k in seen_keys:
                    raise ValueError(f"Key already exists: {k!r}")
                if v in seen_values:
                    raise ValueError(f"Value already exists: {v!r}")
                seen_keys.add(k)
                seen_values.add(v)
                self._forward_map[k] = v
                self._backward_map[v] = k

    # ----- Mapping required methods -----

    def __getitem__(self, key: K) -> V:
        """Get value for key. Raises KeyError if key not found."""
        return self._forward_map[key]

    def __iter__(self) -> Iterator[K]:
        """Iterate over keys."""
        return iter(self._forward_map)

    def __len__(self) -> int:
        """Return number of key-value pairs."""
        return len(self._forward_map)

    def __contains__(self, key: object) -> bool:
        """Test if key exists (standardMapping behavior)."""
        return key in self._forward_map

    # ----- Mapping helpers (typed views) -----

    def keys(self) -> KeysView[K]:
        """Return view of keys."""
        return self._forward_map.keys()

    def values(self) -> ValuesView[V]:
        """Return view of values."""
        return self._forward_map.values()

    def items(self) -> ItemsView[K, V]:
        """Return view of (key, value) pairs."""
        return self._forward_map.items()

    def get(self, key: object, default: V | None = None) -> V | None:
        """Get value for key, returning default if key not found."""
        return self._forward_map.get(key, default)

    # ----- Bidirectional operations -----

    def get_value(self, key: object) -> V:
        """
        Get value for key (alias for __getitem__).

        Args:
            key: The key to look up

        Returns:
            The value associated with the key

        Raises:
            KeyError: If key not found
        """
        return self._forward_map[key]

    def get_key(self, value: object) -> K:
        """
        Get key for value (reverse lookup).

        Args:
            value: The value to look up

        Returns:
            The key associated with the value

        Raises:
            KeyError: If value not found
        """
        return self._backward_map[value]

    def has_value(self, value: object) -> bool:
        """
        Test if value exists in the mapping.

        Args:
            value: The value to test for

        Returns:
            True if value exists, False otherwise
        """
        return value in self._backward_map

    # ----- Conversion and views -----

    def to_dict(self) -> dict[K, V]:
        """
        Extract as standarddictionary (key -> value mapping).

        Returns a shallow copy of the forward mapping as a standarddict.
        Modifications to the returned dict do not affect this BiDirectionalMap.

        Returns:
            A new dict containing all key-value pairs

        Examples:
            >>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
            >>> d = bimap.to_dict()
            >>> d
            {'a': 1, 'b': 2}
            >>> d['c'] = 3  # Does not affect bimap
        """
        return dict(self._forward_map)

    def reversed(self) -> "BiDirectionalMap[V, K]":
        """
        Create new BiDirectionalMap with keys and values swapped.

        Returns a new BiDirectionalMap where the original values become keys
        and the original keys become values. The original mapping is unchanged.

        Returns:
            A new BiDirectionalMap with swapped key-value pairs

        Examples:
            >>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
            >>> rev = bimap.reversed()
            >>> rev.to_dict()
            {1: 'a', 2: 'b'}
            >>> rev[1]
            'a'
        """
        return BiDirectionalMap(self._backward_map)

    # ----- Mutations (keep both maps consistent) -----

    def add(self, key: K, value: V) -> None:
        """
        Add a new key-value pair. Both key and value must be unique.

        Args:
            key: The key to add
            value: The value to add

        Raises:
            ValueError: If key already exists or value already exists
            TypeError: If key or value is not hashable
        """
        if key in self._forward_map:
            raise ValueError(
                f"Key already exists: {fmt_any(key)} maps to {fmt_any(self._forward_map[key])})"
            )
        if value in self._backward_map:
            raise ValueError(
                f"Value already exists: {fmt_any(value)} mapped from {fmt_any(self._backward_map[value])})"
            )
        self._forward_map[key] = value
        self._backward_map[value] = key

    def set(self, key: K, value: V) -> None:
        """
        Set or update mapping for key. Value must be unique across all mappings.

        If key exists, its old value is released. The new value must not be
        used by any other key.

        Args:
            key: The key to set
            value: The value to associate with key

        Raises:
            ValueError: If value already exists (mapped from a different key)
            TypeError: If key or value is not hashable
        """
        if key in self._forward_map:
            old_value = self._forward_map[key]
            if old_value == value:
                return  # no-op
            if value in self._backward_map and self._backward_map[value] != key:
                raise ValueError(
                    f"Value already exists: {fmt_any(value)} mapped from {fmt_any(self._backward_map[value])}"
                )
            del self._backward_map[old_value]
        else:
            if value in self._backward_map:
                raise ValueError(
                    f"Value already exists: {fmt_any(value)} mapped from {fmt_any(self._backward_map[value])}"
                )

        self._forward_map[key] = value
        self._backward_map[value] = key

    @overload
    def pop(self, key: object) -> V: ...

    @overload
    def pop(self, key: object, default: V) -> V: ...

    def pop(self, key: object, default: V = MISSING) -> V:
        """
        Remove mapping for key and return its value.

        Args:
            key: The key to remove
            default: Value to return if key not found

        Returns:
            The value that was associated with key, or default if key not found

        Raises:
            KeyError: If key not found and no default provided
        """
        if key not in self._forward_map:
            if default is not MISSING:
                return default  # type: ignore[return-value]
            raise KeyError(key)
        value = self._forward_map.pop(key)
        del self._backward_map[value]
        return value

    def delete(self, key: object) -> None:
        """
        Remove mapping for key.

        Args:
            key: The key to remove

        Raises:
            KeyError: If key not found
        """
        value = self._forward_map.pop(key)  # type: ignore[arg-type]
        del self._backward_map[value]

    def clear(self) -> None:
        """Remove all mappings."""
        self._forward_map.clear()
        self._backward_map.clear()

    def update(self, other: Mapping[K, V] | Iterable[tuple[K, V]]) -> None:
        """
        Update with key-value pairs from another mapping or iterable.

        Uses set() semantics: existing keys are updated, new keys are added.
        All values must satisfy uniqueness constraints.

        Args:
            other: Mapping or iterable of (key, value) pairs

        Raises:
            ValueError: If any value would violate uniqueness constraints
            TypeError: If any key or value is not hashable
        """
        iterable = other.items() if isinstance(other, abc.Mapping) else other
        for k, v in iterable:
            self.set(k, v)

    # ----- Equality and representation -----

    def __repr__(self) -> str:
        """Return string representation."""
        return f"BiDirectionalMap({self._forward_map!r})"

    def __eq__(self, other: Any) -> bool:
        """Test equality with another mapping based on key-value pairs."""
        if isinstance(other, abc.Mapping):
            return dict(self._forward_map.items()) == dict(other.items())
        return NotImplemented

__contains__(key)

Test if key exists (standardMapping behavior).

Source code in c108/collections.py 
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def __contains__(self, key: object) -> bool:
    """Test if key exists (standardMapping behavior)."""
    return key in self._forward_map

__eq__(other)

Test equality with another mapping based on key-value pairs.

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def __eq__(self, other: Any) -> bool:
    """Test equality with another mapping based on key-value pairs."""
    if isinstance(other, abc.Mapping):
        return dict(self._forward_map.items()) == dict(other.items())
    return NotImplemented

__getitem__(key)

Get value for key. Raises KeyError if key not found.

Source code in c108/collections.py 
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def __getitem__(self, key: K) -> V:
    """Get value for key. Raises KeyError if key not found."""
    return self._forward_map[key]

__init__(initial=None)

Initialize bidirectional map.

Parameters:

Name Type Description Default
initial Mapping[K, V] | Iterable[tuple[K, V]] | None

Optional initial data as mapping or iterable of (key, value) pairs

 None

Raises:

Type Description
ValueError

If initial data contains duplicate keys or values
TypeError

If keys or values are not hashable
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def __init__(self, initial: Mapping[K, V] | Iterable[tuple[K, V]] | None = None) -> None:
    """
    Initialize bidirectional map.

    Args:
        initial: Optional initial data as mapping or iterable of (key, value) pairs

    Raises:
        ValueError: If initial data contains duplicate keys or values
        TypeError: If keys or values are not hashable
    """
    self._forward_map: dict[K, V] = {}
    self._backward_map: dict[V, K] = {}
    if initial:
        self._init_from_data(initial)

__iter__()

Iterate over keys.

Source code in c108/collections.py 
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def __iter__(self) -> Iterator[K]:
    """Iterate over keys."""
    return iter(self._forward_map)

__len__()

Return number of key-value pairs.

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def __len__(self) -> int:
    """Return number of key-value pairs."""
    return len(self._forward_map)

__repr__()

Return string representation.

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def __repr__(self) -> str:
    """Return string representation."""
    return f"BiDirectionalMap({self._forward_map!r})"

add(key, value)

Add a new key-value pair. Both key and value must be unique.

Parameters:

Name Type Description Default
key K

The key to add

 required
value V

The value to add

 required

Raises:

Type Description
ValueError

If key already exists or value already exists
TypeError

If key or value is not hashable
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def add(self, key: K, value: V) -> None:
    """
    Add a new key-value pair. Both key and value must be unique.

    Args:
        key: The key to add
        value: The value to add

    Raises:
        ValueError: If key already exists or value already exists
        TypeError: If key or value is not hashable
    """
    if key in self._forward_map:
        raise ValueError(
            f"Key already exists: {fmt_any(key)} maps to {fmt_any(self._forward_map[key])})"
        )
    if value in self._backward_map:
        raise ValueError(
            f"Value already exists: {fmt_any(value)} mapped from {fmt_any(self._backward_map[value])})"
        )
    self._forward_map[key] = value
    self._backward_map[value] = key

clear()

Remove all mappings.

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def clear(self) -> None:
    """Remove all mappings."""
    self._forward_map.clear()
    self._backward_map.clear()

delete(key)

Remove mapping for key.

Parameters:

Name Type Description Default
key object

The key to remove

 required

Raises:

Type Description
KeyError

If key not found
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def delete(self, key: object) -> None:
    """
    Remove mapping for key.

    Args:
        key: The key to remove

    Raises:
        KeyError: If key not found
    """
    value = self._forward_map.pop(key)  # type: ignore[arg-type]
    del self._backward_map[value]

get(key, default=None)

Get value for key, returning default if key not found.

Source code in c108/collections.py 
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def get(self, key: object, default: V | None = None) -> V | None:
    """Get value for key, returning default if key not found."""
    return self._forward_map.get(key, default)

get_key(value)

Get key for value (reverse lookup).

Parameters:

Name Type Description Default
value object

The value to look up

 required

Returns:

Type Description
K

The key associated with the value

Raises:

Type Description
KeyError

If value not found
Source code in c108/collections.py 
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def get_key(self, value: object) -> K:
    """
    Get key for value (reverse lookup).

    Args:
        value: The value to look up

    Returns:
        The key associated with the value

    Raises:
        KeyError: If value not found
    """
    return self._backward_map[value]

get_value(key)

Get value for key (alias for getitem).

Parameters:

Name Type Description Default
key object

The key to look up

 required

Returns:

Type Description
V

The value associated with the key

Raises:

Type Description
KeyError

If key not found
Source code in c108/collections.py 
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def get_value(self, key: object) -> V:
    """
    Get value for key (alias for __getitem__).

    Args:
        key: The key to look up

    Returns:
        The value associated with the key

    Raises:
        KeyError: If key not found
    """
    return self._forward_map[key]

has_value(value)

Test if value exists in the mapping.

Parameters:

Name Type Description Default
value object

The value to test for

 required

Returns:

Type Description
bool

True if value exists, False otherwise
Source code in c108/collections.py 
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def has_value(self, value: object) -> bool:
    """
    Test if value exists in the mapping.

    Args:
        value: The value to test for

    Returns:
        True if value exists, False otherwise
    """
    return value in self._backward_map

items()

Return view of (key, value) pairs.

Source code in c108/collections.py 
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def items(self) -> ItemsView[K, V]:
    """Return view of (key, value) pairs."""
    return self._forward_map.items()

keys()

Return view of keys.

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def keys(self) -> KeysView[K]:
    """Return view of keys."""
    return self._forward_map.keys()

pop(key, default=MISSING) 

pop(key: object) -> V

pop(key: object, default: V) -> V

Remove mapping for key and return its value.

Parameters:

Name Type Description Default
key object

The key to remove

 required
default V

Value to return if key not found

 MISSING

Returns:

Type Description
V

The value that was associated with key, or default if key not found

Raises:

Type Description
KeyError

If key not found and no default provided
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def pop(self, key: object, default: V = MISSING) -> V:
    """
    Remove mapping for key and return its value.

    Args:
        key: The key to remove
        default: Value to return if key not found

    Returns:
        The value that was associated with key, or default if key not found

    Raises:
        KeyError: If key not found and no default provided
    """
    if key not in self._forward_map:
        if default is not MISSING:
            return default  # type: ignore[return-value]
        raise KeyError(key)
    value = self._forward_map.pop(key)
    del self._backward_map[value]
    return value

reversed()

Create new BiDirectionalMap with keys and values swapped.

Returns a new BiDirectionalMap where the original values become keys and the original keys become values. The original mapping is unchanged.

Returns:

Type Description
BiDirectionalMap[V, K]

A new BiDirectionalMap with swapped key-value pairs

Examples:

>>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
>>> rev = bimap.reversed()
>>> rev.to_dict()
{1: 'a', 2: 'b'}
>>> rev[1]
'a'
Source code in c108/collections.py 
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def reversed(self) -> "BiDirectionalMap[V, K]":
    """
    Create new BiDirectionalMap with keys and values swapped.

    Returns a new BiDirectionalMap where the original values become keys
    and the original keys become values. The original mapping is unchanged.

    Returns:
        A new BiDirectionalMap with swapped key-value pairs

    Examples:
        >>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
        >>> rev = bimap.reversed()
        >>> rev.to_dict()
        {1: 'a', 2: 'b'}
        >>> rev[1]
        'a'
    """
    return BiDirectionalMap(self._backward_map)

set(key, value)

Set or update mapping for key. Value must be unique across all mappings.

If key exists, its old value is released. The new value must not be used by any other key.

Parameters:

Name Type Description Default
key K

The key to set

 required
value V

The value to associate with key

 required

Raises:

Type Description
ValueError

If value already exists (mapped from a different key)
TypeError

If key or value is not hashable
Source code in c108/collections.py 
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def set(self, key: K, value: V) -> None:
    """
    Set or update mapping for key. Value must be unique across all mappings.

    If key exists, its old value is released. The new value must not be
    used by any other key.

    Args:
        key: The key to set
        value: The value to associate with key

    Raises:
        ValueError: If value already exists (mapped from a different key)
        TypeError: If key or value is not hashable
    """
    if key in self._forward_map:
        old_value = self._forward_map[key]
        if old_value == value:
            return  # no-op
        if value in self._backward_map and self._backward_map[value] != key:
            raise ValueError(
                f"Value already exists: {fmt_any(value)} mapped from {fmt_any(self._backward_map[value])}"
            )
        del self._backward_map[old_value]
    else:
        if value in self._backward_map:
            raise ValueError(
                f"Value already exists: {fmt_any(value)} mapped from {fmt_any(self._backward_map[value])}"
            )

    self._forward_map[key] = value
    self._backward_map[value] = key

to_dict()

Extract as standarddictionary (key -> value mapping).

Returns a shallow copy of the forward mapping as a standarddict. Modifications to the returned dict do not affect this BiDirectionalMap.

Returns:

Type Description
dict[K, V]

A new dict containing all key-value pairs

Examples:

>>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
>>> d = bimap.to_dict()
>>> d
{'a': 1, 'b': 2}
>>> d['c'] = 3  # Does not affect bimap
Source code in c108/collections.py 
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def to_dict(self) -> dict[K, V]:
    """
    Extract as standarddictionary (key -> value mapping).

    Returns a shallow copy of the forward mapping as a standarddict.
    Modifications to the returned dict do not affect this BiDirectionalMap.

    Returns:
        A new dict containing all key-value pairs

    Examples:
        >>> bimap = BiDirectionalMap({'a': 1, 'b': 2})
        >>> d = bimap.to_dict()
        >>> d
        {'a': 1, 'b': 2}
        >>> d['c'] = 3  # Does not affect bimap
    """
    return dict(self._forward_map)

update(other)

Update with key-value pairs from another mapping or iterable.

Uses set() semantics: existing keys are updated, new keys are added. All values must satisfy uniqueness constraints.

Parameters:

Name Type Description Default
other Mapping[K, V] | Iterable[tuple[K, V]]

Mapping or iterable of (key, value) pairs

 required

Raises:

Type Description
ValueError

If any value would violate uniqueness constraints
TypeError

If any key or value is not hashable
Source code in c108/collections.py 
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def update(self, other: Mapping[K, V] | Iterable[tuple[K, V]]) -> None:
    """
    Update with key-value pairs from another mapping or iterable.

    Uses set() semantics: existing keys are updated, new keys are added.
    All values must satisfy uniqueness constraints.

    Args:
        other: Mapping or iterable of (key, value) pairs

    Raises:
        ValueError: If any value would violate uniqueness constraints
        TypeError: If any key or value is not hashable
    """
    iterable = other.items() if isinstance(other, abc.Mapping) else other
    for k, v in iterable:
        self.set(k, v)

values()

Return view of values.

Source code in c108/collections.py 
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def values(self) -> ValuesView[V]:
    """Return view of values."""
    return self._forward_map.values()