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Operators

Abstract

Operators are functions that perform operations on Mappings.

combine

Combines two recursive tree structures using a binary operator op to resolve conflicts at the leaf nodes.

This is a powerful generalization of merge. It recursively walks two tree structures and applies the op only when it encounters a conflict at the leaf nodes (e.g., two scalars at the same path, or a structural mismatch like a dict vs a list).

You can pass a custom callable (old, new) -> resolved or use one of the pre-built strategies from the resolver enums: Resolver (structural), LogicalResolver (bitwise/sets), or NumericResolver (math).

If you just need the standard "last-wins" behavior, you can use the simpler merge operator, which is essentially combine(tree1, tree2, op=Resolver.LAST), barring the difference in handling list vs. scalars conflicts.

Combining with different conflict resolutions

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from mappingtools.operators import combine
from mappingtools.resolvers import NumericResolver, Resolver

tree1 = {"a": 1, "b": {"c": 10}}
tree2 = {"a": 2, "b": {"c": 20}, "d": 5}

# 1. Sum conflicts
summed = combine(tree1, tree2, op=NumericResolver.SUM)
print(summed)
# output: {'a': 3, 'b': {'c': 30}, 'd': 5}

# 2. Keep original values on conflict
first_wins = combine(tree1, tree2, op=Resolver.FIRST)
print(first_wins)
# output: {'a': 1, 'b': {'c': 10}, 'd': 5}

distinct

Yields distinct values for a specified key across multiple mappings.

Example

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from mappingtools.operators import distinct

mappings = [
    {'a': 1, 'b': 2},
    {'a': 2, 'b': 3},
    {'a': 1, 'b': 4}
]
distinct_values = list(distinct('a', *mappings))
print(distinct_values)
# output: [1, 2]

flatten

The flatten function takes a nested mapping structure and converts it into a single-level dictionary by flattening the keys into tuples.

Example

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from mappingtools.operators import flatten

nested_dict = {
    'a': {'b': 1, 'c': {'d': 2}},
    'e': 3
}
flat_dict = flatten(nested_dict)
print(flat_dict)
# output: {('a', 'b'): 1, ('a', 'c', 'd'): 2, ('e',): 3}

inverse

Swaps keys and values in a dictionary.

Example

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from mappingtools.operators import inverse

original_mapping = {'a': {1, 2}, 'b': {3}}
inverted_mapping = inverse(original_mapping)
print(inverted_mapping)
# output: defaultdict(<class 'set'>, {1: {'a'}, 2: {'a'}, 3: {'b'}})

merge

A pure function (Monoid operation) to deeply merge two recursive tree structures. The merging strategy resolves conflicts by overwriting existing values with new ones (right-side precedence), unless the conflict is a list vs. scalar, in which case it concatenates (appends/prepends) the list.

Mathematically, this operation forms a composite Monoid:

  • Last Monoid (Scalar Fallback): When resolving conflicts between simple values, the right-hand side (tree2) wins.
  • Pointwise Monoid (Dictionary Merge): If the values are dictionaries, they are merged by key, recursively calling merge on the values.
  • Zip Monoid (List Merge): If both are lists, they are zipped and merged positionally, substituting MISSING for missing indices.
  • Free Monoid (Mixed List/Scalar): If one is a list and the other is a scalar/dict, it concatenates (appends/prepends).

Because it forms a Monoid, this function can be used with functools.reduce to collect an iterable of trees into a single structure.

Merging two trees directly

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from mappingtools.operators import merge

tree1 = {"a": 1, "b": [1, 2]}
tree2 = {"b": [3], "c": 4}

merged = merge(tree1, tree2)
print(merged)
# output: {'a': 1, 'b': [3, 2], 'c': 4}

Reducing an iterable of trees

Using Python's standard functools.reduce, we can easily merge an entire sequence of nested structures.

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from functools import reduce
from mappingtools.operators import merge

trees = [
    {"a": 1, "b": {"c": 2}},
    {"b": {"d": 3}},
    {"a": 10}, # Overwrites previous "a"
]

merged = reduce(merge, trees)
print(merged)
# output: {'a': 10, 'b': {'c': 2, 'd': 3}}

Deep merging with Lenses

If you need to merge data into a specific, deeply nested location of a larger tree, you can compose the merge function with an Optic (Lens). This avoids modifying the pure merge function with path traversal logic.

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from mappingtools.operators import merge
from mappingtools.optics import Lens

system_state = {"system": {"config": {"retries": 3}}}
new_config = {"timeout": 30}

# Focus specifically on the 'config' node inside 'system'
config_lens = Lens.path("system", "config")

# Apply the merge function OVER the focused node
new_state = config_lens.modify(
    system_state, 
    lambda old: merge(old, new_config)
)

print(new_state)
# output: {'system': {'config': {'retries': 3, 'timeout': 30}}}

pivot

Reshapes a list of mappings into a nested dictionary based on index and column keys. Supports different aggregation modes via Aggregation.

Example

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from mappingtools.operators import pivot
from mappingtools.aggregations import Aggregation

data = [
    {"city": "NYC", "month": "Jan", "temp": 10},
    {"city": "NYC", "month": "Feb", "temp": 12},
    {"city": "LON", "month": "Jan", "temp": 5},
    {"city": "NYC", "month": "Jan", "temp": 20}, # Duplicate
]

# Default mode (LAST wins)
result = pivot(data, index="city", columns="month", values="temp")
print(result)
# output: {'NYC': {'Jan': 20, 'Feb': 12}, 'LON': {'Jan': 5}}

# Aggregation mode: ALL (collect list)
result_all = pivot(data, index="city", columns="month", values="temp", aggregation=Aggregation.ALL)
print(result_all["NYC"]["Jan"])
# output: [10, 20]

reshape

A generalization of pivot that creates nested dictionaries (tensors) of arbitrary depth. While pivot is limited to 2 dimensions (Index, Columns), reshape accepts a sequence of keys to define the hierarchy.

Example

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from mappingtools.operators import reshape
from mappingtools.aggregations import Aggregation

data = [
    {"country": "US", "state": "NY", "city": "NYC", "pop": 8.4},
    {"country": "US", "state": "CA", "city": "LA", "pop": 3.9},
    {"country": "UK", "state": "ENG", "city": "LON", "pop": 8.9},
    {"country": "US", "state": "NY", "city": "Albany", "pop": 0.1},
]

# 3-Level Hierarchy: Country -> State -> City
tree = reshape(data, keys=["country", "state", "city"], value="pop")

print(tree["US"]["NY"]["NYC"])
# output: 8.4

# Aggregation: Sum population by Country -> State
# (City is marginalized/ignored)
state_pop = reshape(
    data, 
    keys=["country", "state"], 
    value="pop", 
    aggregation=Aggregation.SUM
)

print(state_pop["US"]["NY"])
# output: 8.5

# Deep Keys (using Lenses or Callables)
# If your data is nested, you can use callables to extract keys.
# This works perfectly with the library's `Lens` or standard `operator.itemgetter`.

nested_data = [
    {"id": 1, "meta": {"region": "US"}, "val": 10},
    {"id": 2, "meta": {"region": "UK"}, "val": 20},
]

# Group by meta.region
deep_tree = reshape(
    nested_data, 
    keys=[lambda x: x["meta"]["region"]], 
    value="val"
)
# output: {'US': 10, 'UK': 20}

rekey

Transforms keys of a mapping based on a factory function of (key, value). This allows "re-indexing" a mapping where the new key depends on the content of the value or a combination of the old key and value. Collisions are handled according to the specified aggregation.

Example

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from mappingtools.operators import rekey
from mappingtools.aggregations import Aggregation

mapping = {
    "alice": {"dept": "IT", "id": 1},
    "bob": {"dept": "HR", "id": 2},
    "charlie": {"dept": "IT", "id": 3},
}

# Re-index by 'id'
by_id = rekey(mapping, lambda k, v: v["id"])
print(by_id[1])
# output: {'dept': 'IT', 'id': 1}

# Group by 'dept' using Aggregation.ALL
by_dept = rekey(mapping, lambda k, v: v["dept"], aggregation=Aggregation.ALL)
print(list(by_dept.keys()))
# output: ['IT', 'HR']
print(len(by_dept["IT"]))
# output: 2

rename

Renames keys in a mapping based on a mapper (Mapping or Callable). If a key is not present in the mapper, it remains unchanged. Collisions are handled according to the specified aggregation.

Example

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from mappingtools.operators import rename

data = {"usr_id": 1, "usr_name": "Alice", "email": "alice@example.com"}

# Using a mapping
renamed = rename(data, {"usr_id": "id", "usr_name": "name"})
print(list(renamed.keys()))
# output: ['id', 'name', 'email']

# Using a callable
renamed_upper = rename(data, str.upper)
print(list(renamed_upper.keys()))
# output: ['USR_ID', 'USR_NAME', 'EMAIL']