col2dict 0.8.1

Create nested dictionaries from columns of tabular data

col2dict

Create nested dictionaries from columns of tabular data.

Python translation of the Wolfram resource function "AssociateColumns".

Installation

pip install col2dict

Overview

Data is well collected in DataFrames, but its manipulation and analysis often requires the use of nested dictionaries. associate_columns allows you to create (possibly nested) dictionaries from columns of a DataFrame or a list of dicts, with control over how duplicate keys are merged at each nesting level.

Usage

from col2dict import associate_columns

Signatures

associate_columns(tab, (col1, col2))
associate_columns(tab, (col1, col2), merge=func)
associate_columns(tab, (col1, col2, col3, ...))
associate_columns(tab, (col1, col2, col3, ...), merge=func)
associate_columns(tab, (col1, col2, col3, ...), merge=[f12, f23, ...])
associate_columns(tab, ([colA, colB], col2))          # multi-column keys
associate_columns(tab, (col1, [colA, colB]))           # multi-column values
associate_columns(tab, ([colA, colB], [colC, colD]))   # both

Parameters

Parameter	Description
tab	A pandas.DataFrame or a list of dicts.
cols	A tuple/list of column specs. Each element is a column name (str) or a list of column names. With 2 elements: first → keys, second → values. With 3+ elements: creates nested dicts.
merge	Merging function(s) for duplicate keys. None (default): auto-merge to list with a warning. A single callable: applied at every level. A list of callables: one per nesting transition.

Options

Option	Default	Description
duplicates_warning	True	Warns when duplicate keys are found and no merge function is provided. Set to False to silence the warning and collect values into lists silently.

Examples

Basic: two columns

import pandas as pd
from col2dict import associate_columns

df = pd.DataFrame({
    "Name": ["Alice", "Bob", "Carol"],
    "Age": [30, 25, 35],
})

associate_columns(df, ("Name", "Age"))
# {'Alice': 30, 'Bob': 25, 'Carol': 35}

Duplicate keys with merging

df = pd.DataFrame({
    "Year": [2020, 2020, 2021, 2021, 2022],
    "Name": ["Alice", "Bob", "Carol", "Dave", "Eve"],
    "Score": [90, 85, 92, 88, 95],
})

associate_columns(df, ("Year", "Name"), merge=sorted)
# {2020: ['Alice', 'Bob'], 2021: ['Carol', 'Dave'], 2022: ['Eve']}

associate_columns(df, ("Year", "Score"), merge=sum)
# {2020: 175, 2021: 180, 2022: 95}

Nested association (3+ columns)

associate_columns(df, ("Year", "Name", "Score"))
# {2020: {'Alice': 90, 'Bob': 85},
#  2021: {'Carol': 92, 'Dave': 88},
#  2022: {'Eve': 95}}

Multi-column keys

df = pd.DataFrame({
    "Store": ["A", "A", "B"],
    "Dept": ["Elec", "Food", "Elec"],
    "Revenue": [500, 100, 450],
})

associate_columns(df, (["Store", "Dept"], "Revenue"))
# {('A', 'Elec'): 500, ('A', 'Food'): 100, ('B', 'Elec'): 450}

Multi-column values

df = pd.DataFrame({
    "Name": ["Alice", "Bob"],
    "Age": [30, 25],
    "City": ["NYC", "LA"],
})

associate_columns(df, ("Name", ["Age", "City"]))
# {'Alice': [30, 'NYC'], 'Bob': [25, 'LA']}

Per-level merge functions

df = pd.DataFrame({
    "A": [1, 1, 1, 2],
    "B": ["x", "x", "y", "z"],
    "C": [100, 200, 300, 400],
})

# sorted at level 1 (sort inner dict keys), sum at level 2 (merge dup values)
associate_columns(df, ("A", "B", "C"), merge=[sorted, sum])
# {1: {'x': 300, 'y': 300}, 2: {'z': 400}}

Deep nesting with mixed multi-column specs

df = pd.DataFrame({
    "Store": ["A", "A", "B", "B"],
    "Dept": ["Elec", "Food", "Elec", "Elec"],
    "Item": ["TV", "Milk", "TV", "TV"],
    "Brand": ["Sony", "Org", "LG", "Sony"],
    "Price": [500, 3, 450, 500],
})

# Store -> {Dept, Item} -> Brand -> Price
associate_columns(df, ("Store", ["Dept", "Item"], "Brand", "Price"))
# {'A': {('Elec', 'TV'): {'Sony': 500}, ('Food', 'Milk'): {'Org': 3}},
#  'B': {('Elec', 'TV'): {'LG': 450, 'Sony': 500}}}

For full documentation and more examples, see the original Wolfram Language resource page, contributed by the same author and vetted by the Wolfram Review Team.

Testing

The test suite lives in tests/test_associate_columns.py and uses pytest. To run it:

pip install col2dict[dev]
pytest

Or from a source checkout:

git clone https://github.com/Daniele-Gregori/PyPI-packages.git
cd PyPI-packages/packages/col2dict
pip install -e ".[dev]"
pytest

The suite includes 68 tests organised in 14 groups covering basic associations, duplicate-key merging, nested dicts up to 5 levels deep, multi-column keys and values, per-level merge functions, input-type compatibility, edge cases, and error handling.

License

MIT — see LICENSE.