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Operator to construct nested rollups from lists of records.

Project description

This module is a Python port of the nest operator from Mike Bostock’s d3.js.

Nest allows elements in an array to be grouped into a hierarchical tree structure; think of it like the GROUP BY operator in SQL, except you can have multiple levels of grouping, and the resulting output is a tree rather than a flat table. The levels in the tree are specified by key functions. The leaf nodes of the tree can be sorted by value, while the internal nodes can be sorted by key. An optional rollup function will collapse the elements in each leaf node using a summary function. The nest operator (the object returned by d3.nest) is reusable, and does not retain any references to the data that is nested.

For example, consider the following tabular data structure of Barley yields, from various sites in Minnesota during 1931-2:

>>> yields = [
    {"yield": 27.00, "variety": "Manchuria", "year": 1931, "site": "University Farm"},
    {"yield": 48.87, "variety": "Manchuria", "year": 1931, "site": "Waseca"},
    {"yield": 27.43, "variety": "Manchuria", "year": 1931, "site": "Morris"},
    {"yield": 43.07, "variety": "Glabron",   "year": 1931, "site": "University Farm"},
    {"yield": 55.20, "variety": "Glabron",   "year": 1931, "site": "Waseca"},
    {"yield": 16.18, "variety": "Glabron",   "year": 1932, "site": "University Farm"},
]

To facilitate visualization, it may be useful to nest the elements first by year, and then by variety, as follows:

>>> from nesting import Nest
>>> (Nest()
...     .key( lambda d: d['year'] )
...     .key( lambda d: d['variety'] )
...     .entries(yields))

Or more concisely:

>>> (Nest()
...     .key('year')
...     .key('variety')
...     .entries(yields))

…as both the key and prop functions will interpret non-callables as they key to look up.

This returns a nested array. Each element of the outer array is a key-values pair, listing the values for each distinct key:

[   {"key": 1931, "values": [
        {"key": "Manchuria", "values": [
            {"yield": 27.00, "variety": "Manchuria", "year": 1931, "site": "University Farm"},
            {"yield": 48.87, "variety": "Manchuria", "year": 1931, "site": "Waseca"},
            {"yield": 27.43, "variety": "Manchuria", "year": 1931, "site": "Morris"}, ]},
        {"key": "Glabron", "values": [
            {"yield": 43.07, "variety": "Glabron", "year": 1931, "site": "University Farm"},
            {"yield": 55.20, "variety": "Glabron", "year": 1931, "site": "Waseca"}, ]},
    ]},
    {"key": 1932, "values": [
        {"key": "Glabron", "values": [
            {"yield": 16.18, "variety": "Glabron", "year": 1932, "site": "University Farm"}, ]},
    ]},
]

The nested form allows easy iteration and generation of hierarchical structures in SVG or HTML.

This port is open-source, freely licensed under the MIT License.

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