transform-tabular

Apply a single function element-wise across an entire DataFrame, or a selection of its columns.

Two special markers extend this to column-aware operations:

• ColumnwiseValue(func) — computes a scalar aggregate per column (e.g. mean, max). The scalar is then used in the element-wise expression, so every row in that column sees the same aggregate.
• ColumnwiseThread(func) — applies a list-to-list transformation per column (e.g. sort, cumulative sum). Each row receives the corresponding element from the transformed list.

This package is a Python port of the Wolfram Language resource function TransformTabular.

Usage

from transform_tabular import transform_tabular, ColumnwiseValue, ColumnwiseThread
import pandas as pd

Syntax

transform_tabular(df, func)                # apply func element-wise to all columns
transform_tabular(df, func, columns)       # apply func only to selected columns
transform_tabular(func)                    # operator form: returns a reusable transformer
transform_tabular(func, columns)           # operator form with column selection

Parameters

The function func is applied element-wise. The optional third argument can be either a list of columns, a list of column indices, a single column, or a slice.

Basic transformation

Increment all numeric columns by 1:

df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
transform_tabular(df, lambda x: x + 1)
#    a  b
# 0  2  5
# 1  3  6
# 2  4  7

Column selection

Transform only specific columns:

df = pd.DataFrame({"a": [1, 2, 3], "b": [10, 20, 30], "c": [100, 200, 300]})
transform_tabular(df, lambda x: x * 2, ["a", "c"])
#    a   b    c
# 0  2  10  200
# 1  4  20  400
# 2  6  30  600

ColumnwiseValue (column-level aggregation)

ColumnwiseValue(func) wraps a function func(column_as_list) -> scalar. The scalar is pre-computed per column and then participates in the element-wise arithmetic — every row in a given column sees that column's aggregate.

Subtract the mean from each element (centering):

df = pd.DataFrame({"x": [1, 2, 3, 4, 5], "y": [10, 20, 30, 40, 50]})
cv_mean = ColumnwiseValue(lambda col: sum(col) / len(col))
transform_tabular(df, lambda x: x - cv_mean)
#      x     y
# 0 -2.0 -20.0
# 1 -1.0 -10.0
# 2  0.0   0.0
# 3  1.0  10.0
# 4  2.0  20.0

ColumnwiseThread (column-level transformation)

ColumnwiseThread(func) wraps a function func(column_as_list) -> list_of_same_length. The transformation is pre-computed per column and each row receives its corresponding element from the resulting list.

Compute a cumulative sum for each column independently:

from itertools import accumulate

df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
ct_acc = ColumnwiseThread(lambda col: list(accumulate(col)))
transform_tabular(df, lambda x: ct_acc)
#    a   b
# 0  1   4
# 1  3   9
# 2  6  15

Combined ColumnwiseValue and ColumnwiseThread

Both markers can be used together. For example, compute the cumulative sum of each column and then subtract its mean:

from itertools import accumulate

df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
ct_acc = ColumnwiseThread(lambda col: list(accumulate(col)))
cv_mean = ColumnwiseValue(lambda col: sum(col) / len(col))
transform_tabular(df, lambda x: ct_acc - cv_mean)
#      a     b
# 0 -1.0  -1.0
# 1  1.0   4.0
# 2  4.0  10.0

Operator form

transform_tabular can be curried to produce a reusable transformer:

double_all = transform_tabular(lambda x: x * 2)
double_all(pd.DataFrame({"a": [1, 2], "b": [3, 4]}))
#    a  b
# 0  2  6
# 1  4  8

See also

For further examples and details, see the documentation for the original Wolfram Language resource function: TransformTabular.

Author

Daniele Gregori

License

MIT