pandas.core.groupby.DataFrameGroupBy.transform

DataFrameGroupBy.transform(func, *args, engine=None, engine_kwargs=None, **kwargs)[source]

Call function producing a same-indexed DataFrame on each group.

Returns a DataFrame having the same indexes as the original object filled with the transformed values.

Parameters:
ffunction

Function to apply to each group. See the Notes section below for requirements.

Can also accept a Numba JIT function with engine='numba' specified.

If the 'numba' engine is chosen, the function must be a user defined function with values and index as the first and second arguments respectively in the function signature. Each group’s index will be passed to the user defined function and optionally available for use.

Changed in version 1.1.0.

*args

Positional arguments to pass to func.

enginestr, default None

  • 'cython' : Runs the function through C-extensions from cython.

  • 'numba' : Runs the function through JIT compiled code from numba.

  • None : Defaults to 'cython' or the global setting compute.use_numba

New in version 1.1.0.

engine_kwargsdict, default None

  • For 'cython' engine, there are no accepted engine_kwargs

  • For 'numba' engine, the engine can accept nopython, nogil and parallel dictionary keys. The values must either be True or False. The default engine_kwargs for the 'numba' engine is {'nopython': True, 'nogil': False, 'parallel': False} and will be applied to the function

New in version 1.1.0.

**kwargs

Keyword arguments to be passed into func.

Returns:
DataFrame

See also

DataFrame.groupby.apply

Apply function func group-wise and combine the results together.

DataFrame.groupby.aggregate

Aggregate using one or more operations over the specified axis.

DataFrame.transform

Call func on self producing a DataFrame with the same axis shape as self.

Notes

Each group is endowed the attribute ‘name’ in case you need to know which group you are working on.

The current implementation imposes three requirements on f:

  • f must return a value that either has the same shape as the input subframe or can be broadcast to the shape of the input subframe. For example, if f returns a scalar it will be broadcast to have the same shape as the input subframe.

  • if this is a DataFrame, f must support application column-by-column in the subframe. If f also supports application to the entire subframe, then a fast path is used starting from the second chunk.

  • f must not mutate groups. Mutation is not supported and may produce unexpected results. See Mutating with User Defined Function (UDF) methods for more details.

When using engine='numba', there will be no “fall back” behavior internally. The group data and group index will be passed as numpy arrays to the JITed user defined function, and no alternative execution attempts will be tried.

Changed in version 1.3.0: The resulting dtype will reflect the return value of the passed func, see the examples below.

Deprecated since version 1.5.0: When using .transform on a grouped DataFrame and the transformation function returns a DataFrame, currently pandas does not align the result’s index with the input’s index. This behavior is deprecated and alignment will be performed in a future version of pandas. You can apply .to_numpy() to the result of the transformation function to avoid alignment.

Examples

>>> df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
...                           'foo', 'bar'],
...                    'B' : ['one', 'one', 'two', 'three',
...                           'two', 'two'],
...                    'C' : [1, 5, 5, 2, 5, 5],
...                    'D' : [2.0, 5., 8., 1., 2., 9.]})
>>> grouped = df.groupby('A')[['C', 'D']]
>>> grouped.transform(lambda x: (x - x.mean()) / x.std())
          C         D
0 -1.154701 -0.577350
1  0.577350  0.000000
2  0.577350  1.154701
3 -1.154701 -1.000000
4  0.577350 -0.577350
5  0.577350  1.000000

Broadcast result of the transformation

>>> grouped.transform(lambda x: x.max() - x.min())
     C    D
0  4.0  6.0
1  3.0  8.0
2  4.0  6.0
3  3.0  8.0
4  4.0  6.0
5  3.0  8.0

Changed in version 1.3.0: The resulting dtype will reflect the return value of the passed func, for example:

>>> grouped.transform(lambda x: x.astype(int).max())
   C  D
0  5  8
1  5  9
2  5  8
3  5  9
4  5  8
5  5  9

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