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# pandas.core.groupby.DataFrameGroupBy.transform

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

  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, str

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

      Accepted inputs are:

      - String

      - Python function

      - Numba JIT function with `engine='numba'` specified.

      Only passing a single function is supported with this engine. 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.

      If a string is chosen, then it needs to be the name of the groupby method you want to use.

    *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`

    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

    **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.

  Changed in version 2.0.0: When using `.transform` on a grouped DataFrame and the transformation function returns a DataFrame, pandas now aligns the result’s index with the input’s index. You can call `.to_numpy()` on 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

      >>> grouped.transform("mean")
          C    D
      0  3.666667  4.0
      1  4.000000  5.0
      2  3.666667  4.0
      3  4.000000  5.0
      4  3.666667  4.0
      5  4.000000  5.0

  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