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

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

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

funcfunction, 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.


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


Keyword arguments to be passed into func.


Series with the same indexes as the original object filled with transformed values.

See also


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


Aggregate using one or more operations.


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


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.


>>> ser = pd.Series([390.0, 350.0, 30.0, 20.0],
...                 index=["Falcon", "Falcon", "Parrot", "Parrot"],
...                 name="Max Speed")
>>> grouped = ser.groupby([1, 1, 2, 2])
>>> grouped.transform(lambda x: (x - x.mean()) / x.std())
    Falcon    0.707107
    Falcon   -0.707107
    Parrot    0.707107
    Parrot   -0.707107
    Name: Max Speed, dtype: float64

Broadcast result of the transformation

>>> grouped.transform(lambda x: x.max() - x.min())
Falcon    40.0
Falcon    40.0
Parrot    10.0
Parrot    10.0
Name: Max Speed, dtype: float64
>>> grouped.transform("mean")
Falcon    370.0
Falcon    370.0
Parrot     25.0
Parrot     25.0
Name: Max Speed, dtype: float64

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())
Falcon    390
Falcon    390
Parrot     30
Parrot     30
Name: Max Speed, dtype: int64