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     slep012/proposal
     slep013/proposal
+    slep018/proposal
 
 .. toctree::
     :maxdepth: 1
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+.. _slep_018:
+
+=======================================================
+SLEP018: Pandas Output for Transformers with set_output
+=======================================================
+
+:Author: Thomas J. Fan
+:Status: Draft
+:Type: Standards Track
+:Created: 2022-06-22
+
+Abstract
+--------
+
+This SLEP proposes a ``set_output`` method to configure the output data container of
+scikit-learn transformers.
+
+Detailed description
+--------------------
+
+Currently, scikit-learn transformers return NumPy ndarrays or SciPy sparse
+matrices. This SLEP proposes adding a ``set_output`` method to configure a
+transformer to output pandas DataFrames::
+
+   scalar = StandardScalar().set_output(transform="pandas")
+   scalar.fit(X_df)
+
+   # X_trans_df is a pandas DataFrame
+   X_trans_df = scalar.transform(X_df)
+
+The index of the output DataFrame must match the index of the input. If the
+transformer does not support ``transform="pandas"``, then it must raise a
+``ValueError`` stating that it does not support the feature.
+
+This SLEP's only focus is dense data for ``set_output``. If a transformer returns
+sparse data, e.g. `OneHotEncoder(sparse=True), then ``transform`` will raise a
+``ValueError`` if ``set_output(transform="pandas")``. Dealing with sparse output
+might be the scope of another future SLEP.
+
+For a pipeline, calling ``set_output`` on the pipeline will configure all steps
+in the pipeline::
+
+   num_prep = make_pipeline(SimpleImputer(), StandardScalar(), PCA())
+   num_preprocessor.set_output(transform="pandas")
+
+   # X_trans_df is a pandas DataFrame
+   X_trans_df = num_preprocessor.fit_transform(X_df)
+
+   # X_trans_df is again a pandas DataFrame
+   X_trans_df = num_preprocessor[0].transform(X_df)
+
+Meta-estimators that support ``set_output`` are required to configure all inner
+transformer by calling ``set_output``. Specifically all fitted and non-fitted
+inner transformers must be configured with ``set_output``. This enables
+``transform``'s output to be a DataFrame before and after the meta-estimator is
+fitted. If an inner transformer does not define ``set_output``, then an error is
+raised.
+
+
+Global Configuration
+....................
+
+For ease of use, this SLEP proposes a global configuration flag that sets the output for all
+transformers::
+
+   import sklearn
+   sklearn.set_config(transform_output="pandas")
+
+The global default configuration is ``"default"`` where the transformer
+determines the output container.
+
+The configuration can also be set locally using the ``config_context`` context
+manager:
+
+   from sklearn import config_context
+   with config_context(transform_output="pandas"):
+      num_prep = make_pipeline(SimpleImputer(), StandardScalar(), PCA())
+      num_preprocessor.fit_transform(X_df)
+
+The following specifies the precedence levels for the three ways to configure
+the output container:
+
+1. Locally configure a transformer: ``transformer.set_output``
+2. Context manager: ``config_context``
+3. Global configuration: ``set_config``
+
+Implementation
+--------------
+
+A possible implementation of this SLEP is worked out in :pr:`23734`.
+
+Backward compatibility
+----------------------
+
+There are no backward compatibility concerns, because the ``set_output`` method
+is a new API. Third party transformers can opt-in to the API by defining
+``set_output``.
+
+Alternatives
+------------
+
+Alternatives to this SLEP includes:
+
+1. `SLEP014 <https://github.com/scikit-learn/enhancement_proposals/pull/37>`__
+   proposes that if the input is a DataFrame than the output is a DataFrame.
+2. Prototype `#20100
+   <https://github.com/scikit-learn/scikit-learn/pull/20100>`__ showcases
+   ``array_out="pandas"`` in `transform`. This API is limited because does not
+   directly support fitting on a pipeline where the steps requires data frames
+   input.
+
+Discussion
+----------
+
+A list of issues discussing Pandas output are: `#14315
+<https://github.com/scikit-learn/scikit-learn/pull/14315>`__, `#20100
+<https://github.com/scikit-learn/scikit-learn/pull/20100>`__, and `#23001
+<https://github.com/scikit-learn/scikit-learn/issueas/23001>`__. This SLEP
+proposes configuring the output to be pandas because it is the DataFrame library
+that is most widely used and requested by users. The ``set_output`` can be
+extended to support support additional DataFrame libraries in the future.
+
+References and Footnotes
+------------------------
+
+.. [1] Each SLEP must either be explicitly labeled as placed in the public
+   domain (see this SLEP as an example) or licensed under the `Open Publication
+   License`_.
+
+.. _Open Publication License: https://www.opencontent.org/openpub/
+
+
+Copyright
+---------
+
+This document has been placed in the public domain. [1]_