ENH Add get_feature_names_out to FunctionTransformer #21569

ageron · 2021-11-06T08:15:04Z

Reference Issues/PRs

Follow-up on #18444.
Part of #21308.
This new feature was discussed in #21079.

What does this implement/fix? Explain your changes.

Adds the get_feature_names_out method and a new parameter feature_names_out to preprocessing.FunctionTransformer. By default, get_feature_names_out returns the input feature names, but you can set feature_names_out to return a different list, which is especially useful when the number of output features differs from the number of input features.

For example, here's a FunctionTransformer that outputs a single feature, equal to the input's mean along axis=1:

import numpy as np
from sklearn.preprocessing import FunctionTransformer

mean_transformer = FunctionTransformer(
    func=lambda X: X.mean(axis=1, keepdims=True),
    feature_names_out=["mean"]
)

X_trans = mean_transformer.fit_transform(np.random.rand(10,2))
print(mean_transformer.get_feature_names_out())  # prints ['mean']

The feature_names_out parameter may also be a callable. This is useful if the output feature names depend on the input feature names, and/or if they depend on parameters like kw_args. Here's an example that uses both. It's a transformer that appends n random features to existing features:

import numpy as np
import pandas as pd
from sklearn.preprocessing import FunctionTransformer

def add_n_random_features(X, n):
    return np.concatenate([X, np.random.rand(len(X), n)], axis=1)

def feature_names_out(transformer, input_features):
    n = transformer.kw_args["n"]
    return list(input_features) + [f"rnd{i}" for i in range(n)]

transformer = FunctionTransformer(
    func=add_n_random_features,
    feature_names_out=feature_names_out,
    kw_args=dict(n=3),
    validate=True,  # IMPORTANT (see discussion below)
)

df = pd.DataFrame({"a": np.random.rand(100), "b": np.random.rand(100)})
X_trans = transformer.fit_transform(df)
print(transformer.get_feature_names_out())  # prints ['a' 'b' 'rnd0' 'rnd1' 'rnd2']

Any other comments?

I have some concerns regarding the fact that validate is False by default, which means that n_features_in_ and feature_names_in_ are not set automatically. So if you create a FunctionTransformer with the default validate=False and feature_names_out=None, then when you call get_feature_names_out without any argument, it will raise an exception (unless transform was called before and func set n_feature_in_ or feature_names_in_). I tried to make this clear in the error message, but I'm worried that this will confuse users. Wdyt?

And if validate=False and you set feature_names_out to a callable, and call get_feature_names_out with no arguments, then the callable will get input_features=None as input (unless transform was called before and func set n_features_in_ or feature_names_in_). Users may be surprised by this. Should we output a warning in this case? Wdyt?

Moreover, as shown in the second code example above, the output feature names may depend on kw_args, so if feature_names_out is a callable, get_feature_names_out passes self to it, plus the input_features. I considered checking feature_names_out.__code__.co_varnames to decide whether to pass no arguments, or just the input_features, or the input_features and self. But __code__ is not used anywhere in the code base, and inspect is not used much, so I'm not sure whether such introspection would be frowned upon? I decided that it was simple enough to require users to always have two arguments: the transformer itself, and the input_features. Wdyt?

Lastly, when users want to create a FunctionTransformer that outputs a single feature, I expect that many will be tempted to set feature_names_out to a string instead of a list. To keep things consistent, I decided to raise an exception in this case, and have a clear error message to tell them to use ["foo"] instead. Wdyt?

thomasjpfan

For validate=False and the feature_names_out parameter is set, I am propose we set feature_names_in_ and n_features_in_, but not validate it during fit or transform.

As for the API, I am thinking of restricting feature_names_out two options at first:

None: No feature names out
callable: User provide function to compute feature names out

Two more options for follow up PRs:

'one-to-one': Feature names out == feature names in
array-like of strings: I am currently unsure about the use case for this option that the callable can not resolve. But we can discuss in a follow up.

ageron · 2021-11-06T20:48:45Z

Thanks @thomasjpfan . I'll remove the option to set feature_names_out to an array-like of strings in this PR (my goal was to have a simple API for what I expect to be a common use case, but it's not much harder to write a callable, and the API may be simpler with just one option). I 'll also replace the default option (currently None) with "one-to-one": I like this idea, it's simple enough to change, and it's more explicit.
Sounds good?

thomasjpfan · 2021-11-06T20:56:12Z

I 'll also replace the default option (currently None) with "one-to-one": I like this idea, it's simple enough to change, and it's more explicit.

I think the default still needs to be None. I do not think the function transformer can assume 'one-to-one' in the general case.

Let's add 'one-to-one' in a future PR and get the callable in first with this PR. The smaller the PR the easier it is to review and get merged.

…ake default 'one-to-one'

ageron · 2021-11-06T21:35:46Z

I just read your message, I had already updated the PR to remove the option to pass an array-like of strings, and I set the default to 'one-to-one'.
Isn't the most common use case for FunctionTransformer something like FunctionTransformer(func=np.log)? If so, then one-to-one is a reasonable default, isn't it? I can revert to None, but what should get_feature_names_out do in this case? Raise an exception saying "you must set 'feature_names_out'"?

thomasjpfan · 2021-11-06T22:06:30Z

Isn't the most common use case for FunctionTransformer something like FunctionTransformer(func=np.log)? If so, then one-to-one is a reasonable default, isn't it?

It is, but I do not think we can assume it. If a user pass a function that creates a column then 'one-to-one' would be wrong. I think ti is better to have the user explicit state it.

I can revert to None, but what should get_feature_names_out do in this case? Raise an exception saying "you must set 'feature_names_out'"?

We can use avaliable_if to conditionally expose get_feature_names_out depending on if feature_names_out is set:

scikit-learn/sklearn/utils/metaestimators.py

Line 140 in 48e83df

def available_if(check):

ageron · 2021-11-06T22:21:04Z

Thanks @thomasjpfan . I updated the PR to make None the default. Right now get_feature_names_out raises a ValueError if feature_names_out is None, but I'll use available_if instead, thanks for the tip.

…feature_names_out=None

ageron · 2021-11-07T04:37:40Z

I ran black, and flake8, make test-coverage, etc., but they didn't catch the issues with the numpydoc (a newline was missing) or with v1.1.rst (someone else had forgotten a `). I looked in the Contributing doc, but I can't find instructions to catch these errors before I push the code to github. Did I miss something?

ageron · 2021-11-14T21:08:29Z

Hi @thomasjpfan, is there anything else you need me to do for this PR?

ogrisel · 2021-11-17T17:19:19Z

I ran black, and flake8, make test-coverage, etc., but they didn't catch the issues with the numpydoc (a newline was missing) or with v1.1.rst (someone else had forgotten a `). I looked in the Contributing doc, but I can't find instructions to catch these errors before I push the code to github. Did I miss something?

For some reason the numpydoc validation was done externally and not part as the main test suite. I am not sure why we do that. We should probably run those checks as part of the main test suite to avoid the confusion.

ogrisel

LGTM. I think the PR in its current state should cover most useful cases. I did not see any particular defect. Just a small improvement suggestion for one of the exception messages below:

sklearn/preprocessing/_function_transformer.py

…geron/scikit-learn into function_transformer_feature_names_out

ageron · 2021-11-17T20:44:11Z

Thanks for reviewing, Olivier. I just made the change you suggested.

ogrisel · 2021-11-23T13:20:12Z

test_calibration_display_default_labels[None-_line1] that is broken on the CI should have been fixed in the main branch.

ageron · 2021-11-24T05:00:08Z

test_calibration_display_default_labels[None-_line1] that is broken on the CI should have been fixed in the main branch.

In such cases, should I pull and merge main into the PR branch?

ogrisel · 2021-11-25T15:47:08Z

In such cases, should I pull and merge main into the PR branch?

That would not hurt, and if the PR is "CI green ticked", it might get a better chance to attract reviewers' attention :)

ageron · 2021-11-25T23:45:12Z

Thanks @ogrisel , I merged main, now there's a beautiful green tick. 😊

thomasjpfan

Thanks for the update @ageron !

sklearn/preprocessing/_function_transformer.py

sklearn/preprocessing/tests/test_function_transformer.py

sklearn/preprocessing/_function_transformer.py

sklearn/preprocessing/tests/test_function_transformer.py

thomasjpfan

LGTM

ageron · 2021-11-30T19:12:16Z

Thanks for the review. 👍

nilslacroix

I copied your function in my scikit enviroment and tried to use it in my enviroment. However I still get the error as below, where preprocessor is my columntransformer and I try the following code:
preprocessor.get_feature_names_out()

Transformer argument looks like this:

('log', FunctionTransformer(np.log1p, validate=True), log_features)

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Input In [10], in <cell line: 3>()
      1 xt = preprocessor.transform(X_test)
      2 #mapie.single_estimator_[1].estimator
----> 3 preprocessor.get_feature_names_out()

File ~\miniconda3\envs\Master_ML\lib\site-packages\sklearn\compose\_column_transformer.py:481, in ColumnTransformer.get_feature_names_out(self, input_features)
    479 transformer_with_feature_names_out = []
    480 for name, trans, column, _ in self._iter(fitted=True):
--> 481     feature_names_out = self._get_feature_name_out_for_transformer(
    482         name, trans, column, input_features
    483     )
    484     if feature_names_out is None:
    485         continue

File ~\miniconda3\envs\Master_ML\lib\site-packages\sklearn\compose\_column_transformer.py:446, in ColumnTransformer._get_feature_name_out_for_transformer(self, name, trans, column, feature_names_in)
    444 # An actual transformer
    445 if not hasattr(trans, "get_feature_names_out"):
--> 446     raise AttributeError(
    447         f"Transformer {name} (type {type(trans).__name__}) does "
    448         "not provide get_feature_names_out."
    449     )
    450 if isinstance(column, Iterable) and not all(
    451     isinstance(col, str) for col in column
    452 ):
    453     column = _safe_indexing(feature_names_in, column)

AttributeError: Transformer log (type FunctionTransformer) does not provide get_feature_names_out.

thomasjpfan · 2022-04-27T13:44:28Z

This feature is not released yet and will be released in v1.1. If you want to try out the feature now, you can install the nightly build:

pip install --pre --extra-index https://pypi.anaconda.org/scipy-wheels-nightly/simple scikit-learn

nilslacroix · 2022-04-28T17:10:04Z

Are you sure this working as intended? I just installed Nightly and I still get exactly this error. The code is in my enviroment, at least function_transformer_.py has this method implemented.

preprocessor = ColumnTransformer(
    transformers=
    [   #('enc_obj', onehot, ["PropertyType"]),    #noworks
        ('pow', PowerTransformer(method="yeo-johnson"), pow_features),   #works
        ('log', FunctionTransformer(np.log1p, validate=True), log_features), #noworks
        ('enc_plz', BinaryEncoder(verbose=1), ["Postcode"]), #noworks
        ('enc_obj', OneHotEncoder(), ["PropertyType"]),    #noworks
        ("target", LeaveOneOutEncoder(), ["MeanPricePostcode"]),      #works
        ('features', "passthrough", unskewed_features)     #works
        
    ],
        n_jobs=-2)

pipeline2 = Pipeline(steps=[("preprocessor", preprocessor),
                           ("scaler", scaler),
                           ('clf', LinearSVR()),
                          ])        
pipeline2.fit(X_train, y_train)
pipeline2[:-1].get_feature_names_out()

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Input In [18], in <cell line: 8>()
      5 pipeline2.fit(X_train, y_train)
      6 #explainer = RegressionExplainer(pipeline2, X_test, y_test, shap="guess", n_jobs = multiprocessing.cpu_count()-1)
      7 #db1 = ExplainerDashboard(explainer)
----> 8 pipeline2[:-1].get_feature_names_out()

File ~\miniconda3\envs\Master_ML\lib\site-packages\sklearn\pipeline.py:733, in Pipeline.get_feature_names_out(self, input_features)
    727     if not hasattr(transform, "get_feature_names_out"):
    728         raise AttributeError(
    729             "Estimator {} does not provide get_feature_names_out. "
    730             "Did you mean to call pipeline[:-1].get_feature_names_out"
    731             "()?".format(name)
    732         )
--> 733     feature_names_out = transform.get_feature_names_out(feature_names_out)
    734 return feature_names_out

File ~\miniconda3\envs\Master_ML\lib\site-packages\sklearn\compose\_column_transformer.py:479, in ColumnTransformer.get_feature_names_out(self, input_features)
    477 transformer_with_feature_names_out = []
    478 for name, trans, column, _ in self._iter(fitted=True):
--> 479     feature_names_out = self._get_feature_name_out_for_transformer(
    480         name, trans, column, input_features
    481     )
    482     if feature_names_out is None:
    483         continue

File ~\miniconda3\envs\Master_ML\lib\site-packages\sklearn\compose\_column_transformer.py:447, in ColumnTransformer._get_feature_name_out_for_transformer(self, name, trans, column, feature_names_in)
    445 # An actual transformer
    446 if not hasattr(trans, "get_feature_names_out"):
--> 447     raise AttributeError(
    448         f"Transformer {name} (type {type(trans).__name__}) does "
    449         "not provide get_feature_names_out."
    450     )
    451 return trans.get_feature_names_out(names)

AttributeError: Transformer log (type FunctionTransformer) does not provide get_feature_names_out.

thomasjpfan · 2022-04-28T17:20:21Z

FunctionTransformer requires setting feature_names_out parameter to work. For simple one-to-one transformations, you can set feature_names_out to "one-to-one":

import numpy as np
import pandas as pd
from sklearn.preprocessing import FunctionTransformer

mean_transformer = FunctionTransformer(
    func=np.log1p,
    feature_names_out="one-to-one",
    validate=True
)

X = pd.DataFrame({"my_feature": [1, 2, 3]})

X_trans = mean_transformer.fit_transform(X)
print(mean_transformer.get_feature_names_out())
# ['my_feature']

nilslacroix · 2022-04-28T17:27:04Z

Thank you Thomas ... sorry for asking all these question that might be totally obvious :(

Add get_feature_names_out to FunctionTransformer

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github-actions bot added the module:preprocessing label Nov 6, 2021

ageron added 4 commits November 6, 2021 21:17

Add PR number

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Remove unused variables

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Add missing ` (unrelated to this PR)

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Remove unreachable elif block

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thomasjpfan reviewed Nov 6, 2021

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Remove option to set feature_names_out to an array-like of str, and m…

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…ake default 'one-to-one'

Default feature_names_out to None instead of 'one-to-one'

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ageron added 3 commits November 7, 2021 11:31

Use available_if to ensure get_feature_names_out is not defined when …

6e7f13b

…feature_names_out=None

Add missing new line

f078eca

Add missing newline in method doc

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Merge branch 'main' into function_transformer_feature_names_out

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ogrisel self-requested a review November 17, 2021 13:31

ogrisel mentioned this pull request Nov 17, 2021

Implement get_feature_names_out for all estimators #21308

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ageron added 2 commits November 18, 2021 09:41

Show value when feature_names_out is invalid

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Merge branch 'function_transformer_feature_names_out' of github.com:a…

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…geron/scikit-learn into function_transformer_feature_names_out

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Merge branch 'main' into function_transformer_feature_names_out

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ageron added 3 commits November 28, 2021 14:46

Replace lambda transformer:... with lambda self:...

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Convert names out to ndarray with dtype=object

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Check names out are ndarray with dtype=object

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thomasjpfan changed the title ~~Add get_feature_names_out to FunctionTransformer~~ ENH Add get_feature_names_out to FunctionTransformer Nov 30, 2021

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thomasjpfan merged commit d23d589 into scikit-learn:main Nov 30, 2021

thomasjpfan mentioned this pull request Dec 13, 2021

Add DateTimeTransformer? #21968

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nilslacroix reviewed Apr 27, 2022

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ageron deleted the function_transformer_feature_names_out branch May 2, 2022 22:59

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ENH Add get_feature_names_out to FunctionTransformer #21569

ENH Add get_feature_names_out to FunctionTransformer #21569

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What does this implement/fix? Explain your changes.

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