This rule raises an issue when an attribute ending with ``++_++`` is set in the ``++__init__++`` method of a class inheriting from
Scikit-Learn `BaseEstimator`


== Why is this an issue?

On a Scikit-Learn estimator, attributes that have a trailing underscore represent attributes that are estimated.
These attributes have to be set in the fit method. Their presence is used to verify if an estimator has been fitted.

[source,python]
----
from sklearn.neighbors import KNeighborsClassifier 

X = [[0], [1], [2], [3]]
y = [0, 0, 1, 1]
knn = KNeighborsClassifier(n_neighbors=1) 
knn.fit(X, y)
knn.n_samples_fit_
----

In the example above the attributes of the `KNeighborsClassifier`, ``++n_samples_fit_++`` 
is set only after the estimator's `fit` method is called. Calling ``++n_samples_fit_++`` before the estimator is fitted
would raise an `AttributeError` exception.

When implementing a custom estimator by subclassing Scikit-Learn's `BaseEstimator`, 
it is important to follow the above convention and not set attributes with a trailing underscore inside the ``++__init__++`` method.

== How to fix it

To fix this issue, move the attributes with a trailing underscore from the ``++__init__++`` method to the `fit` method.

=== Code examples

==== Noncompliant code example

[source,python,diff-id=1,diff-type=noncompliant]
----
from sklearn.base import BaseEstimator

class MyEstimator(BaseEstimator):
    def __init__(self):
        self.estimated_attribute_ = None # Noncompliant: an estimated attribute is set in the __init__ method.
----

==== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
from sklearn.base import BaseEstimator

class MyEstimator(BaseEstimator):
    def fit(self, X, y):
        self.estimated_attribute_ = some_estimation(X) # Compliant
----

== Resources
=== Documentation

* Scikit-Learn documentation - https://scikit-learn.org/stable/developers/develop.html#parameters-and-init[Parameters and init]


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== Implementation specification 

Verify that subclasses of BaseEstimator do not have attributes with a trailing underscore in the __init__ method.
Verify that inherited alongside BaseEstimator do not have attributes with a trailing underscore in the __init__ method.

=== Message 

Primary : Move this estimated attribute inside the `fit` method. 

Secondary:  this attributes is used in this estimator (in the case of mixins)


=== Issue location

Primary : name of the attribute

Secondary : the name of the estimator subclassing the mixin and the BaseEstimator 

=== Quickfix

Possible quickfix if the `fit` method does not exist. (Add the fit method with the estimated attribute) and delete the estimated attributes
from the __init__ method. This is only possible in an estimator directly subclassing BaseEstimator.

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