即使整个管道都安装了管道中的 Sklearn 组件？

Question

我正在尝试从合适的管道中挑选出一个组件/变压器来检查它的行为。但是，当我检索该组件时，该组件显示为未安装，但将管道作为一个整体使用没有问题。这表明管道已安装，组件也已安装。

有人可以解释原因，并建议如何检查已安装管道中的组件吗？

这是一个可重现的例子：

import pandas as pd
import numpy as np

from sklearn.compose import ColumnTransformer
from sklearn.pipeline import Pipeline
from sklearn.impute import SimpleImputer
from sklearn.preprocessing import StandardScaler, OneHotEncoder
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import train_test_split, GridSearchCV

np.random.seed(0)

# Read data from Titanic dataset.
titanic_url = ('https://raw.githubusercontent.com/amueller/'
               'scipy-2017-sklearn/091d371/notebooks/datasets/titanic3.csv')
data = pd.read_csv(titanic_url)

# We create the preprocessing pipelines for both numeric and categorical data.
numeric_features = ['age', 'fare']
numeric_transformer = Pipeline(steps=[
    ('imputer', SimpleImputer(strategy='median')),
    ('scaler', StandardScaler())])

categorical_features = ['embarked', 'sex', 'pclass']
categorical_transformer = Pipeline(steps=[
    ('imputer', SimpleImputer(strategy='constant', fill_value='missing')),
    ('onehot', OneHotEncoder(handle_unknown='ignore'))])

preprocessor = ColumnTransformer(
    transformers=[
        ('num', numeric_transformer, numeric_features),
        ('cat', categorical_transformer, categorical_features)])

# Append classifier to preprocessing pipeline.
# Now we have a full prediction pipeline.
clf = Pipeline(steps=[('preprocessor', preprocessor),
                      ('classifier', LogisticRegression(solver='lbfgs'))])

X = data.drop('survived', axis=1)
y = data['survived']

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2)

clf.fit(X_train, y_train)
print("model score: %.3f" % clf.score(X_test, y_test))

调用任一：

clf.get_params()['preprocessor__cat__imputer'].transform(X)

或

clf.named_steps['preprocessor'].transformers[0][1].named_steps['imputer'].transform(X)

会导致这样的错误：

NotFittedError: This SimpleImputer instance is not fitted yet. Call 'fit' with appropriate arguments before using this method.

Answer 1

ColumnTransformer 属性 transformers 是输入 unfitted 转换器。要访问已安装的变压器，请使用属性transformers_ 或named_transformers_。我想get_params()['preprocessor__cat__imputer'] 也得到了不合适的输入变压器。

（你仍然会得到一个错误，因为 imputer 也会尝试处理字符串数据，strategy='median' 会失败。）