Feature engineering pipelines in MLOps - Mini Project: Build & Apply

Feature Engineering Pipelines

📖 Scenario: You are working on a machine learning project. You need to prepare your data by creating a feature engineering pipeline. This pipeline will help you clean and transform your data automatically before training your model.

🎯 Goal: Build a simple feature engineering pipeline using scikit-learn that scales numeric features and encodes categorical features.

📋 What You'll Learn

Create a dataset dictionary with numeric and categorical features

Define a configuration variable for numeric feature names

Build a pipeline that scales numeric features and encodes categorical features

Print the transformed feature array

💡 Why This Matters

🌍 Real World

Feature engineering pipelines automate data preparation steps, making machine learning workflows faster and less error-prone.

💼 Career

Understanding how to build and use feature engineering pipelines is essential for MLOps engineers and data scientists to deploy reliable ML models.

1

Create the initial dataset dictionary

Create a dictionary called data with these exact entries: 'age': [25, 32, 47], 'salary': [50000, 60000, 80000], and 'department': ['sales', 'engineering', 'marketing'].

MLOps

# Your code here

Hint

Use a dictionary with keys 'age', 'salary', and 'department' and assign lists of values exactly as shown.

2

Define numeric feature names

Create a list called numeric_features containing the strings 'age' and 'salary'.

MLOps

data = {
    'age': [25, 32, 47],
    'salary': [50000, 60000, 80000],
    'department': ['sales', 'engineering', 'marketing']
}
# Your code here

Hint

Define a list named numeric_features with the exact two strings.

3

Build the feature engineering pipeline

Import ColumnTransformer, StandardScaler, and OneHotEncoder from sklearn. Create a ColumnTransformer called preprocessor that applies StandardScaler() to numeric_features and OneHotEncoder() to the 'department' feature.

MLOps

data = {
    'age': [25, 32, 47],
    'salary': [50000, 60000, 80000],
    'department': ['sales', 'engineering', 'marketing']
}
numeric_features = ['age', 'salary']
# Your code here

Hint

Use ColumnTransformer with two transformers: one for numeric features using StandardScaler() and one for categorical feature 'department' using OneHotEncoder().

4

Transform and print the processed features

Convert data to a pandas DataFrame called df. Use preprocessor.fit_transform(df) to transform the data and assign it to features. Print features.

MLOps

from sklearn.compose import ColumnTransformer
from sklearn.preprocessing import StandardScaler, OneHotEncoder
import pandas as pd

data = {
    'age': [25, 32, 47],
    'salary': [50000, 60000, 80000],
    'department': ['sales', 'engineering', 'marketing']
}
numeric_features = ['age', 'salary']

preprocessor = ColumnTransformer(
    transformers=[
        ('num', StandardScaler(), numeric_features),
        ('cat', OneHotEncoder(), ['department'])
    ]
)
# Your code here

Hint

Use pandas.DataFrame to convert data. Then call preprocessor.fit_transform(df) and print the result.

Practice

1. What is the main purpose of a feature engineering pipeline in MLOps?

easy

A. To automate and standardize data preparation steps

B. To deploy machine learning models to production

C. To monitor model performance after deployment

D. To collect raw data from external sources

Solution

1. Step 1: Understand the role of feature engineering pipelines

   Feature engineering pipelines automate the process of transforming raw data into features for model training and testing.

2. Step 2: Differentiate from other MLOps tasks

   Deploying models, monitoring, and data collection are separate tasks from feature engineering pipelines.

3. Final Answer:

   To automate and standardize data preparation steps -> Option A

4. Quick Check:

   Feature engineering pipeline = automate data prep [OK]

Hint: Feature pipelines automate data prep, not deployment or monitoring [OK]

Common Mistakes:

• Confusing feature pipelines with model deployment
• Thinking pipelines collect raw data
• Mixing up monitoring with feature engineering

2. Which of the following is the correct way to define a simple feature engineering pipeline step using scikit-learn's Pipeline?

easy

A. pipeline = Pipeline([('scaler', StandardScaler()), ('pca', PCA(n_components=2))])

B. pipeline = Pipeline('scaler', StandardScaler(), 'pca', PCA(n_components=2))

C. pipeline = Pipeline({'scaler': StandardScaler(), 'pca': PCA(n_components=2)})

D. pipeline = Pipeline(StandardScaler(), PCA(n_components=2))

Solution

1. Step 1: Recall scikit-learn Pipeline syntax

   Pipeline expects a list of tuples, each tuple with a name and a transformer object.

2. Step 2: Check each option's syntax

   pipeline = Pipeline([('scaler', StandardScaler()), ('pca', PCA(n_components=2))]) correctly uses a list of tuples. Options B, C, and D use incorrect argument formats.

3. Final Answer:

   pipeline = Pipeline([('scaler', StandardScaler()), ('pca', PCA(n_components=2))]) -> Option A

4. Quick Check:

   Pipeline needs list of (name, transformer) tuples [OK]

Hint: Pipeline needs list of (name, transformer) tuples [OK]

Common Mistakes:

• Passing arguments without list brackets
• Using dict instead of list of tuples
• Omitting step names in pipeline

3. Given the following pipeline code, what will be the output of pipeline.transform([[0, 0], [1, 1]])?

from sklearn.pipeline import Pipeline
from sklearn.preprocessing import StandardScaler
from sklearn.decomposition import PCA

pipeline = Pipeline([
  ('scaler', StandardScaler()),
  ('pca', PCA(n_components=1))
])
pipeline.fit([[0, 0], [1, 1]])
result = pipeline.transform([[0, 0], [1, 1]])
print(result)

medium

A. Error: PCA requires more than one sample

B. [[0. 0.]
[1. 1.]]

C. [[0.5]
[0.5]]

D. [[-1.41421356]
[ 1.41421356]]

Solution

1. Step 1: Understand pipeline steps

   First, data is scaled to zero mean and unit variance, then PCA reduces to 1 component.

2. Step 2: Calculate transformed output

   Scaling [[0,0],[1,1]] centers data, PCA finds principal component; output is approximately [[-1.41421356],[1.41421356]].

3. Final Answer:

   [[-1.41421356] [ 1.41421356]] -> Option D

4. Quick Check:

   Scaling + PCA output = [[-1.41421356] [ 1.41421356]] [OK]

Hint: Scaling centers data; PCA output is principal component values [OK]

Common Mistakes:

• Expecting original data as output
• Confusing PCA output shape
• Assuming error due to small data

4. You have this pipeline code but it raises an error: ValueError: Expected 2D array, got 1D array instead. What is the likely cause?

pipeline = Pipeline([
  ('scaler', StandardScaler()),
  ('pca', PCA(n_components=1))
])
pipeline.fit([1, 2, 3, 4])

medium

A. Pipeline steps must be functions, not classes

B. Input to fit should be 2D array, not 1D list

C. StandardScaler requires integer inputs only

D. PCA cannot have n_components=1

Solution

1. Step 1: Analyze error message

   The error says input is 1D but 2D is expected for fit method.

2. Step 2: Check input format

   Input [1, 2, 3, 4] is a 1D list; fit expects 2D array like [[1], [2], [3], [4]].

3. Final Answer:

   Input to fit should be 2D array, not 1D list -> Option B

4. Quick Check:

   fit input shape must be 2D [OK]

Hint: fit() needs 2D array shape, not flat list [OK]

Common Mistakes:

• Passing 1D list instead of 2D array
• Misunderstanding PCA parameter limits
• Thinking StandardScaler restricts input types

5. You want to create a feature engineering pipeline that handles missing values by filling them with the median, then scales features, and finally selects the top 3 features using a model-based selector. Which pipeline setup is correct?

hard

A. Pipeline([('scaler', StandardScaler()), ('imputer', SimpleImputer(strategy='median')), ('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3))])

B. Pipeline([('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3)), ('imputer', SimpleImputer(strategy='median')), ('scaler', StandardScaler())])

C. Pipeline([('imputer', SimpleImputer(strategy='median')), ('scaler', StandardScaler()), ('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3))])

D. Pipeline([('imputer', SimpleImputer(strategy='mean')), ('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3)), ('scaler', StandardScaler())])

Solution

1. Step 1: Order pipeline steps logically

   Missing values must be handled first, then scaling, then feature selection.

2. Step 2: Check each option's correctness

   Pipeline([('imputer', SimpleImputer(strategy='median')), ('scaler', StandardScaler()), ('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3))]) follows correct order and uses median for imputation. Pipeline([('scaler', StandardScaler()), ('imputer', SimpleImputer(strategy='median')), ('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3))]) swaps imputer and scaler incorrectly. Pipeline([('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3)), ('imputer', SimpleImputer(strategy='median')), ('scaler', StandardScaler())]) starts with selector which needs complete data. Pipeline([('imputer', SimpleImputer(strategy='mean')), ('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3)), ('scaler', StandardScaler())]) uses mean instead of median and wrong order.

3. Final Answer:

   Pipeline([('imputer', SimpleImputer(strategy='median')), ('scaler', StandardScaler()), ('selector', SelectFromModel(estimator=RandomForestClassifier(), max_features=3))]) -> Option C

4. Quick Check:

   Impute -> scale -> select features [OK]

Hint: Impute missing -> scale -> select features in pipeline order [OK]

Common Mistakes:

• Placing scaler before imputer
• Selecting features before imputing missing values
• Using mean instead of median when median is required