Master Python Pylint: Elevate Your Coding Game


Written by - Deepak Prasad

Introduction to Python Pylint

Pylint is an open-source tool for Python that analyzes your code against an extensive set of coding standards and heuristics. It checks for errors, looks for code smells, offers simple refactoring suggestions, and also provides a score indicating the overall quality of your code. Pylint is widely used in the Python community for maintaining a high level of code quality.

Pylint is commonly integrated into the development workflow to automatically check every new piece of code. It can be configured to run locally on a developer’s machine or as part of a continuous integration (CI) pipeline. Many teams require a certain Pylint score to be met before code can be committed or merged, ensuring a consistent level of quality across a project.

 

Installing Pylint

The easiest way to install Pylint is using pip, which is Python's package manager. Open your terminal or command prompt and run the following command:

pip install pylint

This will download and install the latest version of Pylint along with any required dependencies.

Alternatively you can install using default package manager (assuming pylint is part of the package repository). For Example on Ubuntu:

apt install pylint

After installation, you can check if Pylint is working by running:

pylint --version

sample output:

pylint 2.12.2
astroid 2.9.3
Python 3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0]

 

IDE Integration

Pylint can be easily integrated into various IDEs to provide real-time linting features.

 

VSCode

To use Python Pylint in Visual Studio Code, follow these steps:

  1. Open VSCode and go to the Extensions view by clicking on the Extensions icon in the Activity Bar on the side of the window.
  2. Search for Python and install the Microsoft Python extension.
  3. Open your Python file and the extension should automatically run Pylint on your code. You'll see linting errors and suggestions directly in the editor.

If Pylint is not enabled by default, you can enable it by opening your settings (Ctrl + ,) and adding or modifying the following line:

"python.linting.pylintEnabled": true

 

PyCharm

In PyCharm, Pylint can be set up as follows:

  1. Navigate to File > Settings > Python Integrated Tools.
  2. Under the Linting section, you'll find an option to select the linting tool to use. Choose Pylint from the dropdown menu.
  3. Apply the changes and click OK.

PyCharm will now use Pylint to analyze your Python files, and you'll see warnings, errors, and suggestions directly in the editor.

 

Understanding the Basic Usage

After setting up Pylint, it's time to start using it to analyze your Python code. This section covers how to run Python Pylint on individual files and entire projects, as well as how to interpret the output.

Running Pylint on a Single File

To lint a single Python file, open your terminal, navigate to the directory where your file is located, and run the following command:

pylint my_file.py

Replace my_file.py with the name of the Python file you want to analyze. Pylint will scan the file and display a list of warnings, errors, and suggestions for improvement.

Running Pylint on a Project

To run Pylint on an entire project, navigate to the project's root directory and execute:

pylint my_project/

Replace my_project/ with your project's directory name. Pylint will recursively scan all Python files within the directory and its subdirectories.

Understanding Pylint Output

When you run Python Pylint, the output can be quite extensive depending on the size and complexity of your codebase. Here are some key elements to focus on:

  • Error Codes: Each issue has an associated code, like C0103 for naming convention issues or E1101 for attribute errors. These codes can help you understand what kind of issue Pylint detected.
  • Message: Next to each code, you'll see a message describing the problem or suggesting a change. For example, "Variable name x doesn't conform to snake_case naming style."
  • Line Number: This shows where in your file the issue occurs, making it easier to locate and fix.
  • Pylint Score: At the end of the output, Pylint provides a score out of 10. This is a rough measure of your code quality, based on the number and severity of issues found. The goal is to aim for a score as close to 10 as possible, but don't be overly concerned if you don't hit a perfect score. It's often normal to disable certain rules or accept minor issues for practical reasons.

 

Core Features of Pylint

Python Pylint offers a plethora of features aimed at maintaining a high level of code quality. Here's a deep dive into some of the core aspects that Pylint checks for.

 

1. Check for Code Errors

1.1 Syntax Errors

Python Pylint checks your code for syntax errors such as missing colons, unmatched brackets, or incorrect indentation. It provides instant feedback, so you can fix these issues even before running the program. Syntax errors are basic mistakes that can prevent your code from executing, and Pylint catches these to save you time during the debugging process.

Let's say you have the following line of code with a missing colon:

if x > 10
    print("x is greater than 10")

Python Pylint would catch this syntax error and output:

E0001: SyntaxError: expected ':'

1.2 Logical Errors

While logical errors won't prevent your code from running, they can produce incorrect or unexpected results. Pylint identifies common logical mistakes like unused variables, importing unused modules, or unreachable code after a return statement.

Consider the code where a variable is defined but never used:

x = 10
print("Hello, World!")

Python Pylint would point this out:

W0612: Unused variable 'x'

 

2. Check for Code Style and Formatting

2.1 Naming Conventions

Python Pylint helps enforce Python's PEP 8 naming conventions. It will flag variable names, function names, or class names that do not adhere to the naming guidelines. For example, it will suggest changing a variable name from myVar to my_var to conform to Python's snake_case naming convention.

If you define a function like so:

def myFunction():
    print("Hello, World!")

Python Pylint will advise:

C0103: Function name "myFunction" doesn't conform to snake_case naming style

2.2 Indentation

Incorrect indentation can make Python code hard to read and understand. Python Pylint ensures that your code has consistent indentation, following the PEP 8 guidelines. It checks for tabs mixed with spaces, improper indentation levels, and more.

Inconsistent indentation can be caught easily. For example:

def my_function():
  print("Hello")
    print("World!")

Python Pylint would indicate:

W0312: Found indentation with tabs instead of spaces

 

3. Calculate Code Complexity Metrics

Python Pylint can calculate several metrics related to code complexity, such as Cyclomatic Complexity and the number of lines in a function or method. High complexity might be a sign that you should refactor your code into smaller, more manageable pieces. It offers a helpful way to identify parts of your codebase that might be difficult to maintain or understand.

Python Pylint will produce a message if your function has too many branches or lines. Here's an example:

def complex_function(x):
    if x < 0:
        print("Negative")
    elif x == 0:
        print("Zero")
    else:
        print("Positive")

    if x < -10:
        print("Very negative")
    elif x > 10:
        print("Very positive")

Python Pylint might produce a warning if this function becomes too complex, encouraging you to break it into smaller functions.

 

4. Type Checking

Python 3 introduced optional type hints, and Pylint can use these to perform basic type checking. While it's not as powerful as dedicated tools like mypy, Python Pylint's type checking can catch common errors like passing a string argument where an integer is expected.

If you define a function with type hints and then provide an argument of a different type, Pylint can catch that. Example:

def greet(name: str) -> None:
    print("Hello, " + name)

greet(10)  # Wrong type, should be a string

Pylint output:

E1123: Unexpected keyword argument 'x' in function call

 

5. Refactoring Suggestions

Beyond merely pointing out issues, Python Pylint also offers suggestions for refactoring your code. For example, it can identify repeated code blocks that could be turned into functions or suggest breaking up large functions into smaller, more focused sub-functions.

Python Pylint can point out blocks of code that could be refactored. For instance, if you had the following code with repeated lines:

def greet_morning():
    print("Good Morning!")

def greet_evening():
    print("Good Evening!")

def greet_night():
    print("Good Night!")

Pylint could suggest that the repeated print calls might be better placed in a single function that takes the time of day as an argument.

 

Customization of Pylint

Python Pylint offers a lot of flexibility to customize its behavior according to your specific needs. Here's how you can adjust Pylint to fit your project.

1. Pylint Configuration File

You can create a .pylintrc file in your project root to store Pylint configurations. This allows you to enable or disable specific checks, change scoring methods, and more. Here's a simple example of what such a file might look like:

[MESSAGES CONTROL]
disable=
  too-few-public-methods,
  too-many-arguments

[FORMAT]
indent-string='    '

2. Inline Comments to Disable Checks

If you want to disable a specific Pylint check for a particular line of code, you can do so using inline comments. For example, to disable the "unused-variable" warning for a specific line:

x = 10  # pylint: disable=unused-variable

This will prevent Pylint from flagging this line with an "unused variable" warning.

3. Creating Custom Rules

Python Pylint allows you to create custom rules using Python code. You can write a Python script that defines new checks and then load that script using the --load-plugins command-line option.

Here's a skeleton example of what a custom rule might look like:

from pylint.checkers import BaseChecker
from pylint.interfaces import IAstroidChecker

class MyCustomChecker(BaseChecker):
    __implements__ = IAstroidChecker
    name = 'my-custom-checker'
    msgs = {
        'R9999': (
            'Custom message text.',
            'custom-message-id',
            'Custom message description.')
    }

    def visit_functiondef(self, node):
        if "bad_function" in node.name:
            self.add_message('custom-message-id', node=node)

You would then run Pylint with --load-plugins=my_custom_checker to include this new check.

4. Plugins

Python Pylint supports plugins that you can install separately to check for more specific things, like Django-specific issues or to integrate with other tools. You can install these plugins via pip and then activate them using the --load-plugins option.

5. Ignoring Specific Files or Folders

Sometimes you may want to ignore specific files or folders from being checked by Pylint. This can be done using the --ignore command-line option or by specifying it in the .pylintrc file.

For example, to ignore all test files you can run:

pylint --ignore=test_*.py

Or add in .pylintrc:

[MASTER]
ignore=test_*.py

 

Advanced Usage of Pylint

Once you're comfortable with Pylint's basic features and have customized it to your liking, you may be interested in some more advanced usage scenarios. These include integration with continuous integration tools, use in pre-commit hooks, and specialized configurations for different types of Python projects like Django or Data Science.

1. Continuous Integration (CI) Setup

Using Python Pylint in a CI pipeline can automate the code quality checking process. For example, you can integrate Pylint in a GitHub Actions workflow like this:

Create a .github/workflows/pylint.yml file in your repository:

name: Pylint

on: [push, pull_request]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - name: Check out code
      uses: actions/checkout@v2

    - name: Set up Python
      uses: actions/setup-python@v2
      with:
        python-version: 3.x

    - name: Install dependencies
      run: pip install pylint

    - name: Run Pylint
      run: pylint *.py

This GitHub Action will automatically run Pylint on every push or pull request.

2. Using with Pre-commit Hooks

Python Pylint can be run automatically before you commit your changes, ensuring that only code that passes the checks gets committed. For example, you could use a tool like pre-commit.

First, install pre-commit:

pip install pre-commit

Then add a .pre-commit-config.yaml file:

repos:
-   repo: https://github.com/pre-commit/mirrors-pylint
    rev: master
    hooks:
    -   id: pylint

Run pre-commit install to set up the git hook scripts, and now Pylint will run automatically on git commit.

3. Pylint for Django Projects

Django projects often have specific patterns and practices that a generic Python linter may not handle well. You can use a specialized Pylint plugin for Django:

pip install pylint-django

Then run Pylint like this:

pylint --load-plugins pylint_django your_project_folder/

This will enable Django-specific checks and compatibility.

4. Pylint for Data Science Projects

For Data Science projects that often use libraries like NumPy, SciPy, and Pandas, you can use the pylint-science plugin:

pip install pylint-science

Then run Pylint as follows:

pylint --load-plugins pylint_science your_data_science_folder/

This setup would be more lenient on certain conventions common in data science but not necessarily in general Python programming, such as single-letter variable names for matrices.

 

Performance Considerations for Using Python Pylint

Using Python Pylint can have an impact on the speed of your development process and the resources consumed during code analysis. Here's how you can address these concerns.

1. Speeding up Pylint

Running Pylint on a large codebase can be time-consuming. Here's a simple Python script called example.py:

# example.py

def my_function():
    x = 10
    y = 20
    print(x + y)

You can measure the time it takes to run Pylint on this script using the time command:

time pylint example.py

Suppose the output time is:

real    0m1.237s

Selective Checks

Now run Pylint with selective checks only, like so:

time pylint --disable=all --enable=W0611,W0612 example.py

You may find that the time reduces:

real    0m0.684s

The execution time drops because Pylint has fewer checks to perform.

2. Memory Usage

For measuring memory usage, you can use tools like memory-profiler. First, install it:

pip install memory-profiler

Create a script to run Pylint, let's call it run_pylint.py:

# run_pylint.py
import pylint.lint

pylint_opts = ['example.py']
pylint.lint.Run(pylint_opts)

Now run memory profiling:

mprof run run_pylint.py

You'll get a .dat file that can be visualized using:

mprof plot

This will show you how much memory was used when running Pylint on example.py.

Using fewer checks for lower memory usage

Alter run_pylint.py to run only selected checks:

# run_pylint.py
import pylint.lint

pylint_opts = ['--disable=all', '--enable=W0611,W0612', 'example.py']
pylint.lint.Run(pylint_opts)

Again, run the memory profiler and plot the data. You'll likely observe lower peak memory usage.

3. Parallel Execution

Python Pylint supports parallel execution. You can specify the number of processors to be used for the task by setting the -j flag.

pylint -j 4 my_project/

This will use 4 CPU cores to perform the linting, potentially speeding up the process significantly.

 

Comparison with Other Tools (Flake8, Pyright, Black)

In the Python ecosystem, Pylint is not the only tool available for code analysis and formatting. Below is a comparison of Pylint with other popular tools like Flake8, Pyright, and Black, focusing on their main features, use-cases, and limitations.

Feature/Tool Pylint Flake8 Pyright Black
Purpose Code Analysis Code Linting Type Checking Code Formatting
Customizable Yes Partially Partially No
Exit Codes Multiple Single Multiple Single
Speed Moderate Fast Fast Fast
Type Check Limited No Comprehensive No
Syntax Check Comprehensive Basic Basic Basic
Refactoring Suggestions Provided No No Automatic
Extensible Yes (Plugins) Yes (Plugins) No No
CI Support Strong Strong Strong Strong
PEP 8 Partial Compliance Full Compliance Partial Compliance Full Compliance
IDE Support Extensive Extensive Limited Limited

This table summarizes the core features and functionalities of each tool. Each tool has its own strengths and weaknesses, making them suitable for different aspects of Python development.

 

Frequently Asked Questions (FAQ)

What is Pylint?

Pylint is a static code analysis tool for Python that checks for programming errors, helps enforce a coding standard, and looks for code smells. It also offers features like simple refactoring suggestions.

How do I install Pylint?

Pylint can be easily installed using package managers like pip with the command pip install pylint, or you can also install it from the source.

Is Pylint compatible with Python 3?

Yes, Pylint is fully compatible with Python 3 and is continually updated to ensure ongoing compatibility.

How do I run Pylint on my code?

You can run Pylint from the command line by navigating to your project's directory and executing pylint <filename>.

What are Pylint's exit codes?

Pylint has different exit codes like 0 for a successful run, 1 for fatal messages, 2 for errors, 4 for warnings, etc., which can be useful in automated testing.

Can I customize Pylint checks?

Yes, Pylint is highly customizable. You can either use inline comments or create a .pylintrc configuration file to enable or disable specific checks.

Can Pylint be integrated into IDEs?

Absolutely, many IDEs like VSCode and PyCharm have built-in support or plugins for Pylint.

How does Pylint compare to other tools like Flake8 or Black?

While Pylint offers comprehensive code analysis, Flake8 is more focused on adhering to PEP 8, and Black is a code formatter. Each has its own set of features and limitations.

Can Pylint check for type errors?

Yes, Pylint has limited support for type checking, especially if you use type annotations in your code.

Is Pylint slow?

Pylint can take more time compared to other simpler tools due to its comprehensive checks. However, you can speed it up by only enabling specific checks that are relevant to your project.

 

Summary and Key Takeaways

In the ever-evolving world of Python development, code quality remains a cornerstone for long-term project success. Python Pylint serves as an invaluable tool for maintaining this quality. It goes beyond simple code formatting checks by offering a comprehensive suite of features such as identifying programming errors, enforcing coding standards, and even providing simple refactoring suggestions. Whether you're a beginner looking to learn best practices or an experienced developer aiming to keep your project's codebase in top shape, Pylint has something to offer.

Its exhaustive set of checks can be overwhelming, but they also make the tool highly customizable, allowing you to tailor its behavior to meet your specific needs. The tool integrates seamlessly with many development environments, and its compatibility with continuous integration pipelines makes it an excellent fit for modern DevOps practices.

In summary, Pylint stands as a versatile and robust tool that can help Python developers at any skill level improve their code quality, reduce bugs, and cultivate a more maintainable codebase.

 

Additional Resources

For further information, tutorials, and detailed documentation, you can visit the following official Pylint resources:

 

Deepak Prasad

He is the founder of GoLinuxCloud and brings over a decade of expertise in Linux, Python, Go, Laravel, DevOps, Kubernetes, Git, Shell scripting, OpenShift, AWS, Networking, and Security. With extensive experience, he excels in various domains, from development to DevOps, Networking, and Security, ensuring robust and efficient solutions for diverse projects. You can reach out to him on his LinkedIn profile or join on Facebook page.

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