How to Set Up Visual Studio Code for Python
Python
05.02.2025
Reading time: 10 min
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Creating and debugging programs in Python is easier when using a specialized Integrated Development Environment (IDE). With an IDE, you can quickly and efficiently develop, test, and debug programs.
Visual Studio Code (VS Code) for Python provides full support for the language and offers a wide range of plugins and extensions. In this article, we will install Visual Studio Code on three operating systems (Windows, macOS, Linux) and set it up for Python programming, including the use of popular plugins.
Prerequisites
To install and set up Visual Studio Code for Python, we will need the following:
-
A personal or work computer with Windows 10/11, macOS, or Ubuntu Linux distribution version 24.04 pre-installed. Alternatively, you can rent a dedicated server or a virtual machine with Windows Server 2016/2019/2022. If using regular versions of Windows, you can download your own ISO image in advance. You can also rent a server with Ubuntu.
Installing the Python Interpreter
Before installing VS Code, we need to install the Python interpreter on all three operating systems — Windows, macOS, and Linux.
On Windows
-
Go to the official Python website and download the installer file. In this case, we will be installing Python version 3.13.1.
- Run the installer file.
- You will have two installation options:
-
-
Install Now — This performs a full installation, including documentation files, the package manager pip, the tcl/tk library for graphical interface support, and standard libraries.
-
Customize Installation — This option allows you to choose which components to install.
-
We will use the full installation. Make sure to check the box next to the option Add python.exe to PATH and click Install Now. The installation process will begin.
-
Once the installation is complete, the program will notify you that it has finished.
On macOS
On macOS, the Python interpreter is pre-installed by default. You can verify this by running the following command in the terminal:
python3 --version
However, the installed version may be outdated. If necessary, you can install a newer version. To do this, we will use the Homebrew package manager. First, if Homebrew is not installed on your system, you can install it by running the following command:
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"Next, you need to check which versions of Python are available for installation. Use the following command:
brew search pythonIn our case, several versions of Python are available:
Install the latest available version, Python 3.13, by running the following command:
brew install python@3.13Check the Python version again:
python3 --version
python3.13 --version
As shown in the screenshot above, running the python3 --version still shows the old version (Python 3.9.6). However, the newly installed version (3.13) can be accessed using the command python3.13 --version. If needed, you can change the default Python version to the newly installed one. To do this, first get the full path to the newly installed Python interpreter using the following command:
brew --prefix python@3.13Then, check which shell you are using:
echo $SHELLDepending on the shell used, open the corresponding file for editing:
For bash or sh:
nano ~/.bashrcFor zsh:
nano ~/.zshrcAdd the following line at the end of the file:
export PATH="/opt/homebrew/opt/python@3.13/bin:$PATH"Save the changes and reload the file:
source ~/.zshrcNow, when you check the Python version, it will display the latest installed version:
python3 --version
On Ubuntu
By default, Python is pre-installed on almost all Linux distributions, including Ubuntu. In the latest supported versions of Ubuntu, the current version of Python is installed:
python3 --version
However, if Python is not installed for any reason, you can install it by running the following command:
apt -y install python3Installing Visual Studio Code
You can install Visual Studio Code on your personal computer. You can also rent a dedicated or cloud server with Windows Server or one of the available Linux distributions. If the required distribution is not available in the list of offered images, you can upload your own.
We will cover the installation of Visual Studio Code on three operating systems: Windows, macOS, and Linux (Ubuntu 24.04 distribution).
For Windows
Visual Studio Code supports installation on Windows 10 and Windows 11. It also supports Windows Server distributions, from version 2016 to 2022.
We will install it on Windows 10.
- Go to the official website and download the installer.
- This will download an
.exeinstallation file. Run the installer file. - On the first step, accept the license agreement by selecting the option "I accept the agreement".
- Next, the installer will prompt you to choose an installation location. You can choose the default path suggested by the installer or specify your own.
- If necessary, you can create a shortcut for the program in the Windows menu. If you don’t want to create a shortcut, select the option "Don’t create a Start Menu folder" at the bottom:
- The next step lets you configure additional options by ticking the corresponding checkboxes:
-
-
Create a desktop icon — creates a shortcut on the desktop for quick access to the program.
-
Add “Open with Code” action to Windows Explorer file context menu — adds the "Open with Code" option to the context menu when right-clicking on a file. This option allows you to quickly open any file directly in Visual Studio Code.
-
Add “Open with Code” action to Windows Explorer directory file context menu — similar to the above option but adds the "Open with Code" option to the context menu of directories (folders).
-
Register Code as an editor for supported file types — makes Visual Studio Code the default editor for certain file types (e.g., .c, .cpp, .py, .java, .js, .html files).
-
Add to PATH (require shell restart) — adds Visual Studio Code to the system’s PATH variable so it can be launched from the command line (cmd).
-
- Once all necessary options are set, Visual Studio Code is ready for installation. Click Install.
After the installation is complete, you can launch the program immediately:
For macOS
- Go to the official website and download the installer:
- After downloading, you will have a ZIP archive. Inside the archive, you will find the executable file, which you need to extract to the "Applications" directory.
- On the first launch, the system will notify you that this file was downloaded from the internet and may not have vulnerabilities. Click Open to continue:
For Linux (Ubuntu)
Visual Studio Code supports installation on Linux distributions such as Ubuntu, Debian, Red Hat, Fedora, and SUSE. You need a graphical desktop environment to install Visual Studio Code on Linux (GNOME, KDE, Xfce, etc.).
Let’s consider the installation of Visual Studio Code on Ubuntu 24.04 with the Xfce desktop environment. You can also install Visual Studio Code using Snapcraft.
-
Go to the official website and download the installer for your Linux distribution. In our case, we need the
.debinstaller:
-
Once the file is downloaded, open a terminal (console) and navigate to the directory where the file was downloaded (e.g.,
/root). -
To install, run the following command where
code_1.96.2-1734607745_amd64.debis the name of the downloaded file:
dpkg -i code_1.96.2-1734607745_amd64.deb-
During installation, a message will prompt you to add Microsoft repositories to the system. Select <Yes> and press Enter:
-
Wait for the installation to complete.
-
Once the installation is finished, you can launch the program from the applications menu (for distributions using Xfce, Visual Studio Code is available in the menu: Applications → Development):
Adding Python Interpreter to PATH Variable in Windows
If you haven't checked the Add python.exe to PATH checkbox during the Python installation on Windows, you need to manually add the full path to the interpreter to run Python from the command line.
To do this:
- Press Win+R, type
sysdm.cplin the Run window, and press Enter. - In the window that opens, go to the Advanced tab and click on the Environment Variables button.
- To add a user-level variable, select the Path variable under User variables and click on Edit.
- Double-click on an empty field or click the New button.
- Enter the full path to the Python interpreter file. By default, the Python interpreter is located at the following path:
C:\Users\<Username>\AppData\Local\Programs\Python\Python313For example:
C:\Users\Administrator\AppData\Local\Programs\Python\Python313
- After entering the path, click OK to save the changes.
- To verify, open the command prompt and type
python. If the path to the interpreter is correctly specified, the Python console will open.
Setting Up Python Interpreter in Visual Studio Code
In Windows
Once Python is installed, you need to connect it to Visual Studio Code. To do this:
-
Open Visual Studio Code and click the New File... button on the home page to create a new file.
Alternatively, you can create a Python project in Visual Studio Code by clicking on Open Folder…, where you can select the entire project folder containing the files.
-
Type any name for the file, use the
.pyextension, and press Enter.
-
Save the file in any location. Ensure that the file name ends with the
.pyextension.
- Once the file is saved, the interface in Visual Studio Code will display a prompt at the bottom right, suggesting you install the recommended Python extension.
- To run Python in Visual Studio Code, you first need to select the Python interpreter. A button will appear at the bottom of the panel with a warning: Select Interpreter. Click on it.
- In the menu that appears, select Enter interpreter path… and press Enter.
- Specify the full path to the Python interpreter. By default, it is located at:
C:\Users\Administrator\AppData\Local\Programs\Python\Python313where Administrator is your user account name. Select the file named python and click on Select Interpreter.
To test it, write a simple program that calculates the square root of a number:
import math
num = float(input("Enter a number to find its square root: "))
if num >= 0:
sqrt_result = math.sqrt(num)
print(f"The square root of {num} is {sqrt_result}")
else:
print("Square root of a negative number is not real.")
In Visual Studio Code, to run the Python code, click the Run Python File button at the top-right. If the interpreter is set up correctly, the program will run successfully.
In macOS
On macOS operating systems, the Python interpreter is automatically recognized. Simply create a new .py file as described in the Windows section above and run the program directly.
In Ubuntu
Similarly to macOS, in the Ubuntu distribution, VS Code automatically detects the installed Python interpreter in the system. All you need to do is create a new .py file and run the program directly.
Recommended Extensions for Python in Visual Studio Code
VS Code offers a wide range of Python extensions (plugins) that simplify the development process. Here are some of the most popular ones:
Pylance
Pylance provides code analysis, autocompletion, and IntelliSense support, making Python development more efficient and user-friendly. Key features include fast autocompletion, type checking, and IntelliSense support.
Jupyter
The Jupyter extension is a powerful tool for working with interactive notebooks directly in the editor. It’s especially useful for data analysis, machine learning, visualization, and interactive programming tasks.
autoDocstring — Python Docstring Generator
autoDocstring is a popular extension that helps automatically generate docstrings for Python functions, methods, and classes. Docstrings improve code readability and serve as built-in documentation.
isort
isort is a tool for automatically sorting and organizing imports in Python code. You can configure it in Visual Studio Code to make working with imports easier and to improve code readability.
Conclusion
This article covered the installation and setup of Visual Studio Code for Python development. Visual Studio Code offers full support for Python and provides the ability to extend its functionality through various plugins, making the coding process easier and more efficient.
Python
05.02.2025
Reading time: 10 min
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Python
Command-Line Option and Argument Parsing using argparse in Python
Command-line interfaces (CLIs) are one of the quickest and most effective means of interacting with software. They enable you to provide commands directly which leads to quicker execution and enhanced features. Developers often build CLIs using Python for several applications, utilities, and automation scripts, ensuring they can dynamically process user input. This is where the Python argparse module steps in.
The argparse Python module streamlines the process of managing command-line inputs, enabling developers to create interactive and user-friendly utilities. As part of the standard library, it allows programmers to define, process, and validate inputs seamlessly without the need for complex logic.
This article will discuss some of the most important concepts, useful examples, and advanced features of the argparse module so that you can start building solid command-line tools right away.
How to Use Python argparse for Command-Line Interfaces
This is how to use argparse in your Python script:
Step 1: Import Module
First import the module into your Python parser script:
import argparse
This inclusion enables parsing .py arg inputs from the command line.
Step 2: Create an ArgumentParser Object
The ArgumentParser class is the most minimal class of the Python argumentparser module's API. To use it, begin by creating an instance of the class:
parser = argparse.ArgumentParser(description="A Hostman tutorial on Python argparse.")
Here:
description describes what the program does and will be displayed when someone runs --help.
Step 3: Add Inputs and Options
Define the parameters and features your program accepts via add_argument() function:
parser.add_argument('filename', type=str, help="Name of the file to process")
parser.add_argument('--verbose', action='store_true', help="Enable verbose mode")
Here:
filename is a mandatory option.
--verbose is optional, to allow you to set the flag to make it verbose.
Step 4: Parse User Inputs
Process the user-provided inputs by invoking the parse_args() Python method:
args = parser.parse_args()
This stores the command-line values as attributes of the args object for further use in your Python script.
Step 5: Access Processed Data
Access the inputs and options for further use in your program:
For example:
print(f"File to process: {args.filename}")
if args.verbose:
print("Verbose mode enabled")
else:
print("Verbose mode disabled")
Example CLI Usage
Here are some scenarios to run this script:
File Processing Without Verbose Mode
python3 file.py example.txt
File Processing With Verbose Mode
python3 file.py example.txt --verbose
Display Help
If you need to see what arguments the script accepts or their description, use the --help argument:
python3 file.py --help
Common Examples of argparse Usage
Let's explore a few practical examples of the module.
Example 1: Adding Default Values
Sometimes, optional inputs in command-line interfaces need predefined values for smoother execution. With this module, you can set a default value that applies when someone doesn’t provide input.
This script sets a default timeout of 30 seconds if you don’t specify the --timeout parameter.
import argparse
# Create the argument parser
parser = argparse.ArgumentParser(description="Demonstrating default argument values.")
# Pass an optional argument with a default value
parser.add_argument('--timeout', type=int, default=30, help="Timeout in seconds (default: 30)")
# Interpret the arguments
args = parser.parse_args()
# Retrieve and print the timeout value
print(f"Timeout value: {args.timeout} seconds")
Explanation
Importing Module: Importing the argparse module.
Creating the ArgumentParser Instance: An ArgumentParser object is created with a description so that a short description of the program purpose is provided. This description is displayed when the user runs the program via the --help option.
Including --timeout: The --timeout option is not obligatory (indicated by the -- prefix). The type=int makes the argument for --timeout an integer. The default=30 is provided so that in case the user does not enter a value, then the timeout would be 30 seconds. The help parameter adds a description to the argument, and it will also appear in the help documentation.
Parsing Process: The parse_args() function processes user inputs and makes them accessible as attributes of the args object. In our example, we access args.timeout and print out its value.
Case 1: Default Value Used
If the --timeout option is not specified, the default value of 30 seconds is used:
python file.py
Case 2: Custom Value Provided
For a custom value for --timeout (e.g., 60 seconds), apply:
python file.py --timeout 60
Example 2: Utilizing Choices
The argparse choices parameter allows you to restrict an argument to a set of beforehand known valid values. This is useful if your program features some specific modes, options, or settings to check.
Here, we will specify a --mode option with two default values: basic and advanced.
import argparse
# Creating argument parser
parser = argparse.ArgumentParser(description="Demonstrating the use of choices in argparse.")
# Adding the --mode argument with predefined choices
parser.add_argument('--mode', choices=['basic', 'advanced'], help="Choose the mode of operation")
# Parse the arguments
args = parser.parse_args()
# Access and display the selected mode
if args.mode:
print(f"Mode selected: {args.mode}")
else:
print("No mode selected. Please choose 'basic' or 'advanced'.")
Adding --mode: The choices argument indicates that valid options for the --mode are basic and advanced. The application will fail when the user supplies an input other than in choices.
Help Text: The help parameter gives valuable information when the --help command is executed.
Case 1: Valid Input
To specify a valid value for --mode, utilize:
python3 file.py --mode basic
Case 2: No Input Provided
For running the program without specifying a mode:
python3 file.py
Case 3: Invalid Input
If a value is provided that is not in the predefined choices:
python3 file.py --mode intermediate
Example 3: Handling Multiple Values
The nargs option causes an argument to accept more than one input. This is useful whenever your program requires a list of values for processing, i.e., numbers, filenames, or options.
Here we will show how to use nargs='+' to accept a --numbers option that can take multiple integers.
import argparse
# Create an ArgumentParser object
parser = argparse.ArgumentParser(description="Demonstrating how to handle multiple values using argparse.")
# Add the --numbers argument with nargs='+'
parser.add_argument('--numbers', nargs='+', type=int, help="List of numbers to process")
# Parse the arguments
args = parser.parse_args()
# Access and display the numbers
if args.numbers:
print(f"Numbers provided: {args.numbers}")
print(f"Sum of numbers: {sum(args.numbers)}")
else:
print("No numbers provided. Please use --numbers followed by a list of integers.")
Adding the --numbers Option: The user can provide a list of values as arguments for --numbers. type=int interprets the input as an integer. If a non-integer input is provided, the program raises an exception. The help parameter gives the information.
Parsing Phase: After parsing the arguments, the input to --numbers is stored in the form of a list in args.numbers.
Utilizing the Input: You just need to iterate over the list, calculate statistics (e.g., sum, mean), or any other calculation on the input.
Case 1: Providing Multiple Numbers
To specify multiple integers for the --numbers parameter, execute:
python3 file.py --numbers 10 20 30
Case 2: Providing a Single Number
If just one integer is specified, run:
python3 file.py --numbers 5
Case 3: No Input Provided
If the script is run without --numbers:
python3 file.py
Case 4: Invalid Input
In case of inputting a non-integer value:
python3 file.py --numbers 10 abc 20
Example 4: Required Optional Arguments
Optional arguments (those that begin with the --) are not mandatory by default. But there are times when you would like them to be mandatory for your script to work properly. You can achieve this by passing the required=True parameter when defining the argument.
In this script, --config specifies a path to a configuration file. By leveraging required=True, the script enforces that a value for --config must be provided. If omitted, the program will throw an error.
import argparse
# Create an ArgumentParser object
parser = argparse.ArgumentParser(description="Demonstrating required optional arguments in argparse.")
# Add the --config argument
parser.add_argument('--config', required=True, help="Path to the configuration file")
# Parse the arguments
args = parser.parse_args()
# Access and display the provided configuration file path
print(f"Configuration file path: {args.config}")
Adding the --config Option: --config is considered optional since it starts with --. However, thanks to the required=True parameter, users must include it when they run the script. The help parameter clarifies what this parameter does, and you'll see this information in the help message when you use --help.
Parsing: The parse_args() method takes care of processing the arguments. If someone forgets to include --config, the program will stop and show a clear error message.
Accessing the Input: The value you provide for --config gets stored in args.config. You can then use this in your script to work with the configuration file.
Case 1: Valid Input
For providing a valid path to the configuration file, use:
python3 file.py --config settings.json
Case 2: Missing the Required Argument
For running the script without specifying --config, apply:
python3 file.py
Advanced Features
While argparse excels at handling basic command-line arguments, it also provides advanced features that enhance the functionality and usability of your CLIs. These features ensure your scripts are scalable, readable, and easy to maintain. Below are some advanced capabilities you can leverage.
Handling Boolean Flags
Boolean flags allow toggling features (on/off) without requiring user input. Use the action='store_true' or action='store_false' parameters to implement these flags.
parser.add_argument('--debug', action='store_true', help="Enable debugging mode")
Including --debug enables debugging mode, useful for many Python argparse examples.
Grouping Related Arguments
Use add_argument_group() to organize related arguments, improving readability in complex CLIs.
group = parser.add_argument_group('File Operations')
group.add_argument('--input', type=str, help="Input file")
group.add_argument('--output', type=str, help="Output file")
Grouped arguments appear under their own section in the --help documentation.
Mutually Exclusive Arguments
To ensure users select only one of several conflicting options, use the add_mutually_exclusive_group() method.
group = parser.add_mutually_exclusive_group()
group.add_argument('--json', action='store_true', help="Output in JSON format")
group.add_argument('--xml', action='store_true', help="Output in XML format")
This ensures one can choose either JSON or XML, but not both.
Conclusion
The argparse Python module simplifies creating reliable CLIs for handling Python program command line arguments. From the most basic option of just providing an input to more complex ones like setting choices and nargs, developers can build user-friendly and robust CLIs. Following the best practices of giving proper names to arguments and writing good docstrings would help you in making your scripts user-friendly and easier to maintain.
21 July 2025 · 10 min to read
Python
How to Get the Length of a List in Python
Lists in Python are used almost everywhere. In this tutorial we will look at four ways to find the length of a Python list: by using built‑in functions, recursion, and a loop. Knowing the length of a list is most often required to iterate through it and perform various operations on it.
len() function
len() is a built‑in Python function for finding the length of a list. It takes one argument—the list itself—and returns an integer equal to the list’s length. The same function also works with other iterable objects, such as strings.
Country_list = ["The United States of America", "Cyprus", "Netherlands", "Germany"]
count = len(Country_list)
print("There are", count, "countries")
Output:
There are 4 countries
Finding the Length of a List with a Loop
You can determine a list’s length in Python with a for loop. The idea is to traverse the entire list while incrementing a counter by 1 on each iteration. Let’s wrap this in a separate function:
def list_length(list):
counter = 0
for i in list:
counter = counter + 1
return counter
Country_list = ["The United States of America", "Cyprus", "Netherlands", "Germany", "Japan"]
count = list_length(Country_list)
print("There are", count, "countries")
Output:
There are 5 countries
Finding the Length of a List with Recursion
The same task can be solved with recursion:
def list_length_recursive(list):
if not list:
return 0
return 1 + list_length_recursive(list[1:])
Country_list = ["The United States of America", "Cyprus", "Netherlands","Germany", "Japan", "Poland"]
count = list_length_recursive(Country_list)
print("There are", count, "countries")
Output:
There are 6 countries
How it works. The function list_length_recursive() receives a list as input.
If the list is empty, it returns 0—the length of an empty list.
Otherwise it calls itself recursively with the argument list[1:], a slice of the original list starting from index 1 (i.e., the list without the element at index 0). The result of that call is added to 1. With each recursive step the returned value grows by one while the list shrinks by one element.
length_hint() function
The length_hint() function lives in the operator module. That module contains functions analogous to Python’s internal operators: addition, subtraction, comparison, and so on. length_hint() returns the length of iterable objects such as strings, tuples, dictionaries, and lists. It works similarly to len():
from operator import length_hint
Country_list = ["The United States of America", "Cyprus", "Netherlands","Germany", "Japan", "Poland", "Sweden"]
count = length_hint(Country_list)
print("There are", count, "countries")
Output:
There are 7 countries
Note that length_hint() must be imported before use.
Conclusion
In this guide we covered four ways to determine the length of a list in Python. Under equal conditions the most efficient method is len(). The other approaches are justified mainly when you are implementing custom classes similar to list.
17 July 2025 · 3 min to read
Python
Understanding the main() Function in Python
In any complex program, it’s crucial to organize the code properly: define a starting point and separate its logical components.
In Python, modules can be executed on their own or imported into other modules, so a well‑designed program must detect the execution context and adjust its behavior accordingly.
Separating run‑time code from import‑time code prevents premature execution, and having a single entry point makes it easier to configure launch parameters, pass command‑line arguments, and set up tests. When all important logic is gathered in one place, adding automated tests and rolling out new features becomes much more convenient.
For exactly these reasons it is common in Python to create a dedicated function that is called only when the script is run directly. Thanks to it, the code stays clean, modular, and controllable. That function, usually named main(), is the focus of this article.
All examples were executed with Python 3.10.12 on a Hostman cloud server running Ubuntu 22.04.
Each script was placed in a separate .py file (e.g., script.py) and started with:
python script.py
The scripts are written so they can be run just as easily in any online Python compiler for quick demonstrations.
What Is the main() Function in Python
The simplest Python code might look like:
print("Hello, world!") # direct execution
Or a script might execute statements in sequence at file level:
print("Hello, world!") # action #1
print("How are you, world?") # action #2
print("Good‑bye, world...") # action #3
That trivial arrangement works only for the simplest scripts. As a program grows, the logic quickly becomes tangled and demands re‑organization:
# function containing the program’s main logic (entry point)
def main():
print("Hello, world!")
# launch the main logic
if __name__ == "__main__":
main() # call the function with the main logic
With more actions the code might look like:
def main():
print("Hello, world!")
print("How are you, world?")
print("Good‑bye, world...")
if __name__ == "__main__":
main()
This implementation has several important aspects, discussed below.
The main() Function
The core program logic lives inside a separate function. Although the name can be anything, developers usually choose main, mirroring C, C++, Java, and other languages.
Both helper code and the main logic are encapsulated: nothing sits “naked” at file scope.
# greeting helper
def greet(name):
print(f"Hello, {name}!")
# program logic
def main():
name = input("Enter your name: ")
greet(name)
# launch the program
if __name__ == "__main__":
main()
Thus main() acts as the entry point just as in many other languages.
The if __name__ == "__main__" Check
Before calling main() comes the somewhat odd construct if __name__ == "__main__":.
Its purpose is to split running from importing logic:
If the script runs directly, the code inside the if block executes.
If the script is imported, the block is skipped.
Inside that block, you can put any code—not only the main() call:
if __name__ == "__main__":
print("Any code can live here, not only main()")
__name__ is one of Python’s built‑in “dunder” (double‑underscore) variables, often called magic or special. All dunder objects are defined and used internally by Python, but regular users can read them too.
Depending on the context, __name__ holds:
"__main__" when the module runs as a standalone script.
The module’s own name when it is imported elsewhere.
This lets a module discover its execution context.
Advantages of Using main()
Organization
Helper functions and classes, as well as the main function, are wrapped separately, making them easy to find and read. Global code is minimal—only initialization stays at file scope:
def process_data(data):
return [d * 2 for d in data]
def main():
raw = [1, 2, 3, 4]
result = process_data(raw)
print("Result:", result)
if __name__ == "__main__":
main()
A consistent style means no data manipulation happens at the file level. Even in a large script you can quickly locate the start of execution and any auxiliary sections.
Isolation
When code is written directly at the module level, every temporary variable, file handle, or connection lives in the global namespace, which can be painful for debugging and testing. Importing such a module pollutes the importer’s globals:
# executes immediately on import
values = [2, 4, 6]
doubles = []
for v in values:
doubles.append(v * 2)
print("Doubled values:", doubles)
With main() everything is local; when the function returns, its variables vanish:
def double_list(items):
return [x * 2 for x in items] # create a new list with doubled elements
def main():
values = [2, 4, 6]
result = double_list(values)
print("Doubled values:", result)
if __name__ == "__main__":
main()
That’s invaluable for unit testing, where you might run specific functions (including main()) without triggering the whole program.
Safety
Without the __name__ check, top‑level code runs even on import—usually undesirable and potentially harmful.
some.py:
print("This code will execute even on import!")
def useful_function():
return 42
main.py:
import some
print("The logic of the imported module executed itself...")
Console:
This code will execute even on import!
The logic of the imported module executed itself...
The safer some.py:
def useful_function():
return 42
def main():
print("This code will not run on import")
main() plus the __name__ check guard against accidental execution. Inside main() you can also verify user permissions or environment variables.
How to Write main() in Python
Remember: main() is not a language construct, just a regular function promoted to “entry point.” To ensure it runs only when the script starts directly:
Tools – define helper functions with business logic.
Logic – assemble them inside main() in the desired order.
Check – add the if __name__ == "__main__" guard.
This template yields structured, import‑safe, test‑friendly code—excellent practice for any sizable Python project.
Example Python Program Using main()
# import the standard counter
from collections import Counter
# runs no matter how the program starts
print("The text‑analysis program is active")
# text‑analysis helper
def analyze_text(text):
words = text.split() # split text into words
total = len(words) # total word count
unique = len(set(words)) # unique word count
avg_len = sum(len(w) for w in words) / total if total else 0
freq = Counter(words) # build frequency counter
top3 = freq.most_common(3) # top three words
return {
'total': total,
'unique': unique,
'avg_len': avg_len,
'top3': top3
}
# program’s main logic
def main():
print("Enter text (multiple lines). Press Enter on an empty line to finish:")
lines = []
while True:
line = input()
if not line:
break
lines.append(line)
text = ' '.join(lines)
stats = analyze_text(text)
print(f"\nTotal number of words: {stats['total']}")
print(f"Unique words: {stats['unique']}")
print(f"Average word length: {stats['avg_len']:.2f}")
print("Top‑3 most frequent words:")
for word, count in stats['top3']:
print(f" {word!r}: {count} time(s)")
# launch program
if __name__ == "__main__":
main()
Running the script prints a prompt:
Enter text (multiple lines). Press Enter on an empty line to finish:
Input first line:
Star cruiser Orion glided silently through the darkness of intergalactic space.
Second line:
Signals of unknown life‑forms flashed on the onboard sensors where the nebula glowed with a phosphorescent light.
Third line:
The cruiser checked the sensors, then the cruiser activated the defense system, and the cruiser returned to its course.
Console output:
The text‑analysis program is active
Total number of words: 47
Unique words: 37
Average word length: 5.68
Top‑3 most frequent words:
'the': 7 time(s)
'cruiser': 4 time(s)
'of': 2 time(s)
If you import this program (file program.py) elsewhere:
import program # importing program.py
Only the code outside main() runs:
The text‑analysis program is active
So, a moderately complex text‑analysis utility achieves clear logic separation and context detection.
When to Use main() and When Not To
Use main() (almost always appropriate) when:
Medium/large scripts – significant code with non‑trivial logic, multiple functions/classes.
Libraries or CLI utilities – you want parts of the module importable without side effects.
Autotests – you need to test pure logic without extra boilerplate.
You can skip main() when:
Tiny one‑off scripts – trivial logic for a quick data tweak.
Educational snippets – short examples illustrating a few syntax features.
In short, if your Python program is a standalone utility or app with multiple processing stages, command‑line arguments, and external resources—introduce main().
If it’s a small throw‑away script, omitting main() keeps things concise.
Conclusion
The main() function in Python serves two critical purposes:
Isolates the program’s core logic from the global namespace.
Separates standalone‑execution logic from import logic.
Thus, a Python file evolves from a straightforward script of sequential actions into a fully‑fledged program with an entry point, encapsulated logic, and the ability to detect its runtime environment.
14 July 2025 · 8 min to read
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