Python Tips: How to Call and Retrieve Output & Return Code from an External Program?

If you are a Python developer, you may encounter a scenario where you need to interact with external programs. And when it comes to interacting with these programs, retrieving output and return code is essential. So, if you want to know the most efficient way to call an external program from Python and retrieve both its output and return code, this article is for you.

Many developers face challenges while working with external programs in Python. But have no worries! With the help of Python’s subprocess module, you can easily execute an external command and retrieve its return code and output. Furthermore, the subprocess module is capable of handling complex scenarios, such as running shell commands, piping input, and output, among other things.

If you’re tired of manually executing external programs and tediously retrieving their output and return code, then it’s time to update your knowledge with our Python Tips! Not only will it save you time, but it’ll also make things easier for you. Reading this article will provide you with all the necessary knowledge needed to quickly implement this into your Python code. So, without further ado, let’s dive into the details!

“How To Call An External Program In Python And Retrieve The Output And Return Code?” ~ bbaz

Introduction

Python is a widely used programming language in diverse fields like data science, web development, machine learning, and automation. It offers many built-in modules and libraries that can help you achieve your tasks in a better way. One such module is Python’s subprocess module which can help you execute external programs from within your Python script. This article discusses how you can use Python’s subprocess module to call an external program and retrieve both its output and return code with ease.

What is the Subprocess Module in Python?

The subprocess module is a built-in module in Python that allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. By using the subprocess module, you can execute shell commands, run external programs, and even interact with them from within your Python script. The module provides several classes and functions to achieve these tasks with ease.

Why Use the Subprocess Module in Python?

There are many benefits to using the subprocess module in Python for executing external programs. Some of these are:

• Platform Independence: The subprocess module is cross-platform and works on Windows, Linux, and macOS operating systems.
• Easy Integration: You can easily integrate the subprocess module with your existing Python code and leverage its features.
• Scalability: The subprocess module can handle complex scenarios involving multiple processes and inter-process communication.
• Security: Using the subprocess module eliminates the need for shell injection attacks, making your code more secure.

How to Call an External Program in Python?

To call an external program from Python using subprocess module, you need to follow these steps:

1. Import the subprocess module
2. Call the subprocess.run() function with the desired command as an argument.
3. Retrieve the output and return code from the subprocess using the stdout and returncode attributes.

Example:

Let’s say we want to call the ls command and retrieve its output and return code. Here’s how you can do it:

“`import subprocesscmd = lsresult = subprocess.run(cmd, stdout=subprocess.PIPE)print(result.stdout) # Outputprint(result.returncode) # Return Code“`

Handling Complex Operations using Subprocess Module

The subprocess module provides several classes and functions that can handle complex scenarios like executing shell commands, piping input and output, and even dealing with error messages. Let’s explore some of these features in detail below.

Running Shell Commands

You can execute shell commands by setting shell parameter to True while calling the run() function. It allows you to execute commands like you would in a terminal window.

Example:

Let’s say we want to run a shell command that lists all files and folders in the current directory and sorts them by size. Here’s how you can achieve it:

“`import subprocesscmd = ls -lSresult = subprocess.run(cmd, shell=True, stdout=subprocess.PIPE)print(result.stdout) # Outputprint(result.returncode) # Return Code“`

Piping Input and Output

You can pipe the input and output of a subprocess by setting the stdin and stdout arguments while calling the run() function. You can also use pipes to communicate between multiple subprocesses.

Example:

Let’s say we want to execute a command that takes input from the user and prints it back. Here’s how you can do it:

“`import subprocesscmd = catresult = subprocess.run(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, text=True)input_str = Hello, World!\noutput_str = result.communicate(input=input_str)[0]print(output_str) # Outputprint(result.returncode) # Return Code“`

Error Handling

The subprocess module provides several functions to handle errors like CalledProcessError in case the return code is non-zero. You can also capture the output of error messages using stderr attribute.

Example:

Let’s say we want to execute a command that throws an error. Here’s how you can handle it:

“`import subprocesscmd = ls /non_existent_directorytry: result = subprocess.run(cmd, check=True, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)except subprocess.CalledProcessError as e: print(e.output) # Error Message print(e.returncode) # Return Code“`

Conclusion

The subprocess module is a powerful tool for executing external programs from within your Python script. Whether you want to execute shell commands, run external programs, or handle complex scenarios involving inter-process communication and error handling, the subprocess module provides everything you need. The module is cross-platform, easy to integrate, scalable, and secure, making it an ideal choice for developers who want to streamline their code and enhance productivity. By following the steps outlined in this article, you can quickly learn how to use the subprocess module to call an external program from Python and retrieve both its output and return code with ease.

Here are some frequently asked questions about how to call and retrieve output and return code from an external program using Python:

1. What is an external program?

An external program is any program that is not part of the current Python script, but can be called from within it.

2. How do I call an external program from Python?

You can use the subprocess module in Python to call an external program. This module provides a way to spawn new processes, connect to their input/output/error pipes, and obtain their return codes.

3. How do I retrieve the output of an external program that I called from Python?

You can use the subprocess.check_output function to capture the output of an external program as a byte string. This function runs the command passed to it and returns the output as a byte string.

4. How do I retrieve the return code of an external program that I called from Python?

You can use the subprocess.run function to run an external program and obtain its return code. This function returns a CompletedProcess object with attributes such as returncode that you can use to obtain the return code of the external program.

5. Can I call an external program asynchronously from Python?

Yes, you can use the asyncio.create_subprocess_exec function to call an external program asynchronously in Python. This function creates a subprocess and returns a coroutine that can be awaited.