Python Subprocess: Reading Output Line by Line

What will you learn?

In this tutorial, you will master the art of reading the output of a subprocess line by line in Python. This skill is crucial for efficiently processing real-time outputs from external commands or programs.

Introduction to the Problem and Solution

When executing external commands or programs using Python’s subprocess module, capturing and handling the generated output is essential. To tackle this effectively, we need to read the output line by line as it gets produced, rather than waiting for the entire output to be ready. By adopting techniques like iterating over the subprocess output stream, we can achieve this seamlessly.

One common scenario where reading output incrementally shines is when dealing with long-running processes. Instead of holding up until the command concludes entirely, reading output line by line proves to be more efficient and resourceful.


import subprocess

# Start a subprocess with a command that generates some output
proc = subprocess.Popen(["ls", "-l"], stdout=subprocess.PIPE)

# Read and print each line of output as it becomes available
while True:
    output = proc.stdout.readline()
    if not output:
    print(output.decode().strip())  # Print decoded and stripped line

# Wait for the process to finish execution    

# Close the pipe to release resources

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# Copyright PHD


  • Import the subprocess module to manage external processes.
  • Utilize Popen() function to initiate a new process with specified parameters.
  • Redirect standard output of subprocess using stdout=subprocess.PIPE.
  • Employ a loop to read lines from stdout, decode them, strip trailing whitespace, and print.
  • Continue looping until no more lines are left (readline() returns an empty byte string).
  • Conclude by waiting for child process completion with wait(), closing the pipe, and releasing resources.
  1. How do I handle errors when using subprocess?

  2. To handle errors when working with subprocesses in Python, wrap your code interacting with subprocesses within try-except blocks. This allows you to catch any exceptions raised during subprocess execution.

  3. Can I run multiple commands in a single Popen call?

  4. While technically possible through chaining commands within a shell call (e.g., using ‘&&’ or ‘;’), it’s generally discouraged due to security risks associated with shell injection attacks. Executing separate commands one after another is considered safer and more reliable.

  5. Is there a way to interact with live input/output streams of a running subprocess?

  6. Yes, methods like .communicate() on Popen objects enable interaction with stdin/stdout streams while handling deadlock issues. However, this method waits till execution completion before returning data.

  7. How does decoding work in reading bytes from stdout?

  8. Decoding involves using .decode() on byte strings obtained from stdout as they require conversion into Unicode strings before further processing as text data. Decoding transforms bytes into human-readable characters based on provided encoding rules (e.g., UTF-8).

  9. Why strip trailing whitespaces while printing lines from stdout?

  10. Stripping trailing whitespaces ensures that printed lines are clean without any additional unwanted spaces at their end. This step enhances readability of displayed text outputs.


Mastering how to read subprocess outputs line by line in Python entails creating a child process via subprocess.Popen(), capturing its standard output stream for incremental reading, processing each line as needed until completion or termination of the child process. Remember always properly handle exceptions when working with external processes in order maintain robustness in your codebase.

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