When I was working on a Python script that needed to take input from the command line, display results to the user, and log errors separately.
The issue was… I realized many beginners (and even some experienced developers) are not fully aware of how stdin, stdout, and stderr work in Python.
So, in this tutorial, I’ll share everything I’ve learned about these three streams in my Python development journey. I’ll also cover multiple methods with practical examples so you can start using them right away.
What are stdin, stdout, and stderr in Python?
- stdin (Standard Input): This is where Python reads input from. By default, it’s your keyboard.
- stdout (Standard Output): This is where Python sends normal output. By default, it’s your terminal screen.
- stderr (Standard Error): This is where Python sends error messages. By default, it also goes to your screen, but it can be redirected separately.
Think of them as three communication channels between your Python program and the outside world.
Method 1 – Use input() and print() (Default Behavior)
The simplest way to use stdin and stdout is through the input() and print() methods in Python.
Here’s an example:
# Example 1: Using stdin and stdout
name = input("Enter your name: ") # Reads from stdin (keyboard)
print(f"Hello, {name}! Welcome to Python.") # Writes to stdout (screen)I executed the above example code and added the screenshot below.
When I run this script, Python waits for me to type something. Once I press Enter, it prints the greeting.
Method 2 – Use sys.stdin, sys.stdout, and sys.stderr
For more control, Python provides sys.stdin, sys.stdout, and sys.stderr as file-like objects.
Here’s how I use them:
import sys
# Example 2: Reading and writing with sys.stdin, sys.stdout, sys.stderr
sys.stdout.write("Enter your favorite city in the USA: ")
city = sys.stdin.readline().strip() # Reads input directly
sys.stdout.write(f"You entered: {city}\n")
# Simulating an error
if city.lower() not in ["new york", "los angeles", "chicago", "houston"]:
sys.stderr.write("Warning: That city is not in the top 4 by population.\n")I executed the above example code and added the screenshot below.
This approach is useful when I need to redirect input/output to files or when I want to separate errors from normal output.
Method 3 – Redirect stdout and stderr to a File
Sometimes, I don’t want the output on the screen. For example, when I run a script overnight on a server, I prefer saving logs to a file.
Here’s how I redirect stdout and stderr:
import sys
# Example 3: Redirecting stdout and stderr
with open("output.log", "w") as f_out, open("error.log", "w") as f_err:
sys.stdout = f_out
sys.stderr = f_err
print("This will go to output.log")
raise ValueError("This error will go to error.log")I executed the above example code and added the screenshot below.
When I run this, the normal message goes to output.log, and the error goes to error.log.
Method 4 – Use Subprocess with stdin, stdout, and stderr
In real-world applications, I often need to run external commands (like shell commands) from Python. In such cases, the subprocess module is my go-to.
Here’s an example:
import subprocess
# Example 4: Using subprocess with stdin, stdout, and stderr
process = subprocess.Popen(
["python", "-c", "print('Hello from subprocess!'); raise ValueError('Oops!')"],
stdout=subprocess.PIPE,
stderr=subprocess.PIPE,
text=True
)
out, err = process.communicate()
print("Captured stdout:", out)
print("Captured stderr:", err)I executed the above example code and added the screenshot below.
This lets me capture both stdout and stderr from another program and handle them separately in Python.
Method 5 – Print to stderr Directly
Sometimes, I want to show an error message without raising an exception. In such cases, I print directly to stderr.
import sys
# Example 5: Printing error messages to stderr
print("Processing data...", file=sys.stdout)
print("Error: Invalid data format!", file=sys.stderr)This way, the error message doesn’t get mixed with normal output.
Method 6 – Redirect Streams Temporarily
Another trick I use is redirecting stdout temporarily, especially when I want to capture printed output inside my code.
import sys
import io
# Example 6: Temporarily redirecting stdout
buffer = io.StringIO()
sys_stdout_backup = sys.stdout
sys.stdout = buffer
print("This will be captured instead of printed.")
sys.stdout = sys_stdout_backup # Restore stdout
captured_output = buffer.getvalue()
print("Captured text:", captured_output)This is super handy for testing when I want to check what my function prints without actually displaying it on the screen.
So, that’s how I use stdin, stdout, and stderr in Python.
- For simple scripts, input() and print() are enough.
- For advanced control, sys.stdin, sys.stdout, and sys.stderr give me flexibility.
- For automation and logging, I often redirect streams to files.
- For running external commands, subprocess is my best friend.
Both beginners and experienced developers can benefit from mastering these three streams. They’re the foundation of how Python communicates with the outside world.
You may also read:
- Remove Item from Set in Python
- Check if a Python Set is Empty
- Add Item to Set in Python
- Create an Empty Set in Python
I am Bijay Kumar, a Microsoft MVP in SharePoint. Apart from SharePoint, I started working on Python, Machine learning, and artificial intelligence for the last 5 years. During this time I got expertise in various Python libraries also like Tkinter, Pandas, NumPy, Turtle, Django, Matplotlib, Tensorflow, Scipy, Scikit-Learn, etc… for various clients in the United States, Canada, the United Kingdom, Australia, New Zealand, etc. Check out my profile.
