How do I print to stderr in Python?

Creating a Custom Function for Error Printing

To make your life easier when dealing with error messages, create a special function for this. We'll call this eprint (the "e" is for "error").

Defining eprint

Write this once, use it forever.

import sys

def eprint(*args, **kwargs):
    print(*args, file=sys.stderr, **kwargs)  # basically "print", but edgier
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Using eprint

It's like a regular print, but in red (not really, but let's pretend).

eprint("Error:", "It's not you, it's me.", sep=" - ")
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This will print: "Error: - It's not you, it's me." to stderr.

How eprint Enhances Error Handling

In situations where you need to log an unusual condition or error message, eprint becomes your new best friend.

Proper handling of '\n'

When using sys.stderr.write(), remember to tuck in a '\n' for the message to have a good night's sleep (i.e., line break). The print function takes care of this automatically, so no need to be a helicopter parent.

Using the 'future' features (Yeah, Python is ahead of time)

If your code is in Python 2 but you're afraid of being left in the past, import print_function from __future__.

from __future__ import print_function # living in the future! 🚀
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This tells Python 2 to use the Python 3 print function. This way, you avoid the outdated print >> sys.stderr, "message" syntax.

Making Erroneous Situations More Entertaining with Jokes

Go for Direct Writing for Maximum Thrills

If living on the edge is your thing or if you need maximum performance, write directly to stderr.

sys.stderr.write("'What's up, doc?' - Bugs Bunny, probably\n")
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Print function redirection: The Modern Way

Starting Python 3, the print function got a major upgrade: redirecting its output to stderr directly.

print("Don't you wish your print function was versatile like me?", file=sys.stderr)  # showing off new skills
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Professional Error Tracking with Logging

Basic setup with the logging module

With Python's logging module, you can go professional and ditch the primitive ways of error tracking.

import logging

logging.basicConfig(level=logging.ERROR, format='%(levelname)s: %(message)s')
logger = logging.getLogger('my_logger')  # my_logger, sounds professional!
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Use logger.error("An error clearly happened") to write an error message to stderr.

Using logging for warnings without spamming stdout

Sometimes, non-critical errors occur. They're like when your pet misbehaves: not an immediate crisis, but you'd wish they hadn't done it. Use log.warning() to handle these minor disturbances.

logger.warning('This is the "your pet peed on the floor again" kind of warning')  # 😼 mischief!
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These warnings get sent to stderr, leaving your stdout clean and beautiful.

Advanced stderr Outputs

Direct writing to stderr with os.write()

For more control over the output, use os.write(), which writes directly to a file descriptor.

import os
os.write(2, b'This one goes straight to stderr! \n')  # sending secret messages
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Remember to specify 2 as the first parameter, since that's the file descriptor for stderr.

Prioritizing compatibility

Even if you're in Python 2, use methods compatible with Python 3 for a smooth transition in the future.

import sys
if sys.version_info[0] < 3:
    from __future__ import print_function  # ensures Python 2 behaves like Python 3
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