How do I print to stderr in Python?

To print to stderr (standard error) in Python, you can use one of the following methods:

1. Using print() with sys.stderr

The simplest method for most cases, similar to regular print():

import sys

print("Error: Invalid input", file=sys.stderr)

2. Using sys.stderr.write()

Directly write to stderr (no automatic newline):

import sys

sys.stderr.write("Warning: Resource low\n")  # Add \n explicitly

3. Using the logging Module

For structured logging (recommended for applications):

import logging

logging.basicConfig(level=logging.INFO)
logging.error("Critical error: Connection failed")
# Output: ERROR:root:Critical error: Connection failed

When to Use Each Method

Method	Use Case
print() + sys.stderr	Simple scripts or CLI tools
sys.stderr.write()	Full control over formatting/no newlines
logging	Production code or complex applications

Why Use stderr?

• Separation of Outputs:
• stdout (standard output) for normal program results.
• stderr for errors, warnings, and diagnostics.
• Redirection: In terminals, you can separate logs from data:

  python script.py > output.log 2> errors.log

Example Workflow

import sys

def validate_age(age):
    if age < 0:
        print(f"Invalid age: {age}", file=sys.stderr)
        return False
    return True

validate_age(-5)  # Prints "Invalid age: -5" to stderr

Choose print() for simplicity or logging for robust applications. Use sys.stderr.write() for low-level control.