How can I flush the output of the print function in Python?

To flush the output of the print() function in Python (i.e., force the text to appear immediately in the console instead of waiting for the buffer), use one of the following methods:

1. Use the flush=True Parameter (Python 3.3+)

Add the flush parameter to the print() function to force immediate output:

print("Hello, world!", flush=True)  # Flushes the buffer immediately

Example with a Progress Indicator:

import time

for i in range(5):
    print(f"Progress: {i+1}/5", end='\r', flush=True)  # Overwrite line in real-time
    time.sleep(1)

2. Manually Flush sys.stdout

For older Python versions (or explicit control), flush the buffer manually:

import sys

print("Hello, world!")  # Buffered output
sys.stdout.flush()       # Force buffer flush

3. Disable Buffering Entirely

Run your script with unbuffered output using the -u flag:

python -u your_script.py

Or set the PYTHONUNBUFFERED environment variable:

export PYTHONUNBUFFERED=1  # Linux/macOS
python your_script.py

Key Notes

• Default Behavior: By default, print() flushes the buffer when a newline (\n) is printed or when the program ends.
• Use Cases: Flushing is critical for real-time logging, progress bars, or when interacting with long-running processes.
• Buffering Modes:
• Unbuffered: Immediate output (e.g., python -u).
• Line-Buffered: Flush on newline (default for terminals).
• Block-Buffered: Flush when buffer is full (default for files/redirection).

Example: Real-Time Output in a Loop

Without flushing, this would print all lines at once after the loop ends:

import time

for i in range(3):
    print(f"Update {i}")  # Buffered (no flush)
    time.sleep(1)

With Flushing:

for i in range(3):
    print(f"Update {i}", flush=True)  # Prints immediately
    time.sleep(1)

Summary

• Use flush=True in print() for simplicity (Python 3.3+).
• Use sys.stdout.flush() for compatibility with older versions.
• Run scripts with -u or PYTHONUNBUFFERED to disable buffering globally.