Why is My Python Script Not Printing Immediately?

Output Buffering in Python

In Python, the print function by default buffers output to improve performance. This prevents the terminal from displaying output immediately, which can be problematic when monitoring the progress of a script.

Why it's Buffering

The terminal is a separate program that communicates with your Python script. Buffering reduces the overhead of sending text to the terminal by storing it internally until a specific amount has been accumulated or a newline character is encountered.

Fixing the Issue

Immediate Single Print

To print immediately, you need to flush the output buffer explicitly. In Python 3.x, use the flush argument in the print function:

for step in steps:
    run_simulation(step)
    print('.', end=' ', flush=True)

In Python 2.x, call the .flush method on the standard output stream:

for step in steps:
    run_simulation(step)
    print '.',
    sys.stdout.flush()

Disable Buffering

To disable output line buffering completely, you can modify the buffer size:

import sys
sys.stdout.buffer.write(bytearray('Hello', 'utf-8'))  # Use bytearray for Python 3.x
sys.stdout.buffer.flush()  # Manually flush the buffer as needed

Alternatively, you can use the unbuffered wrapper:

import sys
with open(sys.stdout.fileno(), mode='w', buffering=0) as f:
    # Writes directly to the terminal without buffering
    f.write('Hello')

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