Flushing and buffering concepts in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of print with buffering and flush

What will be the output of this Python code snippet when run in a standard terminal?

Python

import sys
print('Hello', end='')
sys.stdout.flush()
print(' World')

A Hello\n World

B Hello World

C HelloWorld

D Hello

💡 Hint

Think about how flush affects the output buffer and the end parameter in print.

The first print outputs 'Hello' without a newline and flushes the buffer immediately, so 'Hello' appears right away. The second print adds ' World' with a newline, so the combined output is 'Hello World'.

❓ Predict Output

intermediate

Effect of buffering on file write

Consider this code writing to a file. What will be the content of 'output.txt' immediately after running this code?

Python

with open('output.txt', 'w') as f:
    f.write('Line 1\n')
    # No flush or close yet
    f.write('Line 2\n')

A File contains both 'Line 1\nLine 2\n'

B File is empty

C File contains only 'Line 1\n'

D File contains only 'Line 2\n'

💡 Hint

Think about when the file is closed and how Python buffers file writes.

The 'with' block ensures the file is closed at the end, which flushes all buffered writes. So both lines are written to the file.

❓ Predict Output

advanced

Output of print with delayed flush in a loop

What is the output of this code snippet?

Python

import time
for i in range(3):
    print(i, end=' ', flush=False)
    time.sleep(1)
print('Done')

A 0\n1\n2\nDone

B 0 1 2 \nDone

C Done0 1 2

D 0 1 2 Done

💡 Hint

Consider that flush=False means output may be buffered until newline or program ends.

With flush=False (default) and end=' ', the outputs from the loop ('0 1 2 ') are buffered. When print('Done') executes, it appends 'Done\n' to the buffer. Since stdout to terminal is line-buffered, the newline causes the full buffer '0 1 2 Done\n' to flush at once after the sleeps.

❓ Predict Output

advanced

Behavior of sys.stdout with buffering

What will this code print when run in a Python script executed in a terminal?

Python

import sys
sys.stdout.write('Start')
sys.stdout.flush()
sys.stdout.write('End')

A Start\nEnd

B Start

C StartEnd

D End

💡 Hint

Remember that sys.stdout.write does not add newlines and flush controls output buffering.

Both writes output text without newline. The flush after 'Start' forces it to appear immediately. 'End' is written but not flushed explicitly, but the program ends so output is flushed automatically. The combined output is 'StartEnd'.

❓ Predict Output

expert

Output of print with buffering and os.write

What is the output of this code snippet?

Python

import os
import sys
print('Hello', end='')
os.write(sys.stdout.fileno(), b'World\n')

A World\nHello

B Hello\n World

C Hello World

D HelloWorld\n

💡 Hint

os.write bypasses Python buffering and writes directly to the file descriptor.

The print outputs 'Hello' without newline but is buffered. os.write writes 'World\n' directly to the file descriptor, bypassing Python's buffer, so it appears first. At program end, the buffer flushes 'Hello' after it. Output: World\nHello