How to prevent overprinting when using multithreading?

Question

I made my own script in python with multithreading. The problem is my output is completely overprinted, and I need a clear output print. In the picture below you can see the first 15 lines are fine, but after that its not good anymore.

screenshot of output

You can prevent it by redefining the `print()` function as shown near the end of the code in this [answer](https://stackoverflow.com/a/66286326/355230) of mine to another threading question. — martineau, Nov 29 '21 at 20:09
Does this answer your question? [Python - Threads are printing at the same time messing up the text output](https://stackoverflow.com/questions/26688424/python-threads-are-printing-at-the-same-time-messing-up-the-text-output) — Tsyvarev, Nov 29 '21 at 23:11

score 0 · Answer 1 · answered Nov 29 '21 at 19:56

Race conditions

You are most likely running into a race condition where the code is printing the first part of the line before the next part of the line.

Example race

Problem code

print(timestamp)
print(part of log)
print(rest of log)

Example result (with two threads)

T1: prints timestamp

T2: prints timestamp

T2: prints part of log

T1: prints part of log

T1: prints rest of log

T2: prints rest of log

Reason code fails

The prints can interleave, especially if you have a lot of threads running or it has an unexpected spike in data input.

Example code with fix

log_string=f"{timestamp}{part of log}{rest of log}"
print(log_string)

Fix reason

If the print call happens all at once, it won't interleave the writes.