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Tell HN: Python's print function is thread-unsafe

13 points by kerneloops ↗ HN
Some days ago I found that Python's print function is by default thread-unsafe, as the underlying sys.stdout is by default a TextIOWrapper, which is not thread safe. That means, for the code below

  from threading import Thread
  def target(x):
      for _ in range(100000): print(x)
  Thread(target=target, args=['a' * 10]).start()
  Thread(target=target, args=['b' * 10]).start()
Python could print out not only interleaved bytes (aaabbbabab), but also null bytes and uninitialized memory, thanks to non-synchronized buffering.

6 comments

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kerneloops
A survey of some other languages:

C printf: MT-Safe locale.

C++ std::cout: safe, unless you call sync_with_stdio(false).

JVM System.out.println: safe in common JVMs.

C# Console.WriteLine: safe.

Go fmt.Printf: safe.

Rust println!: safe.

Ruby puts: safe.

So it seems that Python is the outlier here.

eesmith
I was able to reproduce your failure (using Python 3.10) after a couple of runs:

  % grep -v '^aaaaaaaaaa$' kerneloops.out | grep -v '^bbbbbbbbbb$' | od -x
  0000000      8130    0db3    0001    0000    bd30    0dad    0001    0000
  *
  0006420      000a
  0006421
I was surprised as I expected possible interleaved bytes but not output other than "a"s, "b"s and new-lines, as I thought the GIL would come to the rescue.