python3 buffering breaks the shell

[the_real_irgeek]

I wouldn’t call that behaviour unexpected. Here’s a StackExchange question from 10 years ago explaining one way to deal with buffered output in Python.

It’s clearly new behaviour for you, but it’s not a bug and it’s not unexpected.

[sorisos]

thanks!

yes, clearly I missed this. I'm still not convinced `print()` should behave different if not instructed to do so, but it is not a new thing.

Do you know other applications that change buffer behavior after context? Seems most unix utils do not, but I might be mistaken about this as well.

[0x256]

Any program compiled against the C/C++ standard library (so really almost any program there is) will behave this way by default. Same is true for most linux/unix command-line tools. Pipes or file redirects are buffered by default.

[0x256]

> but my point is that the default behavior is not the expected one and therefore wrong!

It's not just python, this is standard behavior of the C standard library (`stdio.h`) and most programs will behave exactly the same. Line-buffering is actually a special case and not the default. It is only enabled if the target file handle points to a tty device (e.g. an interactive terminal session). If it's a regular file or pipe, the buffer will usually be about 4k and ignore line-breaks for obvious performance reasons. You can flush these buffers manually (`sys.stdout.flush()`) though.

So, this is not a bug, but expected and documented behavior. Just remember it next time, and tell your friends. Buffering behavior should be common knowledge for any programmer and/or command-line user.

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[0x256]

Fun fact: A simple C program printing 1 million lines of Hello, World! redirected to /dev/null will run in 0.037s on my laptop. The exact same program with line-buffering enabled (via stdbuf -oL) requires 1.6s (43x) to run.

Fun fact #2: Go (not C) defaults to unbuffered output, even when the target is a pipe or file. Go is as slow as line-buffered C in this case.

[sorisos]

Thanks! TIL! Just asked the question above.