TIL #139 – Multiline input in the REPL

A while ago I learned I could use open(0) to open standard input. This unlocks a neat trick that allows you to do multiline input in the REPL:

>>> msg = open(0).read()
Hello,
world!
^D
>>> msg
'Hello,\nworld!\n'

The cryptic ^D is Ctrl+D, which means EOF on Unix systems. If you're on Windows, use Ctrl+Z.

The problem is that if you try to use open(0).read() again to read more multiline input, you get an exception:

OSError: [Errno 9] Bad file descriptor

That's because, when you finished reading the first time around, Python closed the file descriptor 0, so you can no longer use it.

The fix is to set closefd=False when you use the built-in open. With the parameter closefd set to False, the underlying file descriptor isn't closed and you can reuse it:

>>> msg1 = open(0, closefd=False).read()
Hello,
world!
^D
>>> msg1
'Hello,\nworld!\n'

>>> msg2 = open(0, closefd=False).read()
Goodbye,
world!
^D
>>> msg2
'Goodbye,\nworld!\n'

By using open(0, closefd=False), you can read multiline input in the REPL repeatedly.

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