Saving text file with a click: valueerror i/o operation on closed file

[Gribouillis]

You can redirect sys.stdout by defining your own context. The following code prints 'Hello there!' in file 'foo.txt' and then 'Done' to standard output. Don't close files explicitly when using the 'with' statement. They are closed automatically when the context exits

from contextlib import contextmanager
import sys

@contextmanager
def redirect_stdout(file):
    old, sys.stdout = sys.stdout, file
    try:
        yield
    finally:
        sys.stdout = old

def main():
    with open('foo.txt', 'w') as file, redirect_stdout(file):
        print('Hello there!')
    print('Done')

if __name__ == '__main__':
    main()

That said, I don't know how the HeartRateMonitor class works, but you could consider injecting a file argument in this class instead of redirecting the global stdout.