On Fri, Jun 12, 2020 at 09:01:33AM +0100, Rob Cliffe via Python-Dev wrote:
> If I run the following program (using Python 3.8.3 on a Windows 10 laptop):
>
> import sys, time
> for i in range(1,11):
> sys.stdout.write('\r%d' % i)
In Python 2, the 'write()` method returns None, which is suppressed in
the REPR. In Python 3, the `write` method returns the number of bytes
(or characters, I forget which...) actually written, which is not
suppressed.
I've been bitten by this myself, forgetting that in a script any result
not bound to a variable gets silently thrown away, but in the REPL it
gets printed.
> It appears that the requested characters are output, *followed by* the
> number of characters output
> (which is the value returned by sys.stdout.write) and a newline.
> Surely this is not the intended behaviour.
Of course it is. The whole point of the REPL is to evaluate an
expression and have the result printed. (That's the P in REPL :-)
`stdout.write(...)` is an expression that returns a value, so the REPL
prints it.
--
Steven
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