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Interactive scripts (back on topic for once) [was Re: The "loop and a half"]

Terry Reedy tjreedy at udel.edu
Fri Oct 6 16:11:16 EDT 2017 
On 10/6/2017 1:32 PM, Chris Angelico wrote:
> On Sat, Oct 7, 2017 at 4:05 AM, Grant Edwards <grant.b.edwards at gmail.com> wrote:
>> On 2017-10-06, Thomas Jollans <tjol at tjol.eu> wrote:
>>
>>> Seriously? sys.stdin can be None? That's terrifying.
>>
>> Why?
>>
>> Unix daemons usually run with no stdin, stderr, or stdout.
>>
>> And yes, people do write Unix daemons in Python.
> 
> Hmm, but usually I would expect them still to HAVE those streams,
> they're just connected to /dev/null or something. I don't think they
> would actually fail to exist, would they?

On Windows, a file run with pythonw.exe (no console) starts with 
sys.[__]std[in|out|err][__] (6 entries) set to None.  The program can 
reset them as it pleases.  In an IDLE user process, the non-dunder names 
are set to objects that communicate with the IDLE GUI process.

 >>> import sys; sys.__stdin__, sys.stdin
(None, <idlelib.run.PseudoInputFile object at 0x0000014639AA7E10>)

GUI programs interact with the OS and thence users through event streams 
rather than character streams.  (Note that terminal escape sequences are 
character-encoded events.)  That is why automating a GUI usually 
requires a special program than can generate and inject events into the 
event queue read by the GUI program.

-- 
Terry Jan Reedy
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