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Subject: Re: "Latching" variables in function
From: William Ray Wing <wrw@mac.com>
In-reply-to: <li2im2$3ve$1@ger.gmane.org>
Date: Wed, 09 Apr 2014 08:12:32 -0400
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To: Terry Reedy <tjreedy@udel.edu>
Cc: python-list@python.org, "William R. Wing" <wrw@mac.com>
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On Apr 9, 2014, at 12:35 AM, Terry Reedy <tjreedy@udel.edu> wrote:

> On 4/8/2014 4:09 PM, Grawburg wrote:
>>=20
>> I've probably used the wrong term - I'm thinking of what I do when =
writing PLC code - so I can't find how to do this in my reference books.
>> This is part of a project I'm working on with a Raspberry Pi and an =
MCP23017 port expander.
>> I have a N/O pushbutton that I want to "latch" a value to a variable =
when it's been pressed.  I have this function that gets called =
periodically in
>> a 'while True' statement:
>>=20
>> def button():
>>    pushbutton =3D 0
>>   button_value =3D 0
>>    pushbutton=3Dbus.read_byte_data(address,GPIOB)
>>    if pushbutton > 0:
>>         button_value =3D 1
>>    return button_value
>>=20
>> I need button_value to become '1' when the button is pressed and to =
remain '1' until the entire program (only about 25 lines) ends with a =
sys.exit()
>>=20
>> What do I use to 'latch' button_value?
>=20
> It depends on whether you can set up your system so that pushing the =
button generates an interrupt. But I know little about R.Pi and less =
about the 'port expander'. If there were an interrupt, you would just =
have to write an interrupt handler. When possible, this is much better =
than polling.
>=20
> --=20
> Terry Jan Reedy
>=20

I think what the OP was asking for was a way in standard Python to =
=93lock" the value of an instance variable, which of course, you really =
can=92t do. However, what he (I assume it=92s a he) could do is arrange =
his calling program so that after the button method returns a =931=94, =
the method isn=92t called again.  That is, test for truth of button =3D =
1 in the calling program, and if true, skip the call.  The next time the =
program runs, the button value will be re-initialized to zero and =
everything is back to square one.

-Bill=