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Date: Tue, 08 Apr 2014 17:22:12 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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Subject: Re: "Latching" variables in function
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On 04/08/2014 01:09 PM, Grawburg wrote:
>
> 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:
>
>
> def button():
>     pushbutton = 0
>    button_value = 0
>     pushbutton=bus.read_byte_data(address,GPIOB)
>     if pushbutton > 0:
>          button_value = 1
>     return button_value
>
>
>
>
> 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()
>
>
> What do I use to 'latch' button_value?

If I understand correctly, once 'bus.read_byte_data()' has returned a non-zero value, 'button' should continue returning 
1 even if 'bus.read_byte_data()' later returns a 0?

There are a couple options for this:

   - use a default paramater as static storage

     def button(_latched=[0])
         push_button = _latched[0]
         if not push_button:
             button_value = bus.read_byte_data(address, GPIOB)
             if button_value > 0:
                 _latched[0] = push_button = 1
         return push_button

   - use a class

     # implementation left as an exercise for the reader  ;)

--
~Ethan~