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"Latching" variables in function

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  "Latching" variables in function Grawburg <grawburg@myglnc.com> - 2014-04-08 16:09 -0400
    Re: "Latching" variables in function Denis McMahon <denismfmcmahon@gmail.com> - 2014-04-09 00:28 +0000
    Re: "Latching" variables in function Mark H Harris <harrismh777@gmail.com> - 2014-04-09 08:53 -0500
      Re: "Latching" variables in function alister <alister.nospam.ware@ntlworld.com> - 2014-04-09 15:43 +0000

#69908 — "Latching" variables in function

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?




Brian Grawburg
North Carolina

-- 
The truth will set you free . . .but first it will infuriate you.

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#69912

On Tue, 08 Apr 2014 16:09:28 -0400, Grawburg wrote:

> def button():
>    pushbutton = 0
>   button_value = 0
>    pushbutton=bus.read_byte_data(address,GPIOB)
>    if pushbutton > 0:
>         button_value = 1
>    return button_value

Every time your function is called, you start out with button_value of 0.

You may need a global variable that starts out as False (or 0), and once 
flipped to True (or 1) and then stays there:

button_value = False # or: button_value = 0

def button():
    global button_value
    pushbutton = bus.read_byte_data( address, GPIOB )
    if pushbutton > 0:
         button_value = True # or: button_value = 1
    return button_value

Also I think I'd probably pass the IO address as a parameter to the 
button function.

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#69951

On 4/8/14 3:09 PM, Grawburg wrote:

> I have a N/O pushbutton that I want to "latch" a value to a variable when it's been pressed.
> I need button_value to become '1' when the button is pressed and to remain '1' until ...

> What do I use to 'latch' button_value?

Philosophically speaking buttons don't latch. You push the button, an 
event is generated, and the call-back handles the event to do something 
in your project.

You might try setting a global variable on the button-push event.

Or, if I understand you, you might want to use a configurable, like a 
radio button or a check box, either of which are designed to be "latched".


marcus

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#69960

On Wed, 09 Apr 2014 08:53:19 -0500, Mark H Harris wrote:

> On 4/8/14 3:09 PM, Grawburg wrote:
> 
>> I have a N/O pushbutton that I want to "latch" a value to a variable
>> when it's been pressed.
>> I need button_value to become '1' when the button is pressed and to
>> remain '1' until ...
> 
>> What do I use to 'latch' button_value?
> 
> Philosophically speaking buttons don't latch. You push the button, an
> event is generated, and the call-back handles the event to do something
> in your project.
> 
> You might try setting a global variable on the button-push event.
> 
> Or, if I understand you, you might want to use a configurable, like a
> radio button or a check box, either of which are designed to be
> "latched".
> 
> 
> marcus

How familiar are you with oop?
a class would sort this quite nicely

Class Button(object)
	def __init__(self,bit):
		self.pressed=False:
		self.bit=bit

	def check_button(self):
		if bus.read_byte_data(bit):
			self.pressed=True
		

button=Button(address.GPIOB)
while not button.pressed:
	button.check button()

once button.pressed has been set True it will remain True until you reset 
it with button.pressed=False

+ with a bit of tweaking you should be able to create multiple button 
objects for each switch you connect

	
*note this code has been typed direct & not tested so may contain minor 
errors.




-- 
Imbalance of power corrupts and monopoly of power corrupts absolutely.
		-- Genji

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