Detecting change in value

[Sushi]

Heyo all again!

I was wondering how to get python to detect a change in value? Basically I have a loop where its looking at data from somewhere and assigning it to a. I want python to be able to see if a changes and tell me (e.g.print 'ooo it changed'). Ive been looking around, and searched these forums, but couldn't find anything. Its probably very simple but my head's not working today.

Thanks!
Mel

[bartonc]

Heyo all again!

I was wondering how to get python to detect a change in value? Basically I have a loop where its looking at data from somewhere and assigning it to a. I want python to be able to see if a changes and tell me (e.g.print 'ooo it changed'). Ive been looking around, and searched these forums, but couldn't find anything. Its probably very simple but my head's not working today.

Thanks!
Mel

Good morning, Mel. Nice to hear from you again.
I've recently seen the built-in function, property(), used to do as you describe:
property( [fget[, fset[, fdel[, doc]]]])

Return a property attribute for new-style classes (classes that derive from object).
fget is a function for getting an attribute value, likewise fset is a function for setting, and fdel a function for del'ing, an attribute. Typical use is to define a managed attribute x:

class C(object):
    def __init__(self): self.__x = None
    def getx(self): return self.__x
    def setx(self, value): self.__x = value
    def delx(self): del self.__x
    x = property(getx, setx, delx, "I'm the 'x' property.")
 

I haven't used in myself, though.

In an event driven environment (Tk, for example), I'd create a (say) IntVar and assign a trace() to it. Events would then be generated when that instance was accessed via its get() and or set() methods.

[bvdet]

Heyo all again!

I was wondering how to get python to detect a change in value? Basically I have a loop where its looking at data from somewhere and assigning it to a. I want python to be able to see if a changes and tell me (e.g.print 'ooo it changed'). Ive been looking around, and searched these forums, but couldn't find anything. Its probably very simple but my head's not working today.

Thanks!
Mel

Maybe this will give you some ideas:

import time, sys
 
f = sys.stdout
 
fn = 'xdata.txt'
data = False
while True:
    s = open(fn).read().strip()
    if data:
        if s != data:
            print 'The data was %s and now is %s. Exiting loop...' % (data, s)
            break
    else:
        data = s
    f.write(data+'\n')
    f.flush()
    time.sleep(1)
 
'''
>>> 147566349
147566349
147566349
147566349
147566349
The data was 147566349 and now is 0123456789. Exiting loop...
'''

[bartonc]

While this is not 100% useful, it works:

# class to trace writes of its value member
 
class TracedValue:
    def __init__(self, value):
        self.value = value
 
    def get(self):
        return self.value
 
    def set(self, value):
        print "%s changed value from %s to %s" %(repr(self), str(self.value), str(value))
        self.value = value
 
 
a = TracedValue(10)
a.set(11)
print a.get()
 

<__main__.TracedValue instance at 0x009D5C10> changed value from 10 to 11
11

[bartonc]

While this is not 100% useful, it works:

# class to trace writes of its value member
 
class TracedValue:
    def __init__(self, value):
        self.value = value
 
    def get(self):
        return self.value
 
    def set(self, value):
        print "%s changed value from %s to %s" %(repr(self), str(self.value), str(value))
        self.value = value
 
 
a = TracedValue(10)
a.set(11)
print a.get()
 

<__main__.TracedValue instance at 0x009D5C10> changed value from 10 to 11
11

Or you could add a name member:

class TracedValue:
    def __init__(self, name, value):
        self.name = name
        self.value = value
 
    def get(self):
        return self.value
 
    def set(self, value):
        print "%s changed value from %s to %s" %(repr(self.name), str(self.value), str(value))
        self.value = value
 
 
a = TracedValue('a', 10)
a.set(11)
print a.get()

'a' changed value from 10 to 11
11

[bartonc]

Or you could add a name member:

class TracedValue:
    def __init__(self, name, value):
        self.name = name
        self.value = value
 
    def get(self):
        return self.value
 
    def set(self, value):
        print "%s changed value from %s to %s" %(repr(self.name), str(self.value), str(value))
        self.value = value
 
 
a = TracedValue('a', 10)
a.set(11)
print a.get()

'a' changed value from 10 to 11
11

Or how 'bout this one:

class TracedHeap(object):
    def __setattr__(self, name, value):
        try:
            traceStr = "%s had a previous value of %s" %(repr(name), str(getattr(self, name)))
            oldMember = True
        except AttributeError:
            traceStr = "Added %s to the heap. Its value is %s" %(repr(name), str(value))
            oldMember = False
        self.__dict__[name] = value
        print traceStr
        if oldMember:
            print "The new value is %s" %str(value)
 
th = TracedHeap()
th.a = 10
th.a = 11
 

Added 'a' to the heap. Its value is 10
'a' had a previous value of 10
The new value is 11

[bartonc]

Heyo all again!

I was wondering how to get python to detect a change in value? Basically I have a loop where its looking at data from somewhere and assigning it to a. I want python to be able to see if a changes and tell me (e.g.print 'ooo it changed'). Ive been looking around, and searched these forums, but couldn't find anything. Its probably very simple but my head's not working today.

Thanks!
Mel

Hi, Mel. I just found py-notify in the cheese shop.