variable scope of class objects

[sohcahtoa82 at gmail.com]

On Monday, October 19, 2015 at 11:39:59 AM UTC-7, JonRob wrote:
> Hi,
> 
> I've having trouble understanding the self concept as it applies to
> variables.  I think I understand how it affects methods.
> 
> I haven't been able to fully grasp the scope of class variables and
> the effect of the "self"  to the scope of the variable.
> 
> I (think) I understand that in the below case, the word self could be
> replaced with "BME280" to explicitly call out a variable.
> 
> But even still I don't know how explicit call out effects the scope of
> a variable.
> 
> The below pseudo code is distilled from my 1st attempt at a functional
> Python program on the RasPi.
> 
> My questions are:
> What is the scope of class variables?
> does the self. prefix modify this scope?
> 
> Thanks
> 
> Regards
> 
> JonRob
> 
> 
> 
> 
> #!/usr/bin/python
> # -- developed using Python 2.7.3
> 
> class BME280:
> 
> # all the below are class variables
> # those preceded by an underscore are predefined to some constant
> # those without the underscore are to be "working" variables.
> 
>     _regT1       = 0x88
>     _regH6       = 0xE7
>     _coeff_P2    = 0x82
>     _coeff_P6    = 0x32
>     
>     filter       = 0    #should these be "self"?
>     t_fine       = 0
>     
>     def __init__(self, address=0x76, debug=True):
>         self.i2c = Adafruit_I2C(address)
>         self.address = address
>         self.debug = debug
>                 
>     def pressure_calc(self):
>         var1 = self.i2c.readU16(self._regT1,False)
>         p = (1048576.0 - var1) * _coeff_P2
>         return p
>         
>     def read_pressure(self):      #called  by main application
>         pressure_hPa = pressure_calc(self) /10 
>         # apply compensation
>         return pressure_hPa

Class variables are accessible without creating an instance of a class.  Also, changing the value of a class variable affects ALL instances of that class.  This is because the variable belongs to the class itself, not any of the instances of that class.

"self" is used to tell the interpreter that the variable/function you are accessing is a member of an instance of that class.

Here's an example:

class MyObject(object):
    count = 0
    def __init__(value):
        MyObject.count += 1
        self.value = value

    def printStuff():
        print("My value is ", self.value)

print(MyObject.count)   # This will print 0
a = MyObject('a')
print(MyObject.count)   # This will print 1
print(a.count)          # This will also print 1
a.printStuff()          # This will print "My value is a"
b = MyObject('b')
print(a.count)          # This will print 2
print(b.count)          # This will also print 2
b.printStuff()          # This will print "My value is b"