How call method from a method in same class?

Cameron Simpson cs at cskk.id.au
Mon Apr 1 22:29:56 EDT 2019


On 01Apr2019 22:02, Dave <dboland9 at offilive.com> wrote:
>As classes get more complex, it is good to call a function to do some 
>of the processing, and make the code easier to follow.  My question is 
>how to do that?  I've attached some silly code to illustrate the 
>point.  The error is: name 'validScale' is not defined.  Well, yes it 
>is, but maybe not the correct way.  Suggestions?

It is and it isn't. See below:

>class TempConverter():
>    """ Temperature Converter converts a tempeature from one scale
>        to another scale.  For example: 32, F, C will return
>        0 degrees C
>    """
[...]
>    def validScale(self, scaleName):
>        if scaleName.upper == 'F' or 'C' or 'K':
>            return True
>        else:
>            return False
>
>    def convertTemp(self):
>        """ Converts temperature scale if scales valid."""
>        if validScale(self.scale):
>            scaleValid = True
[...]

It is an instance method, so:

    if self.validScale(self.scale)

would resolve the name. However, there are several things worth 
discussing here.

First up, validScale itself returns a Boolean, so just return the test 
result. Change:

    if scaleName.upper == 'F' or 'C' or 'K':
        return True
    else:
        return False

into:

    return scaleName.upper == 'F' or 'C' or 'K'

Second, the condition is buggy. You want this:

    return scaleName.upper() in ('F', 'C', 'K')

i.e. you need to call (the "()") the .upper method, and you need to 
check if the result is in your collection of valid results.

This expression:

    value == A or B or C

means: True if value == A, otherwise B if B is true, otherwise C.

The next thing to observe is that you're testing whether self.scale is 
valid. Normal practice would be to make that test in __init__, and raise 
a ValueError if it is not so:

    def __init__(self, .....scale...):
      if scale.upper() not in ('F', 'C', 'K'):
        raise ValueError("invalid scale %r: expected one of F, C or K" % (scale,)) 

why recite the scale in the message? Because it makes the offending 
value obvious. In particular, if for example you called this incorrectly 
and had the temperature in there instead of the scale that will be 
trivial to debug from the message.

Of course, you actually want to be able to test any scal evalue for 
validity, not just the one stuffed into your instance (.scale). So lets 
revisit the validScale method:

    def validScale(self, scale):
      return scaleName.upper() in ('F', 'C', 'K')

You'll notice that it doesn't depend in "self". Or, for that matter, the 
class. So this is a "static" method: a function defined in the class for 
conceptual clarity, but not with any dependence on the class itself or a 
particular class instance. So:

    @staticmethod
    def validScale(scale):
      return scaleName.upper() in ('F', 'C', 'K')

In __init__, and elsewhere, you can still call this from the instance:

    def __init__(self, .....scale...):
      if not self.validScale(scale):
        raise ValueError("invalid scale %r: expected one of F, C or K" % (scale,)) 

You can also call this from _outside_ the class, for example for other 
validation:

    scale = input("Enter a temperate scale name (F, C or K): ")
    if not TempConverter.validScale(scale):
      print("Bad! Bad user!")

>            newScaleValid = True

Again, validScale returns a Boolean. So you could have just gone:

    newScaleValid = self.validScale(newScale)

>        if scaleValid and newScaleValid:
>            print('Scale converted')
>        else:
>            msg = "There was and error with the scales entered.\n"
>            msg = msg + "You entered: " + self.scale
>            msg = msg + ' ' 'and' + self.newScale
>            print(msg)
>
>if __name__ == "__main__":
>    myclass = TempConverter(32, 'f', 'c')
>    myclass.convertTemp()

My personal inclination would be do define a Temperature class with a 
convert function to be used like this:

    temp = Temperature(32, 'f')
    tempC = temp.convert('c')

This reduces the complexity of the class and IMO makes it easier to use 
elsewhere.


BTW, you get an instance back from tempConverter(...), not a class. So 
don't call it "myclass".

Cheers,
Cameron Simpson <cs at cskk.id.au>


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