Python not updating variable defined as a calculation of other two variables

Question

I am doing a lambda school python pre-course and in the first assignment, I have to calculate a person's BMI. Well, I declared w and h for the weight and height of the individual and set them to some value. Then I declared

w=78
h =1.82
BMI = w/h**2
print(BMI)

Everything was fine up to this point. But the next point asks to calculate the BMI without creating the variables again, just changing their values.

I did this

h = 1.63
w = 59
print(BMI)

Does anyone know why python rescues the stored value of BMI instead of reevaluating it since I defined it as an operation?

Welcome to Stackoverflow! Please post your code as this very difficult to follow. — uttejh, Jan 21 '20 at 13:30
You are probably "updating" the BMI within a function without `return`ing it or using `global` — tobias_k, Jan 21 '20 at 13:32

score 2 · Accepted Answer · answered Jan 21 '20 at 13:33

2

If you did

BMI = w / h**2

then that is an expression, which is immediately evaluated and the result is assigned the name BMI. The relationship is not stored, the resulting value is.

It is quite confusing what you were supposed to achieve, perhaps using a function? That would look like:

def compute_bmi(w, h):
  return w / h ** 2

and you would use it like so:

>>> print(compute_bmi(80, 1.80))
24.691358024691358

answered Jan 21 '20 at 13:33

unwind

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Is there a way to store a relationship of variables without using functions? – klithe Jan 21 '20 at 13:54
1

@klithe Why would you *not* want to use functions? – Gino Mempin Jan 22 '20 at 00:06
There is always `eval`. But don't use that. Use a function. (Even if you don't *want* to, what you *really* don't want to is using `eval`.) – Jongware Jan 22 '20 at 00:09

Gino Mempin · Answer 2 · 2020-01-22T08:22:06.857

As already explained in the other answer, BMI is a variable and it does not get re-evaluated when you change the values originally used to set its value. The simplest solution here would be to put the BMI computation into a function, then just call that function every time w and h is updated.

But, if you want to make it look like the BMI value is "automatically calculated" after updating weight and height, you could also put them all in a class and then use properties to store the weight, height, and BMI values.

class Health():
    def __init__(self):
        self._w = 1  # set defaults
        self._h = 1  # set defaults

    @property
    def weight(self):
        return self._w

    @weight.setter
    def weight(self, value):
        self._w = value

    @property
    def height(self):
        return self._h

    @height.setter
    def height(self, value):
        self._h = value

    @property
    def bmi(self):
        return self._w / self._h ** 2

Then, use the class like this:

myhealth = Health()

myhealth.weight = 78
myhealth.height = 1.82
print(myhealth.bmi)

myhealth.weight = 59
myhealth.height = 1.63
print(myhealth.bmi)

Which outputs something like this:

23.54788069073783
22.20633068613798

This is an overkill solution over using a simple function.
I am quite not sure why you asked is there a way without using functions?
(Though, this solution still uses functions.)

For more information on properties:

From SO: How does the @property decorator work?
From Python docs: property()

Thank you I have got much more information than asked that will serve me for future problems. The problem was with the wording of the task that made think that python could store a variable in function of other variables when that is obviously not the case I just had to rewrite everyhting again and since it was so easy I thought there was something fishy there. Appreciate the help :) — klithe, Jan 22 '20 at 23:23

Python not updating variable defined as a calculation of other two variables

2 Answers2