How to create recalculating variables in Python

Suppose I have the code:

a = 2
b = a + 2
a = 3

The question is: how to keep b updated on each change in a? E.g., after the above code I would like to get: print(b) to be 5, not 4.

Of course, b can be a function of a via def, but, say, in IPython it’s more comfortable to have simple variables. Are there way to do so? Maybe via SymPy or other libraries?

Answer

You can do a lambda, which is basically a function… The only malus is that you have to do b() to get the value instead of just b

>>> a = 2
>>> b = lambda: a + 2
>>> b()
4
>>> a = 3
>>> b()
5