Tell, don’t ask

Procedural code gets information then makes decisions. Object-oriented code tells objects to do things. — Alec Sharp

I have read an article Tell, don’t ask recently and found it a very valuable point for object oriented programming.

Take the following Python code as an example,

# main
person_age = 40
if person_age < 70:
    jump_high()
else:
    jump_low()

This is a typical asking version. The main program takes the responsiblity to decide if a person can jump high or not by checking the variable state, i.e. the age of person here. This does not look like a correct division of responsibility. It also violates the idea of encapsulation as the decision of jump level is made outside the object.

A tell version of the same problem is to encapsulate the behavior jump and the data age into one class, like this

class Person:
    jump_age_threshold = 70
    def __init__(self, age):
        self.age = age

    def jump(self):
        if self.age < self.jump_age_threshold:
            jump_high()
        else:
            jump_low()
# main
person = Person(40)
person.jump()

In the tell version, we hide the inner logic of jump from the main program. This corresponds more to the idea of OOP.

There is another article of Martin Fowler on the same subject tell don’t ask.