"""
Ex. 7.10 from "A primer on..."
Implement a class so that the code lines given in the exercise work
and give the desired output. From the use cases we see that the
code needs the following:
1. Either no constructor or a constructor that takes no arguments
2. A __call__ method which takes a single argument
3. A __str__ special method
The exercise could have been solved differently, for instance with no 
attributes and no constructor, but this is one possible solution.
"""


class Hello:
    def __init__(self):
        self.greeting = "Hello, "

    def __call__(self, arg):
        return self.greeting + arg + '!'

    def __str__(self):
        return self.greeting + "World!"



a = Hello()
print(a("students")) #output: Hello, students!
print(a) #output: Hello, World!