This class isn't very useful, because it can only represent one particular flight. We need to make the flight number configurable at the point a Flight is created. To do that we need to write an initializer method.

If provided, the initializer method is called as part of the process of creating a new object when we call the constructor. The initializer method must be called __init__() delimited by the double underscores used for Python runtime machinery. Like all other instance methods, the first argument to __init__() must be self.

In this case, we also pass a second formal argument to __init__() which is the flight number:

class Flight:

    def __init__(self, number):
        self._number = number

    def number(self):
        return self._number

The initializer should not return anything – it simply modifies the object referred to by self.

If you're coming from a Java, C#, or C++ background it's tempting to think of __init__() as being the constructor. This isn't quite...