Lecture 6 - Modules and Classes

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Modules

• Python projects can be split into multiple files
  • These are known as ‘modules’
  • Code can be imported from one module to another
  • Modules can be used like libraries or like a program
• Importing Modules
  • Import the whole module and access members with .
  • Import specific things from a module
  • The import process requires running imported modules
  • Use if __name__ == '__main__':
    • Lets module work as library and as a program.

Classes

• Generally written in their own module
  • class Dog –> Dog.py
• self is the calling object
  • You must use self to refer to member funcs/vars in class def
  • similar to this in C++/Java

• Member functions are called methods

• There are no private members
  • The convention for “don’t touch this” is naming with _ before the name
• Special methods usually are wrapped with __
  • used to “overload” operators
  • used by constructors

Example:

class Dog:
    """A dog class"""

    def __init__(self, name, weight):
        """Dog constructor"""
        self.name = name
        self.weight = weight

    def bark(self):
        """Make noise"""
        print("BARK I AM", self.name, "BARK BARK!")

    def eat(self, food):
        """Eat some food.

        Gains one pound for every character in the name of the food.

        :param str food: The name of the food.
        """
        self.weight += len(food)

    def as_dict(self):
        return {"name": self.name, "weight": self.weight}

    def __lt__(self, other):
        return self.weight < other.weight

    def __str__(self):
        return "{0} ({1})".format(self.name, self.weight)


def test_dog():
    frank = Dog("Frank", 75)
    frank.bark()
    frank.eat("spaghetti")
    print(frank.as_dict())

    barney = Dog("Barney", 100)

    print("frank < barney:", frank < barney)

    print("barney < frank:", barney < frank)

    print("frank > barney:", frank > barney)

    print("frank >= barney:", frank >= barney)

    s = str(frank)
    print("s:", s)

    print("frank looks like...")
    print(frank)


if __name__ == "__main__":
    test_dog()

• Custom Exception classes
  • often very simple:

class MyError(Exception):
    pass

pass is a no-op placeholder.