Using a Class

Classes provide a pattern for making new objects.

Every object is an example of its class, but the object is NOT a copy of the class. We use a class to build objects, just like we use a cutter to shape separate cookies. The cookie and the cutter are NOT the same thing.

Create New Objects

To create an object from a class, the general syntax is:

object_name = ClassName(arguments)

This creates an instance of the class, which means object_name has the same set of property names as every other object made from the class. However, the values for the properties may differ between objects.

Try It!

Let’s use a Student class to create two objects. Run the following program an examine the output.

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class Student:
   def __init__(self, name, grade, id_number):
      self.name = name
      self.grade_level = grade
      self.id = id_number

dan = Student('Dan', 10, 5234)
jessi = Student('Jessi', 12, 3333)

print(type(dan), type(jessi))
print(dan.id, jessi.id)

repl.it

In lines 7 and 8, we call the Student class twice and pass in different sets of arguments, creating the objects dan and jessi.

The output of line 10 shows that dan and jessi are both the same type of object (Student). Line 11 shows us that the two share the same property names (like id), but they have different values assigned to those names.

After creating a Student object, we can access, modify, or add new properties and values as described in the Object Properties section.

Try It

Play around with modifying and adding properties inside and outside of the Student class.

  1. Add a new property inside the constructor.

  2. On line 12, change the value of the ID number for dan, then print the new information.

  3. On line 15, add the act_score property to jessi and assign it an integer value. Print this value to check your work.

Note that line 15 adds the act_score property to the jessi object, but this change does NOT affect any other objects created with Student.

Default Property Values

If we try to create a new Student object without passing in all of the required arguments, the program crashes.

To avoid issues with missing arguments, we can set a default value for a parameter as follows:

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class Student:
   def __init__(self, name, grade, id_number = 'Missing'):
      self.name = name
      self.grade_level = grade
      self.id = id_number

Now if we call Student but leave out an ID number, the constructor automatically assigns id a value of 'Missing'. If we include an argument for id_number, then the default value is ignored.

Try It!

Return to the example above and modify the Student class to use a default value for one or more of the constructor parameters.

Another Mental Image for Classes

Besides the cookie cutter idea, we can also think of a class as a factory for making objects. The Student class itself isn’t an instance of a student, but it contains all the tools to make student objects. Every time we call the class, we ask the factory to make a new object. As the object goes through the production line, the constructor runs to get the object properly set up.

The statement jessi = Student('Jessi', 12, 3333) essentially says, Hey, Student factory! Please make me a new object with these settings.