20.7. Converting an Object to a String¶
When we’re working with classes and objects, it is often necessary to print an object (that is, to print the state of an object). Consider the example below.
p. The default functionality provided by
Python tells you that
p is an object of type
Point. However, it does not tell you anything about the specific
state of the point.
We can improve on this representation if we include a special method call
__str__. Notice that this method uses the same naming convention as the constructor, that is two underscores before and after the name. It is common that Python
uses this naming technique for special methods.
__str__ method is responsible for returning a string representation as defined by the class creator. In other words, you as the programmer, get to choose what a
Point should look like when it gets printed. In this case, we
have decided that the string representation will include the values of x and y as well as some identifying text. It
is required that the
__str__ method create and return a string.
Whatever string the
__str__ method for a class returns, that is the string that will print when you put any instance of that class in a print statement. For that reason, the string that a class’s
__str__ method returns should usually include values of instance variables. If a point has
x value 3 and
y value 4, but another point has
x value 5 and
y value 9, those two Point objects should probably look different when you print them, right?
Take a look at the code below.
When we run the program above you can see that the
Now, you ask, don’t we already have a
str type converter that can
turn our object into a string? Yes we do!
However, as we saw earlier, these automatic mechanisms do not do exactly what we want. Python provides many default implementations for
methods that we as programmers will probably want to change. When a programmer changes the meaning of a method we
say that we override the method. Note also that the
str type converter function uses whatever
__str__ method we
Check Your Understanding
Create a class called Cereal that accepts three inputs: 2 strings and 1 integer, and assigns them to 3 instance variables in the constructor:
fiber. When an instance of
Cerealis printed, the user should see the following: “[name] cereal is produced by [brand] and has [fiber integer] grams of fiber in every serving!” To the variable name
c1, assign an instance of
Cerealwhose name is
"Corn Flakes", brand is
"Kellogg's", and fiber is
2. To the variable name
c2, assign an instance of
Cerealwhose name is
"Honey Nut Cheerios", brand is
"General Mills", and fiber is
3. Practice printing both!