More Object Details

Objects allow us to collect related data and actions into one place. So far, we have printed the values of properties one at a time using dot notation. What if we try to print the object itself?

Printing Objects

The following code defines a Dog class with a small number of properties and methods.

Try It!

Run the program to see how the print function displays the dog_1 and dog_2 objects.

class Dog:
   def __init__(self, name, age, cute = True):
      self.name = name
      self.age = age
      self.is_cute = cute

   def speak(self, repeat = 1):
      return "Bark! "*repeat

dog_1 = Dog('Spot', 6)
dog_2 = Dog('Fleas', 2)
print(dog_1)
print(dog_2)

repl.it

Instead of giving us the information stored in each object, we see an output similar to <__main__.Dog object at 0x7fa1a167c4c0>. Despite the weird look, this actually gives us some information:

1. The data type (an object of type Dog).

2. A memory address, 0x7fa1a167c4c0, which differs between the two objects.

Usually, this format is not what we want to see when we print an object. Fortunately, Python allows us to decide what an object should look like when it gets printed.

The __str__ Method

Every Python class includes a special method called __str__. Notice that it uses the same naming convention as the initializer: two underscores before and after the name.

The __str__ method is responsible for returning a string designed by the programmer. This means that we get to choose what gets displayed when we print our Dog objects.

In this case, let’s format a string that includes the name and age of our pet. Copy and paste this code into the editor above, making sure to indent each line by the proper amount. Run the program to see the new output.

def __str__(self):
   output = "Name of dog: {0}, Age of dog: {1} years"
   return output.format(self.name, self.age)

Note that the __str__ method MUST create and return a string.

Try It!

Create your own return string in the __str__ method. Feel free to include the is_cute value, or add and display new properties to the class.

Collect All Property Values

The __str__ method allows us to print selected values for our object. Python also gives us a way to quickly collect the property names and their values from an object.

We can use the function vars() to take an object and return a dictionary that contains the properties as key/value paris.

Try It!

Run the program below and examine the output.

class Dog:
   def __init__(self, name, age, cute = True):
      self.name = name
      self.age = age
      self.is_cute = cute

   def speak(self, repeat = 1):
      return "Bark! "*repeat

   def __str__(self):
      output = "Name of dog: {0}, Age of dog: {1} years"
      return output.format(self.name, self.age)

dog_1 = Dog('Spot', 6)
dog_data = vars(dog_1)
print(dog_data)

repl.it

1. Line 14 creates a new Dog object and assigns it to dog_1.

2. vars(dog_1) returns a dictionary and assigns it to dog_data. Each key in the dictionary matches one of the property names defined in the Dog class.

3. Line 16 prints the dog_data dictionary.

Note

We could easily iterate through the dog_data dictionary by keys, values, or both as we saw here.

Property and Method Names

If you want a quick peek of what a an object contains, you can use the dir() function to collect and display the names for all of the items in a given object. This built-in function has a few quirks. Let’s walkthrough to see how to handle one of them.

Try It!

1. Paste this code into the editor above, then run the program.

   print("dog_1 items:")
   for item in dir(dog_1):
      print(item)
   

2. dir(dog_1) returns a (rather long) list of the names for each method and property defined in the Dog class. This includes special methods like __init__ and __str__.

3. Tip: To skip the special names, add a conditional to the loop to check if the name contains a double underscore, __.

   print("dog_1 items:")
   for item in dir(dog_1):
      if '__' not in item:
         print(item)