Type

Python supports classes. With the type built-in, we can directly create types. From these types we can instantiate classes.

With setattr we can add attributes (fields) to our classes. With getattr we retrieve those values. We create types with command statements.

First example

This program creates a type with the type built-in. This is an alternative syntax form to the class keyword. We name this type "Cat."

Tip Our type inherits from object, the base class for Python types. And it has no initial attributes.

Info We use setattr to add a field (or attribute) to our class instance. We pass the class instance, the attribute name, and the value.

Next We call getattr to fetch the value of a class's attribute. Here this call returns the value set by setattr.

Cat = type("Cat", (object,), dict())
cat = Cat()

# Set weight to 4.
setattr(cat, "weight", 10)

# Get the weight.
value = getattr(cat, "weight")
print(value)
10

Dict

We can initialize attributes of a type with a dictionary argument. This is the third argument. Here I set the attribute "paws" to 4, and "weight" to -1.

Tip This is a dictionary instance. It can be created like any other dictionary.

Then We display these values on a Cat instance. We use the print built-in method.

# Create class type with default attributes (fields).
Cat = type("Cat", (object,), {"paws": 4, "weight": -1})
cat = Cat()

# Access attributes.
print("Paws =", cat.paws)
print("Weight =", cat.weight)Paws = 4
Weight = -1

Hasattr

There are two built-in functions other than getattr and setattr. With hasattr we see if an attribute (field) exists on the class instance. It returns True or False.

Note With delattr we remove an attribute from the class. This is another syntax form for the del operator.

Note 2 We use the del operator to remove things (as from a dictionary). This is a special syntax form.

class Box:
    pass

box = Box()

# Create a width attribute.
setattr(box, "width", 15)

# The attribute exists.
if hasattr(box, "width"):
    print(True)

# Delete the width attribute.
delattr(box, "width")

# Width no longer exists.
if not hasattr(box, "width"):
    print(False)True
False

By understanding type, setattr and getattr, we learn more about how Python classes work. This knowledge helps us write better programs.

Statements in Python, like "class" declarations, can be directly translated to built-in method calls like "type." Low-level parts of the language are used to implement high-level parts.