Python Type Checking: When to Use `type()` vs. `isinstance()`?

Understanding the Differences Between type() and isinstance()

In Python, type() and isinstance() are two functions used for type checking. However, they have distinct characteristics that impact their usage scenarios.

Type Checking with type()

type() returns the type of an object, a class in which the object belongs. By comparing the returned type with a known type, you can assess whether an object is of that specific type:

import types

if type(a) is types.DictType:
    do_something()

This code checks if object a is a dictionary type. If it is, the do_something() function is executed.

Type Checking with isinstance()

isinstance() checks if an object is an instance of a specified class or subclass. Unlike type(), it takes the inheritance hierarchy into account:

if isinstance(a, dict):
    do_something()

Here, isinstance() checks if object a is a dictionary or an instance of any class that inherits from dict. If so, the do_something() function is called.

Key Differences

• Inheritance Support: isinstance() considers inheritance, while type() does not. For example:

class Cat(Animal):
    pass

if isinstance(pet, Cat):  # True
    do_something_catty()

type() would return Animal for pet, regardless of whether it was a Cat or another subclass.

• Class vs. Type: isinstance() checks for instances of classes, while type() returns the specific type of the object.
• Performance: isinstance() may be more efficient than type() when inheritance is considered.

Best Practices

In general, isinstance() is preferred when:

• You need to check if an object is an instance of a class or its subclasses.
• You want to avoid the performance implications of inheritance checks using type().

type() is useful when:

• You need the exact type of an object.
• You need to compare an object's type to another type that is not a class.

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