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Python isinstance() Function

How to use Python's isinstance() for type checking, including syntax, parameters, return values, and examples with native types, custom classes, inheritance, and multiple type verification.

The isinstance() function is a function used for type checking: it verifies if an object is of a specified type, returning True if it is, and False otherwise

Syntax

isinstance(object, classinfo)

isinstance() Parameters

Python isinstance() function parameters:

Parameter Condition Description
object Required The object that needs to be checked.
classinfo Required The class, type, or a tuple of classes/types against which the object is checked.

isinstance() Return Value

Python isinstance() function returns:

  • True if the object is an instance or subclass of a class or any element of the tuple
  • False otherwise

Examples

Example 1: basic example with isinstance() function

class MyClass:
  a = 123
  
myClassInstance = MyClass()

print(isinstance(myClassInstance, MyClass))                 # Output: True
print(isinstance(myClassInstance, (list, tuple)))           # Output: False
print(isinstance(myClassInstance, (list, tuple, MyClass)))  # Output: True

output

True
False
True

Notice that:

  • isinstance(myClassInstance, MyClass) checks if myClassInstance is an instance of the MyClass class. Since myClassInstance is created from the MyClass class, this will return True.
  • isinstance(myClassInstance, (list, tuple)) checks if myClassInstance is an instance of either the list or tuple classes. Since myClassInstance is not an instance of either list or tuple, this will return False.
  • isinstance(myClassInstance, (list, tuple, MyClass)) checks if myClassInstance is an instance of any of the classes in the tuple (list, tuple, MyClass). Since myClassInstance is an instance of MyClass, this will return True.

Example 2: Checking for Native Types with isinstance() function

Checking for Native Types Using Python isinstance() function:

a_list = [1,  2,  3,  4]
print(isinstance(a_list, list))   # Output: True
print(isinstance(a_list, tuple))  # Output: False
print(isinstance(a_list, set))    # Output: False

output

True
False
False

Example 3: Checking for Multiple Types with isinstance() function

Check against multiple types using Python isinstance() function:

a_list = [1,  2,  3,  4]
print(isinstance(a_list, (tuple, list)))       # Output: True
print(isinstance(a_list, (tuple, set)))        # Output: False
print(isinstance(a_list, (tuple, list, set)))  # Output: True

output

True
False
True

Example 4: Checking for Custom Class Types with isinstance() function

Check if a certain instance is of a user-defined class using the Python isinstance() function:

class Person:
    def __init__(self, name):
        self.name = name

    def greet(self):
        print(f'Hi! My name is {self.name}')

Nik = Person('Tom')
print(isinstance(Nik, Person)) # Output: True
print(isinstance(Nik, tuple))  # Output: False
print(isinstance(Nik, set))    # Output: False

output

True
False
False

Example 5: Checking for Inherited Class Types with isinstance() function

Checking if an instance is a subclass:

class Developer:
    pass

class PythonDeveloper(Developer):
    pass

class JavaDeveloper(Developer):
    pass


dev = Developer()
devPython = PythonDeveloper()
devJava = JavaDeveloper()

print(isinstance(devPython, Developer))  # Output: True
print(isinstance(devJava, Developer))    # Output: True
print(isinstance(dev, PythonDeveloper))  # Output: False
print(isinstance(dev, JavaDeveloper))    # Output: False

print(isinstance(devPython, PythonDeveloper)) # Output: True
print(isinstance(devPython, JavaDeveloper))   # Output: False
print(isinstance(devJava, PythonDeveloper))   # Output: False
print(isinstance(devJava, JavaDeveloper))     # Output: True

output

True
True
False
False
True
False
False
True

Example 6: Implement a Basic Type Checking Function using isinstance() function

Let's create a function that returns a string message according to the type of the object given to our check_type function:

def check_type(obj):
    if isinstance(obj, int):
        return "It's an integer!"
    elif isinstance(obj, str):
        return "It's a string!"
    else:
        return "I don't know what this is."

print(check_type(42))        # Output: It's an integer!
print(check_type("Hello"))   # Output: It's a string!
print(check_type(3.14))      # Output: I don't know what this is.

output

It's an integer!
It's a string!
I don't know what this is.
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