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Using type() to compare types

The function isinstance is the best-equipped to handle type checking because it supports inheritance (e.g. an instance of a derived class is an instance of a base class, too). Therefore isinstance should be used whenever type comparison is required.

Anti-pattern

The if statement below uses the pattern if type(OBJECT) is types.TYPE to compare a Rectangle object to a built-in type (ListType in this example). This is not the preferred pattern for comparing types.

import types

class Rectangle(object):
    def __init__(self, width, height):
        self.width = width
        self.height = height

r = Rectangle(3, 4)

# bad
if type(r) is types.ListType:
    print("object r is a list")

Note that the following situation will not raise the error, although it should.

import types

class Rectangle(object):
    def __init__(self, width, height):
        self.width = width
        self.height = height

class Circle(object):
    def __init__(self, radius):
        self.radius = radius

c = Circle(2)
r = Rectangle(3, 4)

# bad
if type(r) is not type(c):
    print("object types do not match")

Best practice

Use isinstance to compare types

The preferred pattern for comparing types is the built-in function isinstance.

import types

class Rectangle(object):
    def __init__(self, width, height):
        self.width = width
        self.height = height

r = Rectangle(3, 4)

# good
if isinstance(r, types.ListType):
    print("object r is a list")

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