If the variable contains a value of an immutable type (e.g. a string) then it is okay to assign a default value like this
class Rectangle(object):
def __init__(self, width, height, color='blue'):
self.width = width
self.height = height
self.color = color
def area(self):
return self.width * self.height
# Create some instances of the class
default_rectangle = Rectangle(2, 3)
print(default_rectangle.color) # blue
red_rectangle = Rectangle(2, 3, 'red')
print(red_rectangle.color) # redOne needs to be careful when initializing mutable objects such as lists in the constructor. Consider the following example:
class Rectangle2D(object):
def __init__(self, width, height, pos=[0,0], color='blue'):
self.width = width
self.height = height
self.pos = pos
self.color = color
r1 = Rectangle2D(5,3)
r2 = Rectangle2D(7,8)
r1.pos[0] = 4
r1.pos # [4, 0]
r2.pos # [4, 0] r2's pos has changed as wellThis behavior is caused by the fact that in Python default parameters are bound at function execution and not at function declaration. To get a default instance variable that’s not shared among instances, one should use a construct like this:
class Rectangle2D(object):
def __init__(self, width, height, pos=None, color='blue'):
self.width = width
self.height = height
self.pos = pos or [0, 0] # default value is [0, 0]
self.color = color
r1 = Rectangle2D(5,3)
r2 = Rectangle2D(7,8)
r1.pos[0] = 4
r1.pos # [4, 0]
r2.pos # [0, 0] r2's pos hasn't changedSee also Mutable Default Arguments and “Least Astonishment” and the Mutable Default Argument.