Python: Keyword Argument Default Value Unstable

the hideousness of Python.

Keyword argument's default value is not unstable. Example:

def f(xx, yy=[]):
    yy.append(xx)
    return yy

print f(1)  # [1]
print f(1)  # [1, 1]
print f(1)  # [1, 1, 1]
print f(3, [7,8])   # [7, 8, 3]
print f(4)  # [1, 1, 1, 4]

this example is from the official Python tutorial. 4. More Control Flow Tools — Python v2.7.6 documentation#default-argument-values

explained as follows:

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes.

solution:

def f(xx, yy=None):
    if yy is None:
        yy = []
    yy.append(xx)
    return yy

print f(1)  # [1]
print f(1)  # [1]
print f(1)  # [1]
print f(3, [7,8])   # [7, 8, 3]
print f(4)  # [4]