The default argument value keeps changing

Python's function is first-class__object____code> rather than just a piece of code. In other words, the definition of a function is evaluated as an object.

The function's default parameter is bound to the function's definition, not its execution. Therefore, the default parameter can be written like member data, and the state can be changed every time a function is called.

Please see the following code

def a():
    print "a excuted"
    return 0

def b( x = a() ):
    pass

Result: a cut

Why did "a cut" output when neither a() nor b() were called? This is because in defb(x = a():, the default parameter x is bound to the definition. We didn't run the code like b(), but we initialized x in the definition of the function.

The same goes for this code.

def foo(a=[]):
    a.append(5)
    return a

Every time a function is called

is not running.