How to freeze mutable default parameter values?

Question

Python chose to follow the design decision which evaluates the function's signature only once. So this code:

def test(x=[]):
    x.append(5)
    return x

print(test())
print(test())

Prints:

[5]

[5, 5]

I know it is possible to use None as a default value, and to change the parameter value inside the function if it is None.

---

How can I write some code that change the behavior of test, in a way that the default parameter would be reevaluated every time I call the function? The solution can include decorators and use classes as the default value.

Answer 1

We can use the function's __default__ property to "freeze" defaults (and __kwargs__ to "freeze" default keyword arguments):

def freeze_defaults(func):
     defaults = func.__defaults__
     kwdefaults = func.__kwdefaults__
     @functools.wraps(func) 
     def wrapper(*args, **kwargs): 
         func.__defaults__ = copy.deepcopy(defaults) 
         func.__kwdefaults__ = copy.deepcopy(kwdefaults)
         return func(*args, **kwargs) 
     return wrapper

Now:

@freeze_defaults
def test(x=[]):
    x.append(5)
    return x

print(test())
print(test())

Prints:

[5]

[5]

And even:

@freeze_defaults
def test(x=['hello!']):
    x.append(5)
    return x

print(test())
print(test())

Prints:

['hello!', 5]

['hello!', 5]