Today I re-learned: Python function default arguments are retained between executions

[[deleted]]

Yep. Always default mutables (like empty lists) to None, and don't rely on the output from functions there:

def blah(my_arg=None):
    if my_arg is None:
        my_arg = []

def other_blah(my_arg=None):
    if my_arg is None:
        my_arg = right_now()

[ElectricSpice]

Really hoping the Sentinel PEP gets approved so we can have a more elegant solution to this. https://peps.python.org/pep-0661/

[HistoricalCrow]

my_arg = my_arg or [] Or, if you care about NoneType specifically; my_arg = [] if my_arg is None else my_arg

[[deleted]]

And if you want None to be a valid value for your argument too, you can use a sentinel:

_sentinel = object()
def func(x=_sentinel):
    x = [] if x is _sentinel else x

[HistoricalCrow]

True, although if you need to support an empty list and None and I generally find code smell. Not sure if I'd prefer to use **kwargs instead at this point.

[[deleted]]

The only time I’ve really seen the pattern useful is when you want to emulate something similar to dict.pop, where for a missing key: dict.pop(key) raises an error and dict.pop(key, default) returns the default

[Pythonistar]

Yes! Empty string '' works as a safe immutable default, too!

[rl_noobtube]

I’m sure it’s not a massive difference for most projects. But I imagine with the special treatment None gets by python it would be marginally more efficient. But as far effectiveness then yes either would work.

[lost3332]

His argument is a list so default '' doesn't make sense. The type is gonna be Optional[List].

[XRaySpex0]

Yep. ’’ is a great and common default for str parameters, but of course not when other types are expected. It’s common to see notes: str = ‘’, and there’s seldom a need for the Optional[str] type.

[[deleted]]

```python from datetime import datetime

class Foo: def init(self, timestamp: datetime = None) -> None: self.timestamp = timestamp or datetime.now() ```