traitlets

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The code below shows that even though x is never given a value, it is an empty string.

Why do the docs for Unicode state default_value=traitlets.Undefined yet the type is string when tested?

from traitlets.config import Application
from traitlets import List, Dict, Integer, Unicode

class App(Application):

    aliases = {"x": "App.x", "y": "App.y"}
    x = Unicode().tag(config=True)
    y = Unicode().tag(config=True)

    def start(self):
        print(f"typex = {type(self.x)} val={self.x}")
        print(f"y={self.y}")


if __name__ == "__main__":
    App.launch_instance()

Output

typex = <class 'str'> val=
y=

Even if allow-none=True, an undefined value is not defaulted to None. I changed one of the attributes to have allow_none=True and got this:

from traitlets.config import Application
from traitlets import List, Dict, Integer, Unicode, validate, TraitError


class App(Application):

    aliases = {"x": "App.x", "y": "App.y"}
    x = Unicode().tag(config=True)
    y = Integer(allow_none=True).tag(config=True)

    def start(self):
        print(f"typex = {type(self.x)} val={self.x}")
        print(f"typey = {type(self.y)} val={self.y}")

        self.x = "hi"

c:\programming\traitlets-lab\required-cli-argument>python required_cli.py 
python required_cli.py 
typex = <class 'str'> val=
typey = <class 'int'> val=0

if name == "main": App.launch_instance()

While Undefined is the default value for the default_value argument to the __init__ function, it is never actually used as a value for a trait. It is a placeholder to indicate that there was no override of the default value. When no such override is given, this value is used:

https://github.com/ipython/traitlets/blob/e2c731ef72dd41d4be527d4d93dd87ccc409830d/traitlets/traitlets.py#L2368

I agree that it would be good if the docs surfaced the actual default value in a better way!