How can I make the default value of an argument depend on another argument (in Python)?

The language doesn't support such syntax.

The usual workaround for these situations(*) is to use a default value which is not a valid input.

def func(n=5.0, delta=None):
     if delta is None:
         delta = n/10

(*) Similar problems arise when the default value is mutable.

You can't do it in the function definition line itself, you need to do it in the body of the function:

def func(n=5.0,delta=None):
    if delta is None:
        delta = n/10

You could do:

def func(n=5.0, delta=None):
    if delta is None:
        delta = n / 10
    ...

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