如何在 Python 中使用传递的参数作为默认参数值？

Question

Given a function with multiple arguments, where all but the first one are variable.

Eg: def f(a, b = .., ...)

I am looking for minimalist python-code that realizes the intuitive code below:

def f(a, b = a, ...)

Hence, I could not find any satisfying answers I am asking here although I am without doubts that the answers to this question have been given already somewhere - in that case i apologize. Cheers!

I specify by another example my desired functionality again intuitively by wrong code:

def f(a,b,c, d = 0, e = [], f = b, g = c, h = a):
...

Thank you in advance!

Answer 1

According to the Python docs :

the expression [used as default parameter value] is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call.

So what you are trying to achieve would not work so simply because default parameter values are computed only once at the function definition, not at every call.

Instead, you can set all default parameter values to None and test for this value in the body of the function:

def func(a, b, c, d = 0, e = [], f = None, g = None, h = None):
    f = b if f == None else f
    g = c if g == None else g
    h = a if h == None else h

Answer 2

Thank you for the quick response! You´re totally right with the evaluation of the default parameters. Since I asked my question knowing that already I conclude that python leaves me without sneaky shorthands except the python if-else-shorthand you just used?