Python - notation for redundant lambda parameters

Question

I am trying to build a function

f(x, y, z)

where neither x nor y have any impact on the result. Nonetheless, they must be passed to the function, as the passing is done by a library. I would like to:

• solve this using a lambda expression, and
• ensure that anyone reading my code will immediately know that x and y are redundant.

As I have gathered, using the underscore character for redundant return values from multiple-value-returning functions is common practice, e.g.

a, _, c, d, _, _ = return_six_things(),

however, trying

f = lambda _, _, z: some_operation(z)

crashes with the exception warning the author about using 'duplicate arguments' (two underscore characters).

Are there any other options?

P.S. to avoid the XY problem, the reason I want to use lambdas is to save a few source code characters. I would love to use them anyway, but declaring that "these parameters will not be used" is more important for me.

Answer 1

I'd use a double underscore (__) for the second unused argument:

f = lambda _, __, z: some_operation(z)

Answer 2

Options:

# pass in z as a keyward argument
# like this: f(x,y,z=1)
f = lambda *_,z: some_operation(z)

#Or:
# pass in any number of arguments and only use the last argument 
f = lambda *z: some_operation(z[-1])