Solution: class with call operator

Make a class with function behavior instead of a pure function
The parameters are class attributes
Class instances can be called as ordinary functions, now with x and y as the only formal arguments 
class F:
    def __init__(self, a=1, b=1, c=1):
        self.a = a;  self.b = b;  self.c = c

    def __call__(self, x, y):    # special method!
        return self.a + self.b*x + self.c*y*y

f = F(a=0.5, c=0.01)
# can now call f as
v = f(0.1, 2)
...
gridvalues(f, xcoor, ycoor, somefile)

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