Where's the bug in this function?

Question

def sqrt(a):
    def tryit(x):
        return 0.5 * (a/x + x)
    return fixedPoint(tryit(x), 0.0001)

Answer 1

There are a couple of problems:

• You're calling fixedPoint() - what's that? Is this defined somewhere else?
• You're using x in the function without having defined it.
• You're calling the function sqrt(), but the logic isn't anywhere near something that would calculate the square root of anything.

There is at least one inelegance:

• You're defining a nested function (tryit()). This is not a problem per se, but it doesn't make much sense, and you're relying on a local variable a being defined here instead of passing that variable to a stand-alone function defined on the module level. It's hard to tell what that function is supposed to be doing, though.
• tryit() isn't a good function name.

Answer 2

See Solve this equation with fixed point iteration for working examples of fixed point iteration.

The code in the present question misspells fixed_point, a function that needs to be imported from scipy.optimize. The required arguments of fixed_point include a function and an initial value; for example:

scipy.optimize.fixed_point(tryit, a/2)