So I'm assuming here that fp(x) or f(x) are functions that don't return anything.

Take a look at this,

def fp(x):
    print(x)

print(fp(10))

You might think that the output of this would be 10 but you get this,

10
None

Take a look at this function,

def fp():
    pass
print(fp())

Output:

None

Functions by default return a None type. So in your case when you are doing abs(fp(x)) the problem seems to be that you are doing abs() on None that's why you get the error.

Look at this,

def fp():
        pass
abs(fp())

Output:

TypeError: bad operand type for abs(): 'NoneType' python error

So add a return statement to the function and you can see the error will be gone

def fp(x):
    return x
some_value = abs(fp(-10.5))
print(some_value)

Output:

10.5

Now the error is gone.

I see you've already imported numpy because you're using np.linspace in the code. You are probably confusing numpy's abs, which will happily work on lists and arrays, with __builtin__.abs, which only works for scalars.

Change this:

abs(wf[0:100])

To this:

np.abs(wf[0:100])