human
Solve the following python programming problem given the following code:
def _test_Z(self, kernel, psi2n = False) : 
    def f(p) : 
        psi0 = kernel.psi0 (p, self.qX) 
        psi1 = kernel.psi1 (p, self.qX) 
        psi2 = kernel.psi2 (p, self.qX) 
        if (not psi2n) : 
            psi2 = kernel.psi2 (p, self.qX) 
            return (((self.w1 * psi0).sum () + (self.w2 * psi1).sum ()) + (self.w3 * psi2).sum ()) 
else : 
            psi2 = kernel.psi2n (p, self.qX) 
            return (((self.w1 * psi0).sum () + (self.w2 * psi1).sum ()) + (self.w3n * psi2).sum ()) 
def df(p) : 
        return kernel.gradients_Z_expectations (self.w1, self.w2, (self.w3 if (not psi2n) else self.w3n), p, self.qX) 
from GPy.models import GradientChecker 
    m = GradientChecker (f, self, self.Z.copy ()) 
    self.assertTrue (m.checkgrad ()) 

    
    

    Fix the buggy line: m = GradientChecker (f, self, self.Z.copy ())
assistant
#Here is Python code to fix the buggy line: m = GradientChecker (f, self, self.

m = GradientChecker (f, def df(, self.Z.copy ())