code: def nancorr(a, b, method='pearson', min_periods=None):
      
      if len(a) != len(b):
          raise AssertionError('Operands to nancorr must have same size')
  
      if min_periods is None:
          min_periods = 1
  
      valid = notna(a) & notna(b)
      if not valid.all():
          a = a[valid]
          b = b[valid]
  
      if len(a) < min_periods:
          return np.nan
  
      f = get_corr_func(method)
      return f(a, b)
docs: a, b: ndarrays