code: def precision(self):
          r
          if self._tp + self._fp == 0:
              return float('NaN')
          return self._tp / (self._tp + self._fp)
docs: r"""Return precision.
  
          Precision is defined as :math:`\frac{tp}{tp + fp}`
  
          AKA positive predictive value (PPV)
  
          Cf. https://en.wikipedia.org/wiki/Precision_and_recall
  
          Cf. https://en.wikipedia.org/wiki/Information_retrieval#Precision
  
          Returns
          -------
          float
              The precision of the confusion table
  
          Example
          -------
          >>> ct = ConfusionTable(120, 60, 20, 30)
          >>> ct.precision()
          0.8571428571428571