def for_constant(self, constant):
return self.constants[constant]<EOL>
Returns the ``ChoiceEntry`` for the given constant.

        Parameters
        ----------
        constant: string
            Name of the constant for which we want the choice entry.

        Returns
        -------
        ChoiceEntry
            The instance of ``ChoiceEntry`` for the given constant.

        Raises
        ------
        KeyError
            If the constant is not an existing one.

        Example
        -------

        >>> MY_CHOICES = Choices(('FOO', 1, 'foo'), ('BAR', 2, 'bar'))
        >>> MY_CHOICES.for_constant('FOO')
        ('FOO', 1, 'foo')
        >>> MY_CHOICES.for_constant('FOO').value
        1
        >>> MY_CHOICES.for_constant('QUX')
        Traceback (most recent call last):
        ...
        KeyError: 'QUX'