def binary(self):
    """Returns the binary that builds the pex for this lambda."""
    dependencies = self.dependencies
    if len(dependencies) != 1:
      raise TargetDefinitionException(self, 'An app must define exactly one binary '
                                            'dependency, have: {}'.format(dependencies))
    binary = dependencies[0]
    if not isinstance(binary, PythonBinary):
      raise TargetDefinitionException(self, 'Expected binary dependency to be a python_binary '
                                            'target, found {}'.format(binary))
    return binary
Returns the binary that builds the pex for this lambda.