click.py

from click.types import ParamType                                                
from click.exceptions import BadParameter                                        
                                                                                 
                                                                                 
class FallBackType(ParamType):                                                      
    def __init__(self, fallback, def_type):                                         
        self.fallback = fallback                                                    
        self.def_type = def_type                                                    
                                                                                    
    def convert(self, value, param, ctx):                                           
        try:                                                                        
            return self.def_type(value, param, ctx)                                 
        except BadParameter:
            if callable(fallback):
                return fallback(value, param, ctx)
            return self.fallback