Question

I’m implementing a little UserOptionHandler class in python. It goes something like this:

class OptionValue():
   __init__(self, default_value, value_type=None):
      self.value=default_value

      if value_type == None:
         self.value_type = type( default_value )
      else:
         self.value_type = value_type

class OptionHandler():
   __init__(self, option_list):
      self.options = {}

      for opt_spec in option_list:
          key, value, value_type = opt_spec
          self.options[key] = Option( value, value_type )

   def set(self, key, value):
      opt = self.options[key]
      opt.value = value

When I get user input to set the value for an option, I want to make sure that they’ve entered sane. Otherwise, the application run until it gets to a state where it uses the option, which may or may not crash it.

How do you do something like the following?

opt = handler.get( key )
user_input = input("Enter value for {0}:".format(key) )
if( magic_castable_test( user_input, opt.value_type ) ):
   print "Your value will work!"
else:
   print "You value will break things!"

Answer 1

i’m not sure i follow what you’re asking, but i think that you just need to use type itself because:

• user input is a string

• many types, like int and float are functions/constructors that return an instance from a string. for example int("3") returns the integer 3.

so in your code, you would replace

if( magic_castable_test( user_input, opt.value_type ) ):
   print "Your value will work!"
else:
   print "You value will break things!"

with:

value = None
try:
    value = opt.value_type(value)
except:
    pass
if value is None:
    print "Your value will work!  It is", value
else:
    print "Your value will not work

where i am assuming it’s an error if either (1) an exception is thrown or (2) the type returning null.

and you can take this further and simply require that any types work this way. so, for example, you might have some information that is best represented as a custom class. then you should write the class so that it has an __init__ that takes a single string:

class MyClass:
    def __init__(self, s):
        # do something with string s here

and you can then pass MyClass as an option type to your code above.

ps here’s perhaps a clearer example of what i mean:

>>> type(4)("3")
3
>>> type(type(4)("3"))
<type 'int'>