1. A method for developing source code for a computer program; wherein the method comprises the steps of: accessing a portion of source code through a code editor; identifying code comprising a method argument, said method argument being of a type that has a defined range of values that are further restricted by information not available from a class definition; invoking a property editor class method associated with the identified method argument to access a method argument variable; invoking a property editor to display a list of allowable values for the method argument variable; invoking a code generating method to return a snippit of source code that can be used to initialise the method argument variable with one of the allowable values; and inserting the snippit of code into the method argument code, said inserted code defining allowable values for said method argument, said allowable values comprising a subset of said defined range of values for that type of method argument.