Writing Domain Specific Languages with Python

• The process of writing DSL is the same in all languages.
• You write an ideal spec.
• Then, you try to fit the spec so that it's kind of valid syntax for the language you use.
• An example of ideal domain language (well, maybe not so ideal, but quite good):
  

test '/Login':
 GetReq(uname    = 'a_user',
        password = '')
 expect_tmpl:      'login.tmpl'
 expect_tmpl_has:  'errorMsg'
 expect_tmpl_data: { 'uname':'a_user',
                     'missing_fields' : ['Password']}
 expect_session_lacks: 'uid'

• Now the same but as a valid Python syntax:
  

test_serv('/Login',
   GetReq(uname    = 'a_user',
          password = ''),
   expect_tmpl       = 'login.tmpl',
   expect_tmpl_has   = 'errorMsg',
   expect_tmpl_data  = { 'uname':'a_user',
                         'missing_fields' : ['Password']},
   expect_session_lacks = 'uid')

• Useful tricks inclue:
  
• keyword args (expect_tmpl_data=...)
• unbounded arg lists (*args)
• operator overloading __add__(self, other_form)
• reflection via hasattr, getattr, vars and friends
• Thanks to Dan Milstein for his talk at PyCon about Little Languages in Python.