Implementing DSLs in Django Apps

  • by Mattieu Amiguet

Initial Motivation: Searching Contacts

class Contact(models.Model):

    first_name = models.CharField()
    groups = models.ManyToManyField('Group')
  • Client wants to customize things themselves
  • giving them access to code is dangerous
  • Limit their actions

Other reasons

  • Quick and easy to implement (if you use the right tools)
  • Fun to code!

Not for the end user

  • Only to be used by power users
  • Your DSL could be used as a scripting language

How to make a DSL in Python/Django

  • Basics

    • Lexer (vocabulary)
    • Parser (grammar)
    • Some kind of backend

  • The lexer and parser part are quite generic

    • use code generator

Sample

import ply.lex as lex

tokens = (
    'COMPA', # comparison operator
    'STRING',
    'NUMBER'
)

t_COMPA = r'=|[<>]=?|~~?'

literals = '()'  # shortcut for 1-character functions

def t_STRING(t):
    r'"[^"]*"'
    t.value = t.value[1:-1]

def t_NUMBER(t):
    r'\d+'
    # TODO - finish this function

Note

Not sure how this works. Me need to read up on PLY

Parser - The Grammar

expression : expression B_OP expression expression : U_OP expression expression : ‘(‘ expression ‘)’ value : STRING

NUMBER
DATE

Parser in PLY

  • Grammar rules go into docstrings
  • Special argument p corresponds to rule parts
def p_expression_u_op(p):
    '''expression : U_OP expression'''
    if p[1] == 'NOT':
        p[0] = p[1]