Extending JDOG

It’s easy to add new placeholder.

JDOG instance has two methods which support you to extend behavior.

Each placeholder is represented by some subclass of Placeholder class.

Especially comes handy FuncPlaceholder which can be easily used to introduce new placeholders.

So, how to add a new placeholder?

For example we want to introduce fizzbuzz placeholder which with 50% chance print ‘fizz’ or ‘buzz’.

Its only a few steps and you are good to go.

  1. Think of a new name (or use an existing one).

  2. Create regex pattern.

  3. Add placeholder - use FuncPlaceholder or subclass Placeholder.

  4. Put it together and call add_matcher().

Come up with new name

It should be clear, only by name, what placeholder does. Beside the name think also about arguments.


If you use existing name new behavior will replace odl one.

Create regex pattern

During parsing phase each placeholder is tokenized. Tokenization process use regex to match each placeholder.

Starting regex could look like `^{{token}}$`, that is:

  • It has to start with double {{

  • It has to end with double }}

If you want any arguments for the placeholder, regex has to capture arguments as a group, that is ^{{token((.*))$.

A few examples of existing placeholders:

# age

# number

# name (most complex one, arguments are optional)

# and new one

Simple, isn’t it? (If in doubt, take a look here.)

Add placeholder

Placeholder is special class that holds logic of generating specific values.

The easiest way is to use FuncPlaceholder.

  • Takes argument - function.

  • This function takes one argument - placeholder arguments as list.

So to our fizzbazz example:

def fizzbuzz(args):
    if random.random() > 0.5:
        return 'fizz'
    return 'buzz'

If you want more fine grained functionality, just subclass Placeholder and use it.


If you want to automatically enclose returned value by placeholder within double quotes use FuncStrPlaceholder.

Putting it together

We have name, regex pattern and function which has logic of our fizzbuzz placeholder

On the instance of Jdog call add_matcher() function. Function takes three arguments

  • key - the unique identification of placeholder - name.

  • pattern - our regex pattern.

  • f_placeholder - function which takes two arguments - token, it’s arguments and should return Placeholder subclass.

Putting it together

# our pattern
pattern = r'^{{fizzbuzz}}$'

# placeholder logic
def fizzbuzz(args):
    if random.random() > 0.5:
        return 'fizz'
    return 'buzz'

# helper function to create placeholder
def create_fizzbuzz(token, args):
    return FuncStrPlaceholder(token, args, fizzbuzz)

jdog = Jdog()
jdog.add_matcher('fizzbuzz', pattern, create_fizzbuzz)


We are using FuncStrPlaceholder to automatically enclose value within double quotes. If you generate string values and do not enclose them result is not valid JSON.

Example can be simplified using lambda expressions.

jdog.add_matcher('fizzbuzz',match_fizzbuzz, lambda token, args: FuncStrPlaceholder(token, args, fizzbuzz))

We can go further

# in fizzbuzz logic, we dont really care about arguments
jdog.add_matcher('fizzbuzz',match_fizzbuzz, lambda token, args: FuncStrPlaceholder(token, args,lambda _: 'fizz' if random.random() > 0.5 else 'buzz'))

But remember less lines does not mean more readable code. In this example rather opposite.