Custom Modules

Catcher has a flexible modular system. If built-in modules functionality is not enough you can use your own modules.
Read How to write a test cheat sheet for more info about running and naming steps, registering and using variables.


There are build in modules, shipped with catcher. You can learn about their usage in internal_modules.


There are additional catcher modules available in this repository: Catcher-Modules


Python module

Writing external scripts in Python is a bit easier. All you need is:

  1. Inherit from catcher.steps.external_step.ExternalStep()
  2. Use catcher.steps.step.update_variables() annotation on your action method implementation.
  3. Your function should return variables (from input) and output (optional). Do not modify variables inside function (it is bad practice)
  4. Include path/to/your/module to catcher: catcher -i intentory.yaml test.yaml -m my.python.package

You can use catcher.steps.external_step.ExternalStep.simple_input() to fill all templates. example:

from catcher.steps.external_step import ExternalStep
from catcher.steps.step import update_variables

class HelloStep(ExternalStep):
    Very important and useful step. Says hello to input. Return as a string.
    Example usage.
            say: 'John Doe'
            register: {greeting='{{ OUTPUT }}'}

    def action(self, includes: dict, variables: dict) -> (dict, str):
        body = self.simple_input(variables)
        person = body['say']
        return variables, 'hello {}'.format(person)

Other languages - executable

You can easily write your own modules and plug them to catcher:

  1. write your module (it should be executable)
  2. use it in your test script
  3. tell catcher where it is

For example you’ve developed a math module:

one=$(echo ${1} | jq -r '.add.the')
two=$(echo ${1} | jq -r '')
echo $((${one} + ${two}))

to sum the and to. Then in test script you will write:

  one: 1
  two: 2
    - math:
        add: {the: '{{ one }}', to: '{{ two }}'}
        register: {sum: '{{ OUTPUT }}'}
To run it just tell the Catcher where your script is:
catcher your_test.yaml -m path/to/your/module
Catcher will find your math module (by the step name) in path/to/your/module directory and call it with {“add”: {“the”: “1”,”to”: “2”}} argument. Your file’s stdout will be the step’s OUTPUT.

Other languages - compiled/VM

Catcher also supports not-executable (can’t be run as ./your_step) external steps. F.e. java files compiled in a Jar:

    person: 'John Doe'
    - MyClass.jar:
        say: '{{ person }}'
        register: {greeting: '{{ OUTPUT }}'}
    - check: {equals: {the: '{{ greeting.strip() }}', is: 'hello John Doe'}}

Catcher will run MyClass.jar (should be in resources directory) with local java (should be installed). In this case all arguments (like say in this example) will be passed as command-line arguments in JSON format (argv in main function on Java-side) while all variables will be accessible as environmental variables (System.getenv on Java-side).

To output any value stdout should be used. In case of Json you can access fields directly (f.e. OUTPUT.your_field). If Catcher fails to parse json - it will use the whole output as a plain text.

In case of java script based step Catcher will run nodeJs (should be installed)

In case of Java/Kotlin source code files - Catcher will compile them first (javac/kotlinc should be installed). It is extremely useful when you debug your step. Source code files should be placed in the root of resources folder. In case of multiple classes - Catcher will compile them all.


package jtest;

import jtest.OtherClass;

public class MyClass {

    public static void main(String[] args) {
        // works only with {"say": "<name>"}
        String name = args[0].split(":")[1].split("\"")[1];
        System.out.println(new OtherClass("hello " + name).arg);


package jtest;

public class OtherClass {

    public String arg;

    public OtherClass(String arg) {
        this.arg = arg;


    person: 'John Doe'
        say: '{{ person }}'
        register: {greeting: '{{ OUTPUT }}'}
    - check: {equals: {the: '{{ greeting.strip() }}', is: 'hello John Doe'}}