Hooks

Cookiecutter hooks are scripts executed at specific stages during the project generation process. They are either Python or shell scripts, facilitating automated tasks like data validation, pre-processing, and post-processing. These hooks are instrumental in customizing the generated project structure and executing initial setup tasks.

Types of Hooks

Hook	Execution Timing	Working Directory	Template Variables	Version
pre_prompt	Before any question is rendered.	A copy of the repository directory	No	2.4.0
pre_gen_project	After questions, before template process.	Root of the generated project	Yes	0.7.0
post_gen_project	After the project generation.	Root of the generated project	Yes	0.7.0

Creating Hooks

Hooks are added to the hooks/ folder of your template. Both Python and Shell scripts are supported.

Python Hooks Structure:

cookiecutter-something/
├── {{cookiecutter.project_slug}}/
├── hooks
│   ├── pre_prompt.py
│   ├── pre_gen_project.py
│   └── post_gen_project.py
└── cookiecutter.json

Shell Scripts Structure:

cookiecutter-something/
├── {{cookiecutter.project_slug}}/
├── hooks
│   ├── pre_prompt.sh
│   ├── pre_gen_project.sh
│   └── post_gen_project.sh
└── cookiecutter.json

Python scripts are recommended for cross-platform compatibility. However, shell scripts or .bat files can be used for platform-specific templates.

Hook Execution

Hooks should be robust and handle errors gracefully. If a hook exits with a nonzero status, the project generation halts, and the generated directory is cleaned.

Working Directory:

• pre_prompt: Scripts run in the root directory of a copy of the repository directory. That allows the rewrite of cookiecutter.json to your own needs.

• pre_gen_project and post_gen_project: Scripts run in the root directory of the generated project, simplifying the process of locating generated files using relative paths.

Template Variables:

The pre_gen_project and post_gen_project hooks support Jinja template rendering, similar to project templates. For instance:

module_name = '{{ cookiecutter.module_name }}'

Examples

Pre-Prompt Sanity Check:

A pre_prompt hook, like the one below in hooks/pre_prompt.py, ensures prerequisites, such as Docker, are installed before prompting the user.

import sys
import subprocess

def is_docker_installed() -> bool:
    try:
        subprocess.run(["docker", "--version"], capture_output=True, check=True)
        return True
    except Exception:
        return False

if __name__ == "__main__":
    if not is_docker_installed():
        print("ERROR: Docker is not installed.")
        sys.exit(1)

Validating Template Variables:

A pre_gen_project hook can validate template variables. The following script checks if the provided module name is valid.

import re
import sys

MODULE_REGEX = r'^[_a-zA-Z][_a-zA-Z0-9]+$'
module_name = '{{ cookiecutter.module_name }}'

if not re.match(MODULE_REGEX, module_name):
    print(f'ERROR: {module_name} is not a valid Python module name!')
    sys.exit(1)

Conditional File/Directory Removal:

A post_gen_project hook can conditionally control files and directories. The example below removes unnecessary files based on the selected packaging option.

import os

REMOVE_PATHS = [
    '{% if cookiecutter.packaging != "pip" %}requirements.txt{% endif %}',
    '{% if cookiecutter.packaging != "poetry" %}poetry.lock{% endif %}',
]

for path in REMOVE_PATHS:
    path = path.strip()
    if path and os.path.exists(path):
        os.unlink(path) if os.path.isfile(path) else os.rmdir(path)