Extending Virtualenvwrapper

Long experience with home-grown solutions for customizing a development environment has proven how valuable it can be to have the ability to automate common tasks and eliminate persistent annoyances. Carpenters build jigs, software developers write shell scripts. virtualenvwrapper continues the tradition of encouraging a craftsman to modify their tools to work the way they want, rather than the other way around.

There are two ways to attach your code so that virtualenvwrapper will run it: End-users can use shell scripts or other programs for personal customization, e.g. automatically performing an action on every new virtualenv (see Per-User Customization). Extensions can also be implemented in Python by using Setuptools entry points, making it possible to share common behaviors between systems and developers.

Use the hooks provided to eliminate repetitive manual operations and streamline your development workflow. For example, set up the pre_activate and post_activate hooks to trigger an IDE to load a project file to reload files from the last editing session, manage time-tracking records, or start and stop development versions of an application server. Use the initialize hook to add entirely new commands and hooks to virtualenvwrapper. And the pre_mkvirtualenv and post_mkvirtualenv hooks give you an opportunity to install basic requirements into each new development environment, initialize a source code control repository, or otherwise set up a new project.

Defining an Extension


Virtualenvwrapper is delivered with a plugin for creating and running the user customization scripts (user_scripts). The examples below are taken from the implementation of that plugin.

Code Organization

The Python package for virtualenvwrapper is a namespace package. That means multiple libraries can install code into the package, even if they are not distributed together or installed into the same directory. Extensions can (optionally) use the virtualenvwrapper namespace by setting up their source tree like:

  • virtualenvwrapper/

    • __init__.py

    • user_scripts.py

And placing the following code in __init__.py:

"""virtualenvwrapper module



Extensions can be loaded from any package, so using the virtualenvwrapper namespace is not required.

Extension API

After the package is established, the next step is to create a module to hold the extension code. For example, virtualenvwrapper/user_scripts.py. The module should contain the actual extension entry points. Supporting code can be included, or imported from elsewhere using standard Python code organization techniques.

The API is the same for every extension point. Each uses a Python function that takes a single argument, a list of strings passed to the hook loader on the command line.

def function_name(args):
    # args is a list of strings passed to the hook loader

The contents of the argument list are defined for each extension point below (see Extension Points).

Extension Invocation

Direct Action

Plugins can attach to each hook in two different ways. The default is to have a function run and do some work directly. For example, the initialize() function for the user scripts plugin creates default user scripts when virtualenvwrapper.sh is loaded.

def initialize(args):
    for filename, comment in GLOBAL_HOOKS:
        make_hook(os.path.join('$WORKON_HOME', filename), comment)

Modifying the User Environment

There are cases where the extension needs to update the user’s environment (e.g., changing the current working directory or setting environment variables). Modifications to the user environment must be made within the user’s current shell, and cannot be run in a separate process. To have code run in the user’s shell process, extensions can define hook functions to return the text of the shell statements to be executed. These source hooks are run after the regular hooks with the same name, and should not do any work of their own.

The initialize_source() hook for the user scripts plugin looks for a global initialize script and causes it to be run in the current shell process.

def initialize_source(args):
    return """
# Run user-provided scripts
[ -f "$WORKON_HOME/initialize" ] && source "$WORKON_HOME/initialize"


Because the extension is modifying the user’s working shell, care must be taken not to corrupt the environment by overwriting existing variable values unexpectedly. Avoid creating temporary variables where possible, and use unique names where variables cannot be avoided. Prefixing variables with the extension name is a good way to manage the namespace. For example, instead of temp_file use user_scripts_temp_file. Use unset to release temporary variable names when they are no longer needed.


virtualenvwrapper works under several shells with slightly different syntax (bash, sh, zsh). Take this portability into account when defining source hooks. Sticking to the simplest possible syntax usually avoids problems, but there may be cases where examining the SHELL environment variable to generate different syntax for each case is the only way to achieve the desired result.

Registering Entry Points

The functions defined in the plugin need to be registered as entry points in order for virtualenvwrapper’s hook loader to find them. Entry points are configured in the packaging instructions for your package by mapping the entry point name to the function in the package that implements it.

This partial copy of virtualenvwrapper’s pyproject.toml illustrates how the initialize() and initialize_source() entry points are configured.

user_scripts = "virtualenvwrapper.user_scripts:initialize"
project = "virtualenvwrapper.project:initialize"

user_scripts = "virtualenvwrapper.user_scripts:initialize_source"

Each entry points section maps the group names to lists of entry point specifiers. A different group name is defined by virtualenvwrapper for each extension point (see Extension Points).

The entry point specifiers are strings with the syntax name = "package.module:function". By convention, the name of each entry point is the plugin name, but that is not required (the names are not used).

The Hook Loader

Extensions are run through a command line application implemented in virtualenvwrapper.hook_loader. Because virtualenvwrapper.sh is the primary caller and users do not typically need to run the app directly, no separate script is installed. Instead, to run the application, use the -m option to the interpreter:

$ python -m virtualenvwrapper.hook_loader -h
Usage: virtualenvwrapper.hook_loader [options] <hook> [<arguments>]

Manage hooks for virtualenvwrapper

  -h, --help            show this help message and exit
  -s, --source          Print the shell commands to be run in the current
  -l, --list            Print a list of the plugins available for the given
  -v, --verbose         Show more information on the console
  -q, --quiet           Show less information on the console
  -n NAMES, --name=NAMES
                        Only run the hook from the named plugin

To run the extensions for the initialize hook:

$ python -m virtualenvwrapper.hook_loader -v initialize

To get the shell commands for the initialize hook:

$ python -m virtualenvwrapper.hook_loader --source initialize

In practice, rather than invoking the hook loader directly it is more convenient to use the shell function, virtualenvwrapper_run_hook to run the hooks in both modes.:

$ virtualenvwrapper_run_hook initialize

All of the arguments given to shell function are passed directly to the hook loader.


The hook loader configures logging so that messages are written to $WORKON_HOME/hook.log. Messages also may be written to stderr, depending on the verbosity flag. The default is for messages at info or higher levels to be written to stderr, and debug or higher to go to the log file. Using logging in this way provides a convenient mechanism for users to control the verbosity of extensions.

To use logging from within your extension, simply instantiate a logger and call its info(), debug() and other methods with the messages.

import logging
log = logging.getLogger(__name__)

def pre_mkvirtualenv(args):
    log.debug('pre_mkvirtualenv %s', str(args))
    # ...

Extension Points

The extension point names for native plugins follow a naming convention with several parts: virtualenvwrapper.(pre|post)_<event>[_source]. The <event> is the action taken by the user or virtualenvwrapper that triggers the extension. (pre|post) indicates whether to call the extension before or after the event. The suffix _source is added for extensions that return shell code instead of taking action directly (see Modifying the User Environment).


The virtualenvwrapper.get_env_details hooks are run when workon is run with no arguments and a list of the virtual environments is printed. The hook is run once for each environment, after the name is printed, and can be used to show additional information about that environment.


The virtualenvwrapper.initialize hooks are run each time virtualenvwrapper.sh is loaded into the user’s environment. The initialize hook can be used to install templates for configuration files or otherwise prepare the system for proper plugin operation.


The virtualenvwrapper.pre_mkvirtualenv hooks are run after the virtual environment is created, but before the new environment is activated. The current working directory for when the hook is run is $WORKON_HOME and the name of the new environment is passed as an argument.


The virtualenvwrapper.post_mkvirtualenv hooks are run after a new virtual environment is created and activated. $VIRTUAL_ENV is set to point to the new environment.


The virtualenvwrapper.pre_activate hooks are run just before an environment is enabled. The environment name is passed as the first argument.


The virtualenvwrapper.post_activate hooks are run just after an environment is enabled. $VIRTUAL_ENV is set to point to the current environment.


The virtualenvwrapper.pre_deactivate hooks are run just before an environment is disabled. $VIRTUAL_ENV is set to point to the current environment.


The virtualenvwrapper.post_deactivate hooks are run just after an environment is disabled. The name of the environment just deactivated is passed as the first argument.


The virtualenvwrapper.pre_rmvirtualenv hooks are run just before an environment is deleted. The name of the environment being deleted is passed as the first argument.


The virtualenvwrapper.post_rmvirtualenv hooks are run just after an environment is deleted. The name of the environment being deleted is passed as the first argument.

Adding New Extension Points

Plugins that define new operations can also define new extension points. No setup needs to be done to allow the hook loader to find the extensions; documenting the names and adding calls to virtualenvwrapper_run_hook is sufficient to cause them to be invoked.

The hook loader assumes all extension point names start with virtualenvwrapper. and new plugins will want to use their own namespace qualifier to append to that. For example, the project extension defines new events around creating project directories (pre and post). These are called virtualenvwrapper.project.pre_mkproject and virtualenvwrapper.project.post_mkproject. These are invoked with:

virtualenvwrapper_run_hook project.pre_mkproject $project_name


virtualenvwrapper_run_hook project.post_mkproject