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¶
Note
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
"""
__import__('pkg_resources').declare_namespace(__name__)
Note
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('$WORKON_HOME', filename), comment)
return
Modifying the User Environment¶
There are cases where the extension needs to update the user’s environment (e.g., changing the current working directory or setting environment variables). Modifications to the user environment must be made within the user’s current shell, and cannot be run in a separate process. To have code run in the user’s shell process, extensions can define hook functions to return the text of the shell statements to be executed. These source hooks are run after the regular hooks with the same name, and should not do any work of their own.
The initialize_source()
hook for the user scripts plugin looks for
a global initialize script and causes it to be run in the current
shell process.
def initialize_source(args):
return """
#
# Run user-provided scripts
#
[ -f "$WORKON_HOME/initialize" ] && source "$WORKON_HOME/initialize"
"""
Warning
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.
Warning
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.
[project.entry-points."virtualenvwrapper.initialize"]
user_scripts = "virtualenvwrapper.user_scripts:initialize"
project = "virtualenvwrapper.project:initialize"
[project.entry-points."virtualenvwrapper.initialize_source"]
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).
See also
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
Options:
-h, --help show this help message and exit
-s, --source Print the shell commands to be run in the current
shell
-l, --list Print a list of the plugins available for the given
hook
-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.
Logging¶
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).
get_env_details¶
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.
initialize¶
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.
pre_mkvirtualenv¶
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.
post_mkvirtualenv¶
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.
pre_activate¶
The virtualenvwrapper.pre_activate
hooks are run just before an
environment is enabled. The environment name is passed as the first
argument.
post_activate¶
The virtualenvwrapper.post_activate
hooks are run just after an
environment is enabled. $VIRTUAL_ENV
is set to point to the
current environment.
pre_deactivate¶
The virtualenvwrapper.pre_deactivate
hooks are run just before an
environment is disabled. $VIRTUAL_ENV
is set to point to the
current environment.
post_deactivate¶
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.
pre_rmvirtualenv¶
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.
post_rmvirtualenv¶
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
and:
virtualenvwrapper_run_hook project.post_mkproject
respectively.