Looksee Python Module Scanner

Looksee is a tool for scanning Python packages for objects that match a custom predicate, and when there's a match, performing a custom callback, like adding each object to a global registry of some kind or triggering logic as an import side-effect.

It's similar to Venusian but far less annoying. Venusian can be a pain in the arse when it comes to handling errors. Often times, you're left scratching your head, trying to figure out what, if anything, went wrong.

Install Looksee

Clone the repo or just run...

pip install looksee

Basic Example

Here's an example of looksee's Scanner being used to scan a fictitious package, called pooply, for all PooplyObject subclasses. When found, we add each one to a growing "context" dict. Note that the scan method returns a shallow copy of the context dict, memoizing the original in scanner.context.

from looksee import Scanner
from pooply.base import PooplyObject

scanner = Scanner(
    predicate=lambda: obj: issubclass(obj, PooplyObject),
    callback=lambda name, obj, ctx: ctx.update({name: obj})
)

found = scanner.scan('pooply')

for name, class_obj in found.items():
    print(f'detected {name} class: {class_obj}...')

Use-cases

Class Factory Pattern

Suppose you have a config file that specifies the name of a class to use in your application. To translate the name of the class to an actual class object, you write code like this:

from project.models.user import User
from project.models.account import Account

class Model:

    @classmethod
    def factory(cls, class_name: Text) -> Type:
        if class_name == 'User':
            return User
        if class_name == 'Account':
            return Account
        else:
            raise TypeError(class_name)

By itself, this seems fine, but when you try to run it, you realize that you've caused a bunch of cyclic import errors by trying to import each subclass into the module containing its base class. Not only that, but the if-statement must be keep up-to-date manually with each new class added to the application. To fix the import errors, you could try moving each import into the factory function itself, but this would be bad form, and you would still have to update the if-statement with each new class.

By using looksee, you could avoid this altogether by doing something like:

from looksee import Scanner

class Model:

    # lazy loaded global registry of model subclasses
    model_classes = {}

    # scanner that detects Model subclasses
    scanner = Scanner(
        predicate=lambda obj: issubclass(obj, cls),
        callback=lambda name, obj, ctx: ctx.update({name: obj})
    )

    @classmethod
    def factory(cls, class_name: Text) -> Type:
        # lazily scan resource modules, picking up subclasses
        if not cls.model_classes:
            cls.model_classes = cls.scanner.scan('project.resources')

        # return the named class from the model_classes dict
        model_class = cls.model_classes.get(class_name)
        if model_class is None:
            raise TypeError(class_name)

        return model_class

Endpoint/Function Registration in a Framework

Most Python web frameworks use decorators to register endpoints. It's likely that you've seen some form of...

@app.get(url='/users/{user_id}')
def get_user(request, user_id):
    return User.get(user_id)

Internally, app.get() adds the get_user function to the app as a route. In order to detect each endpoint, the framework must be able to scan the modules contained a project, evaluating decorators as a side-effect. Looksee can be used for cases like this. Here is a sketch of how you might achieve this:

class Application(HttpServer):
    def __init__(self, package: Text):
        self.routes = []
        self.scanner = Scanner()
        self.package = package

    def start(self):
        self.scanner.scan(self.package)
        self.serve_forever()

    def get(self, url: Text):
        """ Register an HTTP GET route """
        return Decorator(self, 'GET', url)

    def post(self, url: Text):
        """ Register an HTTP POST route """
        return Decorator(self, 'POST', url)


class Decorator:
    def __init__(self, app, method: Text, url: Text):
        self.app = app
        self.method = method
        self.url = url

    def __call__(self, func: Callable):
        self.app.routes.append(Route(url, func))


class Route:
    def __init__(self, method: Text, url: Text, func: Callable):
        self.method = method
        self.url = url
        self.func = func

Now, when someone using your framework calls app.start(), the app's scanner will trigger each decorator in whatever module it resides, simply as a side-effect of being imported. This might look something like:

# file: app.py

app = Application('my_project')
app.start()

# file: routes.py

from .app import app

@app.get('/users/{user_id}')
def get_user(request, user_id):
    return User.get(user_id)

Working Example

A complete working example in which a Scanner is used to scan a fictitious project is located in the example/ directory. To run it, just do:

python ./example/scan.py

Event Hooks

A couple things can go wrong while performing a scan: either (1) a module cannot be imported due to an error or (2) your callback raises an exception while processing an object. You can override Scanner hook base methods to deal with both of these cases.

Import Error Hook

def on_import_error(
    self, exc: Exception, module_path: Text, context: Dict
):
    """
    Executes if the scanner can't import a module because of an error.
    """

Callback Error Hook

def on_callback_error(
    self, exc: Exception, module: ModuleType, context: Dict, name: Text, obj: Any
):
    """
    Execute if scanner encountered error inside custom callback function
    """

Ignore Directory Hook

In addition to handling errors, the scanner can be directed to skip certain directories. To do so, it looks for a .looksee JSON file in the directory to skip. That file should contain a JSON object like:

{
  "ignore": true
}

If the scanner determines that it should ignore a directory, it triggers its on_ignore_directory hook:

def on_ignore_directory(self, dirpath: Text):
    """
    Executes when scanner skips a directory because of .looksee file.
    """

Logging

You can easily toggle the internal log level by either setting the LOOKSEE_LOG_LEVEL environment variable or by passing a custom logger into the Scanner initializer.

Questions & Comments

If you have any questions or comments, feel free to open an issue or email us directly. We appreciate your contributions and support!