Lazy, Cached, Singleton Imports

Using Dunder Methods

Created: 2022-01-06

Problem: You need to make a resource available for import, but it requires some additional processing before being ready.

Examples:

• ORM Sessions/Engines
  • Different modules should use the same ORM connection
• Loggers
  • You want to use a logger, but with each import the logging handlers are duplicated

How We Approach with Cached Properties

This is a common pattern to cache properties

class MyResource:
    def __init__(self):
        self.cached_ = None

    @property
    def cached(self):
        if self.cached_:
            return self.cached_
        self.cached_ = one_time_load()
        return self.cached_

Notice we could have used a Class Attribute which would be shared across all instances of MyResource

Imports

Here's how to do something similar with a module (typing is helpful but optional)

from typing import Any

_MY_RESOURCE=None

def _load_resource():
    global _MY_RESOURCE
    if _MY_RESOURCE:
        # We've already loaded our resource
        return _MY_RESOURCE
    # one time operation
    _MY_RESOURCE = some_value
    return _MY_RESOURCE

# This is the good part

def __getattr__(name: str) -> Any:
    if name == "MY_RESOURCE":
        return _load_resource()
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

And now we can simply import MY_RESOURCE knowing we're getting _MY_RESOURCE!

We're defining a dunder method for a module?!

If you found this use of descriptors interesting, check out the official Descriptor HowTo Guide