nr.proxy

The nr.proxy module provides utilities for creating Python object proxies which will forward almost any interaction with the object to another object. This allows for the creation of very convenient APIs in Python applications (e.g. à la Flask request object).

This package requires Python 3.5 or higher.

Example: proxy

Command-line interfaces built with Click can use the Context.ensure_object() method to attach information to a context. Retrieving that information from the context include a couple of lines which over all reduce the readability of the code. Using a proxy allows you to access that information once it is initialized in the context as if the data was available globally.

import click
import nr.proxy
from pathlib import Path
from .config import Configuration

config = nr.proxy.proxy[Configuration](lambda: click.get_current_context().obj['config'])

@click.group()
@click.option('-c', '--config', 'config_file', type=Path, default='config.toml',
  default='Path to the configuration file.')
@click.pass_context
def cli(ctx: click.Context, config: Path) -> None:
  ctx.ensure_object(dict)['config'] = Configuration.load(config)

@cli.command()
def validate():
  # No need to use @click.pass_context or access ctx.obj['config'].
  config.validate()

Example: threadlocal

The below is an example for creating a globally accessible SqlAlchemy session. Inside a with-context using make_session(), the global session object can be accesses like a normal instance of the Session class. Outside of the context, accessing the session object results in a RuntimeError with the specified error message.

The advantage of this method is that the Session object does not need to be passed around, but can instead just be accessed globally.

import contextlib
import nr.proxy
from sqlalchemy.orm import sessionmaker
from typing import Generator

Session = sessionmaker()
session = nr.proxy.threadlocal[Session](
  name=__name__ + '.session',
  error_message=
    '({name}) No SqlAlchemy session is available. Ensure that you are using the '
    'make_session() context manager before accessing the global session proxy.',
)

@contextlib.contextmanager
def make_session() -> Generator[None, None, None]:
  """
  A context manager that creates a new #Session object and makes it available in the global
  #session proxy object.
  """

  nr.proxy.push(session, Session())
  try:
    yield
  except:
    session.rollback()
    raise
  else:
    session.commit()
  finally:
    nr.proxy.pop(session)

Copyright © 2020 Niklas Rosenstein