Created
February 18, 2019 07:43
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# Copyright 2019 Google LLC | |
# | |
# Licensed under the Apache License, Version 2.0 (the "License"); | |
# you may not use this file except in compliance with the License. | |
# You may obtain a copy of the License at | |
# | |
# https://www.apache.org/licenses/LICENSE-2.0 | |
# | |
# Unless required by applicable law or agreed to in writing, software | |
# distributed under the License is distributed on an "AS IS" BASIS, | |
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | |
# See the License for the specific language governing permissions and | |
# limitations under the License. | |
"""A really (really) simple dependency injector. | |
This is mostly just to keep code that requires client libraries & auth | |
relatively sane. | |
""" | |
import functools | |
from typing import Any, Dict, List | |
_registry: Dict[str, Any] = dict() | |
def set(name: str, value: Any): | |
"""Sets a value to be injected. | |
value can also be a callable, which will mean that it is treated as a | |
factory function. It will be invoked *once* at runtime to create the | |
needed value. | |
""" | |
_registry[name] = value | |
_default_sentinel = object() | |
def get(name: str, default=_default_sentinel): | |
try: | |
value = _registry[name] | |
if callable(value): | |
value = value() | |
# Update the value so we only call factories once. | |
_registry[name] = value | |
return value | |
except KeyError: | |
if default is not _default_sentinel: | |
return default | |
raise KeyError(f"{name} has not been provided.") | |
def needs(*things): | |
"""An injector that injects the requested dependences into a function's | |
arguments. | |
""" | |
def decorator(f): | |
@functools.wraps(f) | |
def invocation(*args, **kwargs): | |
for req in things: | |
kwargs.setdefault(req, get(req)) | |
return f(*args, **kwargs) | |
return invocation | |
return decorator | |
# Avoid shadowing below | |
_needs = needs | |
def provides(name=None, needs: List[str] = None): | |
"""A shortcut for defining a factory function that also needs dependencies | |
itself.""" | |
if not needs: | |
needs = [] | |
def decorator(f): | |
decorated = _needs(*needs)(f) | |
set(name or f.__name__, decorated) | |
return f | |
return decorator |
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