src/pyapp/conf/helpers/plugins.py
"""
*Configuration based Instance factories*
Inversion of control style plugin factories that return an object instance
based on a named definition from configuration. Makes use of
:py:mod:`pyapp.conf` to store instance definitions in settings.
An instance definition includes both the object type as well as arguments
provided at instantiation, this can be used to provide for example connections
to different multiple different database instances (that use the same database
engine and connection library).
The default instance factory also integrates with the :py:mod:`pyapp.checks`
framework to allow checks to be executed on the instances.
Usage::
>>> foo_factory = NamedPluginFactory('FOO')
>>> instance = foo_factory.create()
or taking advantage of the factory being callable we can create a singleton
factory::
>>> get_bar_instance = NamedSingletonPluginFactory('BAR')
>>> # Get iron bar instance
>>> bar = get_bar_instance.create('iron')
"""
import threading
from abc import ABCMeta
from typing import Type, TypeVar
from pyapp import checks
from pyapp.conf import settings
from pyapp.conf.helpers.bases import (
DefaultCache,
FactoryMixin,
SingletonFactoryMixin,
ThreadLocalSingletonFactoryMixin,
)
from pyapp.exceptions import (
BadAlias,
CannotImport,
InvalidSubType,
NotFound,
NotProvided,
)
from pyapp.utils import cached_property, import_type
__all__ = (
"NamedPluginFactory",
"NamedSingletonPluginFactory",
"ThreadLocalNamedSingletonPluginFactory",
"NoDefault",
)
PT_co = TypeVar("PT_co", covariant=True)
NoDefault = "__NoDefault__" # pylint: disable=invalid-name
class NamedPluginFactory(FactoryMixin[PT_co], metaclass=ABCMeta):
"""
Factory object that generates a named instance from a definition in
settings. Can optionally verify an instance type against a specified ABC
(Abstract Base Class). The factory will cache imported types.
Named instances are defined in settings using the following definition::
MY_TYPE = {
'default': (
'my.module.MyClass', {
'param1': 123,
'param2': 'foo',
}
)
}
The :py:class:`NamedPluginFactory` is thread safe.
"""
def __init__(
self, setting: str, *, abc: Type[PT_co] = None, default_name: str = "default"
):
"""
Initialise a named factory.
:param setting: Setting attribute that holds the definition of a
instance, this value should be an upper case.
:param abc: The absolute base class that any new instance should be
based on.
:param default_name: The key within the settings to be returned as the
default instance type if a value is not supplied.
"""
if not (isinstance(setting, str) and setting.isupper()):
raise ValueError(f"Setting `{setting}` must be upper-case")
self.setting = setting
self.abc = abc
self.default_name = default_name
self._type_definitions = DefaultCache(self._get_type_definition)
self._type_definitions_lock = threading.RLock()
self._register_checks()
@cached_property
def _instance_definitions(self): # pylint: disable=method-hidden
return getattr(settings, self.setting, {})
@cached_property
def has_default(self) -> bool:
"""
This plugin does not have a default instance type
"""
return self.default_name != NoDefault
@property
def available(self):
"""
Defined names available in settings.
"""
return self._instance_definitions.keys()
def _resolve_instance_definition(self, name):
alias_names = {name}
while True:
try:
type_name, kwargs = self._instance_definitions[name]
except KeyError:
raise NotFound(f"Setting definition `{name}` not found") from None
if type_name.upper() == "ALIAS":
# Alias found
name = kwargs.get("name")
if not name:
raise BadAlias(f"Name not defined for alias `{name}`")
# Circular alias protection
if name in alias_names:
raise BadAlias(f"Circular aliases detected: {alias_names!r}")
alias_names.add(name)
else:
return type_name, kwargs
def _get_type_definition(self, name: str):
type_name, kwargs = self._resolve_instance_definition(name)
try:
type_ = import_type(type_name)
except (ImportError, AttributeError) as ex:
raise CannotImport(
f"Cannot import type `{type_name}` from config '{name}'."
) from ex
if self.abc and not issubclass(type_, self.abc):
raise InvalidSubType(
f"Setting definition `{type_name}` is not a subclass of `{self.abc}`"
)
return type_, kwargs
def create(self, name: str = None) -> PT_co:
"""
Get a named instance.
:param name: Named instance definition; default value is defined by the
``default_name`` instance argument.
:returns: New instance of the named type.
"""
if self.has_default: # pylint: disable=using-constant-test
name = name or self.default_name
elif not name:
raise NotProvided("A name is required if no default is specified.")
with self._type_definitions_lock:
instance_type, kwargs = self._type_definitions[name]
return instance_type(**kwargs)
def _register_checks(self):
checks.register(self)
def checks(self, **kwargs):
"""
Run checks to ensure settings are valid, secondly run checks against
individual definitions in settings.
"""
settings_ = kwargs["settings"]
# Check settings are defined
if not hasattr(settings_, self.setting):
return checks.Critical(
"Instance definitions missing from settings.",
hint=f"Add a {self.setting} entry into settings.",
obj=f"settings.{self.setting}",
)
instance_definitions = getattr(settings_, self.setting)
if instance_definitions is None:
return None # Nothing is defined so end now.
if not isinstance(instance_definitions, dict):
return checks.Critical(
"Instance definitions defined in settings not a dict instance.",
hint=f"Change setting {self.setting} to be a dict in settings file.",
obj=f"settings.{self.setting}",
)
# Take over the lock while checks are being performed
with self._type_definitions_lock:
# Backup definitions, replace and clear cache, this is to make it
# easier to write checks for instances.
instance_definitions_orig = self._instance_definitions
try:
self._instance_definitions = instance_definitions
self._type_definitions.clear()
messages = []
# Check default is defined
if self.has_default and self.default_name not in instance_definitions:
messages.append(
checks.Warn(
"Default definition not defined.",
hint=f"The default instance type `{self.default_name}` is not defined.",
obj=f"settings.{self.setting}",
)
)
# Check instance definitions
for name in instance_definitions:
message = self.check_instance(instance_definitions, name, **kwargs)
if isinstance(message, checks.CheckMessage):
messages.append(message)
elif message:
messages += message
return messages
finally:
# Put definitions back and clear cache.
self._instance_definitions = instance_definitions_orig
self._type_definitions.clear()
checks.check_name = "{obj.setting}.check_configuration"
def check_instance(self, instance_definitions, name, **_):
"""
Checks for individual instances.
"""
definition = instance_definitions[name]
if not isinstance(definition, (tuple, list)):
return checks.Critical(
"Instance definition is not a list/tuple.",
hint="Change definition to be a list/tuple (type_name, kwargs) in settings.",
obj=f"settings.{self.setting}[{name}]",
)
if len(definition) != 2: # noqa: PLR2004
return checks.Critical(
"Instance definition is not a type name, kwarg (dict) pair.",
hint="Change definition to be a list/tuple (type_name, kwargs) in settings.",
obj=f"settings.{self.setting}[{name}]",
)
type_name, kwargs = definition
messages = []
if type_name.upper() == "ALIAS":
target_name = kwargs.get("name")
if not target_name:
messages.append(
checks.Critical(
"Name of alias target not defined",
hint="An alias entry must provide a `name` value that refers to another entry.",
obj=f"settings.{self.setting}[{name}]",
)
)
elif target_name not in instance_definitions:
messages.append(
checks.Critical(
"Alias target not defined",
hint="The target specified by the alias does not exist, check the `name` value.",
obj=f"settings.{self.setting}[{name}][{target_name}]",
)
)
if len(kwargs) > 1:
messages.append(
checks.Warn(
"Alias contains unknown arguments",
hint="An alias entry must only provide a `name` value.",
obj=f"settings.{self.setting}[{name}]",
)
)
else:
try:
import_type(type_name)
except (ImportError, ValueError, AttributeError):
messages.append(
checks.Error(
f"Unable to import type `{type_name}`.",
hint="Check the type name in definition.",
obj=f"settings.{self.setting}[{name}]",
)
)
if not isinstance(kwargs, dict):
messages.append(
checks.Critical(
"Instance kwargs is not a dict.",
hint="Change kwargs definition to be a dict.",
obj=f"settings.{self.setting}[{name}]",
)
)
return messages
class NamedSingletonPluginFactory(SingletonFactoryMixin, NamedPluginFactory[PT_co]):
"""
:py:class:`NamedPluginFactory` that provides a single instance of an object.
This instance factory type is useful for instance types that only require
a single instance eg database connections, web service agents.
If your instance types are not thread safe it is recommended that the
:py:class:`ThreadLocalNamedSingletonPluginFactory` is used.
"""
class ThreadLocalNamedSingletonPluginFactory(
ThreadLocalSingletonFactoryMixin, NamedPluginFactory[PT_co]
):
"""
:py:class:`NamedPluginFactory` that provides a single instance of a plugin per
thread.
This instance factory type is useful for instance types that only require
a single instance eg database connections, web service agents and that are
not thread safe.
"""