Implement instantiated services option.

This introduces an option to enable instantiated services while
maintaining backwards compatibility with the singleton pattern.

It also allows passing caller arguments into the `Endpoint` declaration.

Resolve #119. While I think the singleton pattern should be deprecated,
put under a feature flag, and discouraged, the first step is probably to
give instantiated services some field experience as an optional feature.

docs/advanced-usage.rst

@@ -113,3 +113,40 @@ This is useful for workflows where cookies or other information need to persist
 It's often more useful in logs to know which module initiated the code doing the logging.
 ``apiron`` allows for an existing logger object to be passed to an endpoint call using the ``logger`` argument
 so that logs will indicate the caller module rather than :mod:`apiron.client`.
+
+
+**********************
+Instantiated endpoints
+**********************
+
+While the other documented usage patterns implement the singleton pattern, you may wish to use instantiated
+services for reasons such as those mentioned `in this issue <https://github.com/ithaka/apiron/issues/119>`_.
+
+This feature can be enabled by setting ``APIRON_INSTANTIATED_SERVICES=1`` either in the shell in which your
+program runs or early in the entrypoint to your program, prior to the evaluation of your service classes.
+
+Endpoints should then be called on instances of ``Service`` subclasses, rather than the class itself:
+
+As an additional benefit, arguments passed into the constructor will be passed through to the endpoint as arguments
+to the caller that forms the request. See ``aprion.client.call`` for all the available options.
+
+.. code-block:: python
+
+    import os
+
+    import requests
+
+    from apiron import JsonEndpoint, Service
+
+    os.environ['APIRON_INSTANTIATED_SERVICES'] = "1"
+
+
+    class GitHub(Service):
+        domain = 'https://api.github.com'
+        user = JsonEndpoint(path='/users/{username}')
+        repo = JsonEndpoint(path='/repos/{org}/{repo}')
+
+
+    service = GitHub(session=requests.Session())
+    response = service.user(username='defunkt')
+    print(response)

src/apiron/endpoint/endpoint.py

@@ -2,21 +2,8 @@
 
 import logging
 import string
-import sys
 import warnings
-from functools import partial, update_wrapper
-from typing import Optional, Any, Callable, Dict, Iterable, List, TypeVar, Union, TYPE_CHECKING
-
-if TYPE_CHECKING:
-    if sys.version_info >= (3, 10):
-        from typing import Concatenate, ParamSpec
-    else:
-        from typing_extensions import Concatenate, ParamSpec
-
-    from apiron.service import Service
-
-    P = ParamSpec("P")
-    R = TypeVar("R")
+from typing import Optional, Iterable, Any, Dict, List, Union
 
 import requests
 
@@ -27,26 +14,13 @@
 LOGGER = logging.getLogger(__name__)
 
 
-def _create_caller(
-    call_fn: Callable["Concatenate[Service, Endpoint, P]", "R"],
-    instance: Any,
-    owner: Any,
-) -> Callable["P", "R"]:
-    return partial(call_fn, instance, owner)
-
-
 class Endpoint:
     """
     A basic service endpoint that responds with the default ``Content-Type`` for that endpoint
     """
 
-    def __get__(self, instance, owner):
-        caller = _create_caller(client.call, owner, self)
-        update_wrapper(caller, client.call)
-        return caller
-
-    def __call__(self):
-        raise TypeError("Endpoints are only callable in conjunction with a Service class.")
+    def __call__(self, service, *args, **kwargs):
+        return client.call(service, self, *args, **{**self.kwargs, **service._kwargs, **kwargs})
 
     def __init__(
         self,
@@ -55,6 +29,7 @@ def __init__(
         default_params: Optional[Dict[str, Any]] = None,
         required_params: Optional[Iterable[str]] = None,
         return_raw_response_object: bool = False,
+        **kwargs,
     ):
         """
         :param str path:
@@ -72,8 +47,11 @@ def __init__(
             Whether to return a :class:`requests.Response` object or call :func:`format_response` on it first.
             This can be overridden when calling the endpoint.
             (Default ``False``)
+        :param kwargs:
+            Default arguments to pass through to `apiron.client.call`.
         """
         self.default_method = default_method
+        self.kwargs = kwargs
 
         if "?" in path:
             warnings.warn(

src/apiron/endpoint/stub.py

@@ -11,8 +11,8 @@ class StubEndpoint(Endpoint):
     before the endpoint is complete.
     """
 
-    def __get__(self, instance, owner):
-        return self.stub_response
+    def __call__(self, service, *args, **kwargs):
+        return self.stub_response(*args, **kwargs)
 
     def __init__(self, stub_response: Optional[Any] = None, **kwargs):
         """

src/apiron/service/base.py

@@ -1,3 +1,5 @@
+import os
+import types
 from typing import Any, Dict, List, Set
 
 from apiron import Endpoint
@@ -8,24 +10,70 @@ class ServiceMeta(type):
     def required_headers(cls) -> Dict[str, str]:
         return cls().required_headers
 
-    @property
-    def endpoints(cls) -> Set[Endpoint]:
-        return {attr for attr_name, attr in cls.__dict__.items() if isinstance(attr, Endpoint)}
+    @classmethod
+    def _instantiated_services(cls) -> bool:
+        setting_variable = "APIRON_INSTANTIATED_SERVICES"
+        false_values = ["0", "false"]
+        true_values = ["1", "true"]
+        environment_setting = os.getenv(setting_variable, "false").lower()
+        if environment_setting in false_values:
+            return False
+        elif environment_setting in true_values:
+            return True
+
+        setting_values = false_values + true_values
+        raise ValueError(
+            f'Invalid {setting_variable}, "{environment_setting}"\n',
+            f"{setting_variable} must be one of {setting_values}\n",
+        )
 
     def __str__(cls) -> str:
         return str(cls())
 
     def __repr__(cls) -> str:
         return repr(cls())
 
+    def __new__(cls, name, bases, namespace, **kwargs):
+        klass = super().__new__(cls, name, bases, namespace, **kwargs)
+
+        # Behave as a normal class if instantiated services are enabled or if
+        # this is an apiron base class.
+        if cls._instantiated_services() or klass.__module__.split(".")[:2] == ["apiron", "service"]:
+            return klass
+
+        # Singleton class.
+        if not hasattr(klass, "_instance"):
+            klass._instance = klass()
+
+        # Mask declared Endpoints with bound instance methods. (singleton)
+        for k, v in namespace.items():
+            if isinstance(v, Endpoint):
+                setattr(klass, k, types.MethodType(v, klass._instance))
+
+        return klass._instance
+
 
 class ServiceBase(metaclass=ServiceMeta):
     required_headers: Dict[str, Any] = {}
     auth = ()
     proxies: Dict[str, str] = {}
 
-    @classmethod
-    def get_hosts(cls) -> List[str]:
+    def __setattr__(self, name, value):
+        """Transform assigned Endpoints into bound instance methods."""
+        if isinstance(value, Endpoint):
+            value = types.MethodType(value, self)
+        super().__setattr__(name, value)
+
+    @property
+    def endpoints(self) -> Set[Endpoint]:
+        endpoints = set()
+        for attr in self.__dict__.values():
+            func = getattr(attr, "__func__", None)
+            if isinstance(func, Endpoint):
+                endpoints.add(func)
+        return endpoints
+
+    def get_hosts(self) -> List[str]:
         """
         The fully-qualified hostnames that correspond to this service.
         These are often determined by asking a load balancer or service discovery mechanism.
@@ -35,7 +83,7 @@ def get_hosts(cls) -> List[str]:
         :rtype:
             list
         """
-        return []
+        return [self.domain]
 
 
 class Service(ServiceBase):
@@ -47,21 +95,21 @@ class Service(ServiceBase):
 
     domain: str
 
-    @classmethod
-    def get_hosts(cls) -> List[str]:
-        """
-        The fully-qualified hostnames that correspond to this service.
-        These are often determined by asking a load balancer or service discovery mechanism.
+    @property
+    def domain(self):
+        return self._domain if self._domain else self.__class__.domain
 
-        :return:
-            The hostname strings corresponding to this service
-        :rtype:
-            list
-        """
-        return [cls.domain]
+    def __init__(self, domain=None, **kwargs):
+        self._domain = domain
+        self._kwargs = kwargs
+
+        # Mask declared Endpoints with bound instance methods. (instantiated)
+        for name, attr in self.__class__.__dict__.items():
+            if isinstance(attr, Endpoint):
+                setattr(self, name, types.MethodType(attr, self))
 
     def __str__(self) -> str:
-        return self.__class__.domain
+        return self.domain
 
     def __repr__(self) -> str:
-        return f"{self.__class__.__name__}(domain={self.__class__.domain})"
+        return f"{self.__class__.__name__}(domain={self.domain})"

tests/conftest.py

@@ -0,0 +1,38 @@
+import os
+
+import pytest
+
+import apiron
+
+
+def instantiated_service(returntype="instance"):
+    os.environ["APIRON_INSTANTIATED_SERVICES"] = "1"
+
+    class SomeService(apiron.Service):
+        pass
+
+    if returntype == "instance":
+        return SomeService(domain="http://foo.com")
+    elif returntype == "class":
+        return SomeService
+
+    raise ValueError('Expected "returntype" value to be "instance" or "class".')
+
+
+def singleton_service():
+    os.environ["APIRON_INSTANTIATED_SERVICES"] = "0"
+
+    class SomeService(apiron.Service):
+        domain = "http://foo.com"
+
+    return SomeService
+
+
+@pytest.fixture(scope="function", params=["singleton", "instance"])
+def service(request):
+    if request.param == "singleton":
+        yield singleton_service()
+    elif request.param == "instance":
+        yield instantiated_service()
+    else:
+        raise ValueError(f'unknown service type "{request.param}"')

tests/service/test_base.py

@@ -1,20 +1,7 @@
-import pytest
-
-from apiron import Endpoint, Service, ServiceBase
-
-
-@pytest.fixture
-def service():
-    class SomeService(Service):
-        domain = "http://foo.com"
-
-    return SomeService
+from apiron import Endpoint
 
 
 class TestServiceBase:
-    def test_get_hosts_returns_empty_list_by_default(self):
-        assert [] == ServiceBase.get_hosts()
-
     def test_required_headers_returns_empty_dict_by_default(self, service):
         assert {} == service.required_headers
 

tests/service/test_instantiated.py

@@ -0,0 +1,35 @@
+import os
+
+import pytest
+
+from apiron.service.base import ServiceMeta
+
+from .. import conftest
+
+
+class TestInstantiatedServices:
+    @pytest.mark.parametrize("value,result", [("0", False), ("false", False), ("1", True), ("true", True)])
+    def test_instantiated_services_variable_true(self, value, result):
+        os.environ["APIRON_INSTANTIATED_SERVICES"] = value
+
+        assert ServiceMeta._instantiated_services() is result
+
+    @pytest.mark.parametrize("value", ["", "YES"])
+    def test_instantiated_services_variable_other(self, value):
+        os.environ["APIRON_INSTANTIATED_SERVICES"] = value
+
+        with pytest.raises(ValueError, match="Invalid"):
+            ServiceMeta._instantiated_services()
+
+    def test_singleton_constructor_arguments(self):
+        """Singleton services do not accept arguments."""
+        service = conftest.singleton_service()
+
+        with pytest.raises(TypeError, match="object is not callable"):
+            service(foo="bar")
+
+    def test_instantiated_services_constructor_arguments(self):
+        """Instantiated services accept arguments."""
+        service = conftest.instantiated_service(returntype="class")
+
+        service(foo="bar")

tests/test_endpoint.py

@@ -6,14 +6,6 @@
 import apiron
 
 
-@pytest.fixture
-def service():
-    class SomeService(apiron.Service):
-        domain = "http://foo.com"
-
-    return SomeService
-
-
 @pytest.fixture
 def stub_function():
     def stub_response(**kwargs):