feat: Improve default postgres driver selection heuristics.

docs/source/postgres.rst

@@ -7,13 +7,13 @@ Postgres
    and `asyncpg` for async fixtures. If you want to use a different driver, you
    can configure the `drivername` field using the `pmr_postgres_config` fixture:
 
-   ```python
-   from pytest_mock_resources import PostgresConfig
+   .. code-block:: python
 
-   @pytest.fixture
-   def pmr_postgres_config():
-       PostgresConfig(drivername='postgresql+psycopg2')  # but whatever drivername you require.
-   ```
+      from pytest_mock_resources import PostgresConfig
+
+      @pytest.fixture(scope='session')
+      def pmr_postgres_config():
+          return PostgresConfig(drivername='postgresql+psycopg2')  # but whatever driver you require.
 
    Note however, that the `asyncpg` driver **only** works with the async fixture, and the
    `psycopg2` driver **only** works with the synchronous fixture. These are inherent

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "pytest-mock-resources"
-version = "2.10.1"
+version = "2.10.2"
 description = "A pytest plugin for easily instantiating reproducible mock resources."
 authors = [
     "Omar Khan <oakhan3@gmail.com>",

src/pytest_mock_resources/container/postgres.py

@@ -1,4 +1,5 @@
-from typing import ClassVar, Iterable
+from typing import ClassVar, Iterable, Optional
+from importlib.metadata import Distribution, PackageNotFoundError
 
 import sqlalchemy
 import sqlalchemy.exc
@@ -48,7 +49,7 @@ class PostgresConfig(DockerContainerConfig):
         "username": "user",
         "password": "password",
         "root_database": "dev",
-        "drivername": "postgresql+psycopg2",
+        "drivername": None,
     }
 
     @fallback
@@ -94,9 +95,7 @@ def check_fn(self):
 def get_sqlalchemy_engine(config, database_name, async_=False, autocommit=False, **engine_kwargs):
     # For backwards compatibility, our hardcoded default is psycopg2, and async fixtures
     # will not work with psycopg2, so we instead swap the default to the preferred async driver.
-    drivername = config.drivername
-    if async_ and drivername.endswith("psycopg2"):
-        drivername = drivername.replace("psycopg2", "asyncpg")
+    drivername = detect_driver(config.drivername, async_=async_)
 
     url = URL(
         drivername=drivername,
@@ -118,3 +117,23 @@ def get_sqlalchemy_engine(config, database_name, async_=False, autocommit=False,
         engine = sqlalchemy.create_engine(url, **engine_kwargs)
 
     return engine
+
+
+def detect_driver(drivername: Optional[str] = None, async_: bool = False) -> str:
+    if drivername:
+        return drivername
+
+    if any(Distribution.discover(name="psycopg")):
+        return "postgresql+psycopg"
+
+    if async_:
+        if any(Distribution.discover(name="asyncpg")):
+            return "postgresql+asyncpg"
+    else:
+        if any(Distribution.discover(name="psycopg2")):
+            return "postgresql+psycopg2"
+
+    raise ValueError(  # pragma: no cover
+        "No suitable driver found for Postgres. Please install `psycopg`, `psycopg2`, "
+        "`asyncpg`, or explicitly configure the `drivername=` field of `PostgresConfig`."
+    )