use sql catalog for test and add doc

Author: Jack Ye

mkdocs/docs/configuration.md

@@ -139,7 +139,13 @@ For the FileIO there are several configuration options available:
 
 ## Catalogs
 
-PyIceberg currently has native support for REST, SQL, Hive, Glue and DynamoDB.
+PyIceberg currently has native catalog type support for REST, SQL, Hive, Glue and DynamoDB.
+You can also set the catalog implementation explicitly:
+
+| Key             | Example                      | Description                                                                                      |
+| --------------- | ---------------------------- | ------------------------------------------------------------------------------------------------ |
+| type            | rest                         | Type of catalog, one of `rest`, `sql`, `hive`, `glue`, `dymamodb`. Default to `rest`             |
+| py-catalog-impl | mypackage.mymodule.MyCatalog | Sets the catalog explicitly to an implementation, and will fail explicitly if it can't be loaded |
 
 There are three ways to pass in configuration:
 
@@ -379,6 +385,18 @@ catalog:
 
 <!-- prettier-ignore-end -->
 
+### Custom Catalog Implementations
+
+If you want to load any custom catalog implementation, you can set catalog configurations like the following:
+
+```yaml
+catalog:
+  default:
+    py-catalog-impl: mypackage.mymodule.MyCatalog
+    custom-key1: value1
+    custom-key2: value2
+```
+
 ## Unified AWS Credentials
 
 You can explicitly set the AWS credentials for both Glue/DynamoDB Catalog and S3 FileIO by configuring `client.*` properties. For example:

tests/catalog/test_glue.py

@@ -22,7 +22,6 @@
 import pytest
 from moto import mock_aws
 
-from pyiceberg.catalog import load_catalog
 from pyiceberg.catalog.glue import GlueCatalog
 from pyiceberg.exceptions import (
     NamespaceAlreadyExistsError,
@@ -47,23 +46,6 @@
 )
 
 
-@mock_aws
-def test_load_catalog_from_impl() -> None:
-    assert isinstance(
-        load_catalog(
-            "catalog",
-            **{
-                "py-catalog-impl": "pyiceberg.catalog.glue.GlueCatalog",
-                "glue.region_name": "us-east-1",
-                "glue.aws_access_key_id": "access-key",
-                "glue.aws_secret_access_key": "secret-key",
-                "glue.aws_session_token": "session-token",
-            },
-        ),
-        GlueCatalog,
-    )
-
-
 @mock_aws
 def test_create_table_with_database_location(
     _glue: boto3.client,

tests/catalog/test_sql.py

@@ -27,6 +27,7 @@
 
 from pyiceberg.catalog import (
     Catalog,
+    load_catalog,
 )
 from pyiceberg.catalog.sql import DEFAULT_ECHO_VALUE, DEFAULT_POOL_PRE_PING_VALUE, SqlCatalog
 from pyiceberg.exceptions import (
@@ -210,6 +211,17 @@ def test_creation_with_pool_pre_ping_parameter(catalog_name: str, warehouse: Pat
         )
 
 
+def test_creation_from_impl(catalog_name: str, warehouse: Path) -> None:
+    assert isinstance(
+        load_catalog(
+            catalog_name,
+            uri=f"sqlite:////{warehouse}/sql-catalog.db",
+            warehouse=f"file://{warehouse}",
+        ),
+        SqlCatalog,
+    )
+
+
 @pytest.mark.parametrize(
     "catalog",
     [