docs: testing section and override features (#54)

* Create testing section and description for override features

* Add sphinx-copybutton package

---------

Co-authored-by: ivan <ivan.belyaev@ailet.com>

docs/conf.py

@@ -10,6 +10,7 @@
 
 # -- General configuration
 extensions = [
+    "sphinx_copybutton",
     "sphinx.ext.duration",
     "sphinx.ext.doctest",
     "sphinx.ext.autodoc",

docs/index.md

@@ -26,6 +26,12 @@
     providers/collections
     providers/selector
     providers/object
+   
+.. toctree::
+    :maxdepth: 1
+    :caption: Testing
+    
+    testing/provider-overriding
 
 .. toctree::
     :maxdepth: 1

docs/requirements.txt

@@ -1,3 +1,4 @@
 sphinx==7.*
 sphinx-rtd-theme==2.*
 myst-parser
+sphinx-copybutton

docs/testing/provider-overriding.md

@@ -0,0 +1,107 @@
+# Provider overriding
+
+DI container provides, in addition to direct dependency injection, another very important functionality: 
+**dependencies or providers overriding**.
+
+Any provider registered with the container can be overridden. 
+This can help you replace objects with simple stubs, or with other objects.
+**Override affects all providers that use the overridden provider (_see example_)**.
+
+## Example
+
+```python
+from pydantic_settings import BaseSettings
+from sqlalchemy import create_engine, Engine, text
+from testcontainers.postgres import PostgresContainer
+from that_depends import BaseContainer, providers, Provide, inject
+
+
+class SomeSQLADao:
+    def __init__(self, *, sqla_engine: Engine):
+        self.engine = sqla_engine
+        self._connection = None
+
+    def __enter__(self):
+        self._connection = self.engine.connect()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self._connection.close()
+
+    def exec_query(self, query: str):
+        return self._connection.execute(text(query))
+
+
+class Settings(BaseSettings):
+    db_url: str = 'some_production_db_url'
+
+
+class DIContainer(BaseContainer):
+    settings = providers.Singleton(Settings)
+    sqla_engine = providers.Singleton(create_engine, settings.db_url)
+    some_sqla_dao = providers.Factory(SomeSQLADao, sqla_engine=sqla_engine)
+
+
+@inject
+def exec_query_example(some_sqla_dao=Provide[DIContainer.some_sqla_dao]):
+    with some_sqla_dao:
+        result = some_sqla_dao.exec_query('SELECT 234')
+
+    return next(result)
+
+
+def main():
+    pg_container = PostgresContainer(image='postgres:alpine3.19')
+    pg_container.start()
+    db_url = pg_container.get_connection_url()
+
+    """
+    We override only settings, but this override will also affect the 'sqla_engine' 
+    and 'some_sqla_dao' providers because the 'settings' provider is used by them!
+    """
+    local_testing_settings = Settings(db_url=db_url)
+    DIContainer.settings.override(local_testing_settings)
+
+    try:
+        result = exec_query_example()
+        assert result == (234,)
+    finally:
+        DIContainer.settings.reset_override()
+        pg_container.stop()
+
+
+if __name__ == '__main__':
+    main()
+
+```
+
+The example above shows how overriding a nested provider ('_settings_') 
+affects another provider ('_engine_' and '_some_sqla_dao_').
+
+## Override multiple providers
+
+The example above looked at overriding only one settings provider, 
+but the container also provides the ability to override 
+multiple providers at once with method ```override_providers```. 
+
+The code above could remain the same except that 
+the single provider override could be replaced with the following code:
+
+```python
+def main():
+    pg_container = PostgresContainer(image='postgres:alpine3.19')
+    pg_container.start()
+    db_url = pg_container.get_connection_url()
+
+    local_testing_settings = Settings(db_url=db_url)
+    providers_for_overriding = {
+        'settings': local_testing_settings,
+        # more values...
+    }
+    with DIContainer.override_providers(providers_for_overriding):
+        try:
+            result = exec_query_example()
+            assert result == (234,)
+        finally:
+            pg_container.stop()
+```