new tip

index.md

@@ -133,6 +133,7 @@ This file gets generated by [this script](index.py).
 ## Designpatterns
 
 - [Composition over inheritance](notes/20240509111232.md)
+- [Depndency injection use case](notes/20240614172758.md)
 - [Repository pattern](notes/20240614165322.md)
 
 ## Developer

notes/20240614172758.md

@@ -0,0 +1,33 @@
+# Depndency injection use case
+
+Having an engine object in module scope can lead to accidentally using your dev database during tests, leading to unwanted data and potential bugs! 🐛😱
+
+One solution = _encapsulation_: inject the db string into your DB class. This way, you can use another isolated DB for testing 🔒📈
+
+```python
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, declarative_base
+
+DEFAULT_DB_STRING = "sqlite:///todo.db"
+
+Base = declarative_base()
+
+class DB:
+    def __init__(self, *, db_string=None):
+        if db_string is None:
+            db_string = DEFAULT_DB_STRING
+        self._engine = create_engine(db_string)
+        self._Session = sessionmaker(bind=self._engine)
+        self.session = self._Session()
+
+
+if __name__ == "__main__":
+    db = DB()
+    print(db._engine)  # Engine(sqlite:///todo.db)
+
+    # but you can Inject in-memory test DB in tests
+    test_db = DB(db_string="sqlite:///:memory:")
+    print(test_db._engine)  # Engine(sqlite:///:memory:)
+```
+
+#designpatterns