Fix a few issues in the introduction (#580)

* Fix a statement about coupling and cohesion that is not always true

https://enterprisecraftsmanship.com/posts/cohesion-coupling-difference/#_types_of_code_from_a_cohesion_and_coupling_perspective

* Fix a typing issue in an example

`ApiClient` expects timeout to be an integer (based on a type hint), but `os.getenv` returns a string when `TIMEOUT` is set.

* Specify the `None` return type where it is missed

* Fix typing issues in some other places

* Edit a statement about coupling and cohesion

Co-authored-by: Roman Mogylatov <rmogilatov@gmail.com>

Co-authored-by: Roman Mogylatov <rmogilatov@gmail.com>

README.rst

@@ -100,7 +100,7 @@ Key features of the ``Dependency Injector``:
 
 
    @inject
-   def main(service: Service = Provide[Container.service]):
+   def main(service: Service = Provide[Container.service]) -> None:
        ...
 
 

docs/index.rst

@@ -106,7 +106,7 @@ Key features of the ``Dependency Injector``:
 
 
    @inject
-   def main(service: Service = Provide[Container.service]):
+   def main(service: Service = Provide[Container.service]) -> None:
        ...
 
 

docs/introduction/di_in_python.rst

@@ -49,7 +49,8 @@ Coupling and cohesion are about how tough the components are tied.
 - **High cohesion**. High cohesion is like using the screws. Very easy to disassemble and
   assemble back or assemble a different way. It is an opposite to high coupling.
 
-When the cohesion is high the coupling is low.
+Cohesion often correlates with coupling. Higher cohesion usually leads to lower coupling, and vice
+versa.
 
 Low coupling brings a flexibility. Your code becomes easier to change and test.
 
@@ -66,14 +67,14 @@ Before:
 
    class ApiClient:
 
-       def __init__(self):
+       def __init__(self) -> None:
            self.api_key = os.getenv("API_KEY")  # <-- dependency
-           self.timeout = os.getenv("TIMEOUT")  # <-- dependency
+           self.timeout = int(os.getenv("TIMEOUT"))  # <-- dependency
 
 
    class Service:
 
-       def __init__(self):
+       def __init__(self) -> None:
            self.api_client = ApiClient()  # <-- dependency
 
 
@@ -94,18 +95,18 @@ After:
 
    class ApiClient:
 
-       def __init__(self, api_key: str, timeout: int):
+       def __init__(self, api_key: str, timeout: int) -> None:
            self.api_key = api_key  # <-- dependency is injected
            self.timeout = timeout  # <-- dependency is injected
 
 
    class Service:
 
-       def __init__(self, api_client: ApiClient):
+       def __init__(self, api_client: ApiClient) -> None:
            self.api_client = api_client  # <-- dependency is injected
 
 
-   def main(service: Service):  # <-- dependency is injected
+   def main(service: Service) -> None:  # <-- dependency is injected
        ...
 
 
@@ -114,7 +115,7 @@ After:
            service=Service(
                api_client=ApiClient(
                    api_key=os.getenv("API_KEY"),
-                   timeout=os.getenv("TIMEOUT"),
+                   timeout=int(os.getenv("TIMEOUT")),
                ),
            ),
        )
@@ -137,7 +138,7 @@ Now you need to assemble and inject the objects like this:
        service=Service(
            api_client=ApiClient(
                api_key=os.getenv("API_KEY"),
-               timeout=os.getenv("TIMEOUT"),
+               timeout=int(os.getenv("TIMEOUT")),
            ),
        ),
    )
@@ -182,7 +183,7 @@ the dependency.
 
 
    @inject
-   def main(service: Service = Provide[Container.service]):
+   def main(service: Service = Provide[Container.service]) -> None:
        ...
 
 

examples/demo/after.py

@@ -3,18 +3,18 @@ import os
 
 class ApiClient:
 
-    def __init__(self, api_key: str, timeout: int):
+    def __init__(self, api_key: str, timeout: int) -> None:
         self.api_key = api_key  # <-- dependency is injected
         self.timeout = timeout  # <-- dependency is injected
 
 
 class Service:
 
-    def __init__(self, api_client: ApiClient):
+    def __init__(self, api_client: ApiClient) -> None:
         self.api_client = api_client  # <-- dependency is injected
 
 
-def main(service: Service):  # <-- dependency is injected
+def main(service: Service) -> None:  # <-- dependency is injected
     ...
 
 
@@ -23,7 +23,7 @@ if __name__ == "__main__":
         service=Service(
             api_client=ApiClient(
                 api_key=os.getenv("API_KEY"),
-                timeout=os.getenv("TIMEOUT"),
+                timeout=int(os.getenv("TIMEOUT")),
             ),
         ),
     )

examples/demo/with_di.py

@@ -23,7 +23,7 @@ class Container(containers.DeclarativeContainer):
 
 
 @inject
-def main(service: Service = Provide[Container.service]):
+def main(service: Service = Provide[Container.service]) -> None:
     ...