Python webapps for Java Devs

Unlike Java, Python doesn't have mutlithreading built in. In The Quick Python Book, we read: "Python doesn’t use multiple processors well... the standard implementation of Python is not designed to use multiple cores. This is due to a feature called the global interpreter lock, or GIL... if you need concurrency out of the box, Python may not be for you." Multithreaded code is not a first class citizen in Python unlike Java so things are somewhat complicated. 

If you're running a FastAPI application, you're probably using Docker. But if you want to debug it on your laptop, you might need to run it by hand with uvicorn. A similar technique can be employed with Flask [SO].

[Uvicorn is an ASGI compliant web server. ASGI is the Asynchronous Server Gateway Interface, "a standard interface between async-capable Python web servers, frameworks, and applications".]

Uvicorn allows hot deployment of code, much like Tomcat. You can run it with something like:

env $(grep -v "#" .env | xargs) poetry run uvicorn src.api.main:app --reload

where your file src.api.main has a field called app that's a fastapi.FastAPI.

[Poetry is a dependency management tool that's a bit like SBT in that it's interactive. You start it off with poetry shell]

Another web framework supporting another standard (WSGI, "a specification that describes how a web server communicates with web applications") is Gunicorn. This is a pre-fork framework, that is to say, it is the layer above a call to a POSIX fork kernel call.

Simple Python webapps often use Alembic for DB creation and migration. To get yourself going, you call

alembic init db

to create a directory called 'db' of all the revisions, tell the autogenerated env.py files where to find the classes that represent tables etc, run 

alembic revision --autogenerate -m "snapshot"

(where snapshot is essentially a comment) and you'll find a file somewhere under db that contains Python code to create SQL tables. Now, just run it with 

alembic upgrade head

and you'll see the files in your database if you've correcly populated your .env file with a DATABASE_URL. (DotEnv seems to be the standard Python way of reading properties files or accessing environment variables if it can't find what it's looking for.) 

There's more to Alembic than that but you get the idea. It's like Java's Flyway.

It uses the SQLAlchemy library that is Python's answer to Java's Hibernate. I was pleasantly surprised to see that I had to call session.flush() to populate my POJO-equivalents with keys generated by the DB. Takes me back to the mid-noughties :)

For streaming, Python uses the smart-open library. Think of this as the Pythonic equivalent of FS2.

Finally, there's a Python equivalent of Java's Lombok that auto-generates getters and setters. It's the DataClasses libary.