Python for JS Devs · Reference

Glossary

The nomenclature of this course. Adhered to in every lesson.

Runtime & internals

CPython

The reference Python implementation, written in C — what you run via python/uv run. The analogue of V8 for Node.

bytecode

The intermediate instructions your source compiles to, executed by the interpreter. Inspect with dis; cached as .pyc in __pycache__/.

ceval loop

CPython's bytecode interpreter — the core execution loop. No optimizing JIT by default (unlike V8's TurboFan).

specializing interpreter

PEP 659 (Python 3.11+): the interpreter swaps in type-specialized bytecodes with inline caches at runtime — V8's inline-caching idea without a full JIT.

GIL (Global Interpreter Lock)

A lock ensuring only one thread runs Python bytecode at a time. Blocks CPU parallelism via threads (use multiprocessing); released during I/O. Optional in 3.14's free-threaded build.

reference counting

CPython's primary memory management: each object tracks references; freed immediately when the count hits 0. A separate cyclic GC reclaims reference cycles. (V8 uses a tracing generational GC instead.)

PyObject

The C struct behind every Python value (even an int) — carries a type pointer and refcount. Attribute access goes through a dict, not V8-style hidden-class slots.

PyPy

Alternative Python implementation with a real tracing JIT — often much faster than CPython (like SpiderMonkey/JSC vs V8).

Language & tooling

uv

Fast all-in-one project/package/Python-version manager (npm + nvm + npx in one). Our standard tool.

venv (virtual environment)

Per-project isolated folder (.venv/) holding that project's dependencies and its own interpreter — Python's node_modules. With uv you never manually activate it.

pyproject.toml

Project + dependency manifest (Python's package.json).

ruff

Fast linter + formatter from Astral. Our standard.

REPL

Interactive Python shell (uv run python) — the fastest feedback loop.

module / package

A .py file is a module; a folder of modules is a package. Imported with dotted paths (app.routers.users).

dunder method

A "double underscore" special method (__init__, __repr__) that hooks into language features.

type hint

An annotation like x: int. Advisory at runtime (checked by tools), except Pydantic/FastAPI read them to validate.

comprehension

One-expression collection builder: [f(x) for x in xs if g(x)]. Replaces most map/filter.

dataclass

@dataclass-decorated class that auto-generates __init__/__repr__/__eq__ — a typed struct.

coroutine

The object returned by calling an async def function. Lazy — does nothing until awaited.

event loop

The single-threaded scheduler running async tasks. Blocking it freezes all concurrency.

async generator

An async def with yield — a lazy async iterator; the producer behind streaming responses.

Web & FastAPI

FastAPI

Type-hint-driven async web framework. Reads your hints to validate requests, serialize responses, and generate docs.

uvicorn

The ASGI server that runs a FastAPI app (its app.listen).

Pydantic

Runtime validation/serialization library (like Zod). A BaseModel validates data against type hints. v2 uses .model_dump().

pydantic-settings

Loads env vars into a typed, validated Settings object; fails fast if required values are missing.

path / query / body param

FastAPI infers a param's source: named in the route → path; scalar → query; Pydantic model → body.

response_model

Declares and enforces a route's output shape; strips any field not on the model (e.g. secrets).

dependency injection (Depends)

A route declares what it needs as a parameter; FastAPI supplies it. Composable, typed, testable middleware.

HTTPException

Raise it to abort a handler and return an error response with the right status code.

APIRouter

Groups related routes in a module, mounted via include_router (Express's Router).

blocking-I/O trap

Putting a synchronous blocking call inside an async def route — freezes the event loop. Use def (threadpooled) or an async client.

SQLAlchemy 2.0

Standard ORM; modern typed style uses DeclarativeBase, Mapped[], mapped_column, and select().

Alembic

SQLAlchemy's migration tool (the Prisma-migrate / django-migrate equivalent).

StreamingResponse

FastAPI response that streams an (async) generator's chunks to the client — used for token streaming and SSE.

SSE (Server-Sent Events)

One-way server→client stream over HTTP; each frame is data: ...\n\n, media type text/event-stream.