Python

Use pipx to install environment management tools and scripts. (Execute binaries installed in isolated virtual environments.)
Use Poetry to manage packaging and dependencies.

Hey, there's already a Hitchhiker's Guide to Python! This is quite comprehensive on the Python development basics.

GIL

GIL required to run bytecode, but not when waiting on I/O - can switch to different thread. CPython has macros to release and re-aquire GIL which can be used if extension does not need to run python bytecode/do any kind of ref-count changes (e.g. sleep, read/write files, network socket, numpy, image processing(?)). Cannot share GIL in CPU-bound code. CPU-bound "check" every 100 ticks to allow switching. Note: I/O doesn't often block unless you are flushing - OS buffers. Note: Threads/Processes result in CPU context switches, while async does not.

Pure CPU: Process <---> Threads <---> Async: Pure I/O (Many "Connections"/Slow I/O).

Async I/O clearly delineates the context switch locations, in theory.

Processes	Threads	Async I/O
		No CPU Context Switches
	Shared memory can result in race conditions.	No race conditions.. usually.
		No dead locks.. usually.
Costly.	Each thread has its own stack.	Shared Stack. Uses an executor pool to run sync tasks in a background thread (this uses additional resources)

Python 2 -> 3: Strings/Bytes, Print, Super() - new style classes, division.

Linting

black (how-to)
flake8
- pycodestyle (formerly pep8)
- pyflakes
- mccabe
pylint
bandit/dodgy/safety(pipenv)
prospector
isort
Layer Linter (not tried)
pydocstyle
mypy
vulture
pyroma for libraries (setup.py)

Libraries

passlib - high-level secrets library
pysnooper - line-by-line print debugging
boltons - "boltons should be buitins" (stdlib additions)
[freezegun]

Pytest

See my cheat sheet.

Useful plugins:

pytest-randomly
pytest-datadir
pytest-bdd

Stop network calls:

import socket
def stop_network_calls(monkeypatch)
    def _socket(*_, **__):
        raise Exception()
    monkeypatch.setattr(socket, "socket", _socket)

Randomise time-zones:

import os, random, pytest
from _pytest.monkeypatch import MonkeyPatch

@pytest.fixture(scope="session")
def monkeysession():
    mpatch = MonkeyPatch()
    yield mpatch
    mpatch.undo()

@pytest.fixture(autouse=True, scope="session")
def randomise_timezone(monkeysession):
    monkeysession.setenv("TZ", random.choice(["Europe/London"]))

SQLAlchemy

Remember to index on ForeignKeys. Postgres does not do this automatically. MySQL always does this.
Remember to set onupdate and ondelete cascades on ForeignKeys.

To prevent spurious joins: (this cannot be used with NULLable outer joins)

session.query(Model).options(
    sa.orm.joinedload("child").joinedload("grandchild"),
    sa.orm.raiseload("*"),
)

Pandas

Understanding SettingWithCopyWarning.

Raising it as an error (setup.cfg)

[tool:pytest]
filterwarnings =
    error::pandas.core.common.SettingWithCopyWarning
    error::FutureWarning

Gotchas

Probably the last one you will learn:

x = [1,2,3,4]
lambdas = [
   lambda: x[i]
   for i in range(4)
]

[f() for f in lambdas]

The Programmer's Compendium