Python

Local setup

Install pyenv and pyenv-virtualenv. Respectively for Python version management and virtualenvs management.

pyenv-virtualenv uses python -m venv (Python 3.3+) if it is available and the virtualenv command if not

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On Mac

brew install pyenv-virtualenv

On Linux (Debian)

sudo apt-get update; sudo apt-get install make build-essential libssl-dev zlib1g-dev \
  libbz2-dev libreadline-dev libsqlite3-dev wget curl llvm \
  libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev

curl https://pyenv.run | bash

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echo '''
# pyenv-virtualenv
# see https://github.com/pyenv/pyenv-virtualenv
export PYENV_ROOT=$HOME/.pyenv
export PATH=$PATH:$PYENV_ROOT/bin
eval "$(pyenv init --path)"
eval "$(pyenv init -)"
eval "$(pyenv virtualenv-init -)"
''' >> ~/.zshrc

source ~/.zshrc

Configure a Python 3 virtualenv

pyenv install --list | grep 3.9 | less # list possible python versions

pyenv install 3.9.6 # install a python version
pyenv virtualenv 3.9.6 python3 # create a new virtualenv
pyenv activate python3 # activate virtualenv

Other useful pyenv commands:

pyenv deactivate # deactivate current virtualenv

pyenv versions # list installed python versions and available virtualenvs
pyenv which python # see path to python executable
pyenv version-name # see current virtualenv

pyenv uninstall python3 # remove virtualenv / python version

Generate a .python-version file

pyenv version-name > .python-version 

.python-version is then used by pyenv-virtualenv to automatically switch to correct virtualenv.

Generate a requirements.txt file

pyenv virtualenv 3.9.6 [VENV_NAME] # create a new virtualenv
pip install [DEPENDENCY_1] [...] # install dependencies
pip freeze > requirements.txt # freeze requirements

Install dependencies from a requirements.txt file

pip install -r requirements.txt

Containerized Python development

• run-python-script
• Read the Docker blog post series

Docs

• The Python Language Reference
• The Python Standard Library

Package registry

• PyPi

Snippets

#!/usr/bin/env python

Consult object help

help(set)

Parse arguments

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('-v', '--verbose', action='store_true')
parser.add_argument('-i', '--requests-interval', type=float, default=5)
args = parser.parse_args()

print(args.verbose, args.requests_interval)

Reference: - https://docs.python.org/3/howto/argparse.html - https://docs.python.org/3/library/argparse.html

Performance counter

time.perf_counter()

import time
start = time.perf_counter()
# ...
end = time.perf_counter()
print('timing: {time}'.format(time=(end - start)))

Reference count

import ctypes
def ref_count(address):
    return ctypes.c_long.from_address(address).value

Interning

a = 10
b = int('1010', 2)
print(hex(id(a)))
print(hex(id(b)))
a is b # True, in [-5, 256]

a = 500
b = 500
a is b # False

String interning: don't do this unless optimization is needed

import sys
a = 'foo'
b = 'foo'
a is b # True, looks like an identifier so gets interned... but don't count on it

a = 'foo bar'
b = 'foo bar'
a is b # False

a = sys.intern('foo bar')
b = sys.intern('foo bar')
a is b # True

Peephole

def my_func():
    a = 24 * 60
    b = 'foo'
    c = 'foo' * 2
    d = ['foo', 'bar']
    e = ('foo', 'bar')
    f = {'foo', 'bar'}

print(my_func.__code__.co_consts) 
# (None, 24, 60, 'foo', 2, 'bar', 1440, 'foofoo', ('foo', 'bar'))

Unpacking zen

Remove list duplicates

a = [1, 1, 2, 2, 3, 3]
*a, = {*a} # a = [1, 2, 3]

Overwrite dictionary values

a = {'foo': 'bar', 'baz': 'qux'}
b = {'foo': 'quux', 'quuz': 'courge'}
b = {**a, **b} # b = {'foo': 'quux', 'baz': 'qux', 'quuz': 'courge'}

Nested unpacking

a, *b, (c, d, e) = [1, 2, 3, 'XYZ']
# a = 1, b = [2, 3], c = 'X', d = 'Y', e = 'Z'