Exploring Py 3.8 Walrus + F-string Tweet By Raymond Hettinger

walrus_fstring_example.py

#!/usr/bin/env python3
"""
Requires Python 3.8

Expanding on a Tweet from RH using walrus and f-strings

https://twitter.com/raymondh/status/1153085050650423296

"""
import logging
import os
import sys
from math import radians, sin

log = logging.getLogger(__name__)

FUNCS = []


def register(f):
    FUNCS.append(f)

    def w(*args, **kwargs):
        return f(*args, **kwargs)
    return w


@register
def example_rh_tweet():
    """Raw example from RH to show usage of := (walrus) with f-strings"""
    # Note that \N means look up the unicode by id here. e.g., \N{Greek Capital Letter Theta}
    # https://en.wikipedia.org/wiki/List_of_Unicode_characters
    n = 4
    for angle in range(4):
        log.info(f'{angle=}\N{degree sign} {(theta:=radians(angle))=:.3f} {sin(theta)=:.3f}')


@register
def example_02():
    """
    Notice this also works outside of a loop.
    """
    angle = 90.0
    log.info(f'{angle=}\N{degree sign} {(theta:=radians(angle))=:.3f} {sin(theta)=:.3f}')
    log.info(theta)


@register
def example_03():
    """
    Calling the raw function call outside of the f-string.

    Note, the parentheses are extremely important
    """

    angle = 90.0

    # This is a SyntaxError
    # theta := radians(angle)

    # This is valid python 3.8
    (theta := radians(angle))

    log.info(f"Theta={theta}")
    log.info(f'{angle=}\N{degree sign} {theta=:.3f} {sin(theta)=:.3f}')
    log.info(theta)


@register
def example_04():
    """
    Values defined within the f-string don't need to be used within the f-string.
    """
    angle = 90.0
    log.info(f'{angle=}\N{degree sign} {(theta:=radians(angle))=:.3f} {(sin_theta:=sin(theta))=:.3f}')
    log.info(f'{theta=}')
    log.info(f'{sin_theta=}')


def run_func(func):
    print("Running func {}".format(func.__name__))
    return func()


def run_main():
    _ = logging.basicConfig(level=logging.INFO, stream=sys.stdout)
    _ = list(map(run_func, FUNCS))
    return 0


if __name__ == '__main__':
    sys.exit(run_main())

for anyone who was confused, the "reverse walrus" (=:) is really just a combination of two operators or language features. f-string formatting[1] (float in this case, theta:.3f) and the new self documenting expression[2] (theta=).

for example log.info(f'{angle=}\N{degree sign} {theta=:.3f} {sin(theta)=:.3f}') from line 65 can be rewritten as

log.info(f'angle={angle}\N{degree sign} theta={theta:.3f} sin(theta)={sin(theta):.3f}')

top/bottom comparison

log.info(f'{angle=}\N{degree sign} {theta=:.3f} {sin(theta)=:.3f}')
log.info(f'angle={angle}\N{degree sign} theta={theta:.3f} sin(theta)={sin(theta):.3f}')

[1] https://www.python.org/dev/peps/pep-0498/#format-specifiers

[2] https://docs.python.org/3/whatsnew/3.8.html#f-strings-support-for-self-documenting-expressions-and-debugging

[3] BONUS: https://docs.python.org/3.3/howto/unicode.html#the-string-type \N lookup examples from python docs