FormatDefault

str.format but with default values

Ideal for building a CLI program with format-able filepath arguments

Many existing string formatting tools use regex to find sections like %(param)s, but regex can be fiddly and is not as flexible as str.format when it comes to validating format spec.

Why reinvent the wheel when one can just pass an fstring-looking argument and allow Python (and this mini library) to do the heavy lifting, no regex required!

Example Usage

from FormatDefault import format_default

s = "Hello {x:#02x} {y} {y:#02x} {z}"
params = {"x": 100, "y": "hi"}

print(format_default(s, params))
# Hello 0x64 hi NA NA

import argparse
from FormatDefault import format_default

parser = argparse.ArgumentParser()
parser.add_argument("-o",
    default = "YouTube/{uploader}/{title} - {camera_type} - {id}.{ext}"
)
args = parser.parse_args()

video_data = {
    "id": "jNQXAC9IVRw", "title": "Me at the zoo",
    "uploader": "jawed", "ext": "webm",
}

print(format_default(args.o, video_data))
# YouTube/jawed/Me at the zoo - NA - jNQXAC9IVRw.webm

How it works

format_default(s, params, default = "NA") tries str.format in a while-loop with a copy of params.

format_default catches KeyErrors from missing parameters and the parameter name is added to params with the value default.

The values in params are wrapped in a wrapper class _DefaultWrapper before being passed to str.format. This wrapper class implements the __format__(self, fmt) dunder method to intercept the format specifiers (parts after the colon, eg "#02x" in "{foo = :#02x}").

_DefaultWrapper.__format__ catches ValueErrors from invalid format specifiers for its wrapped value and returns default in this case.