app.py

from flask import Flask, render_template, request

from conversion_functions import conversion_functions
from utils import format_result

app = Flask(__name__)


@app.route("/", defaults={"unit": "length"})
@app.route("/<unit>", methods=["GET", "POST"])
def converter(unit):
    target_template = f"{unit}.html"

    if request.method == "POST":
        try:
            value = float(request.form[unit])
        except ValueError:
            return render_template(target_template, unit=unit, error="Invalid value")

        convert_from = request.form["convert-from"]
        convert_to = request.form["convert-to"]

        if convert_from == convert_to:
            return render_template(
                target_template,
                unit=unit,
                result=format_result(value),
                value=value,
                convert_from=convert_from,
                convert_to=convert_to,
            )

        conversion_function = conversion_functions[
            f"convert_{convert_from.replace('-', '_')}"
        ]
        result = format_result(conversion_function(value, convert_to))

        return render_template(
            target_template,
            unit=unit,
            result=result,
            value=value,
            convert_from=convert_from,
            convert_to=convert_to,
        )

    return render_template(target_template, unit=unit)