Bill payment API integration with Python

# Python 3.11+. Use from your server, not from a browser.
import json
import os
import re
import urllib.error
import urllib.parse
import urllib.request

BASE_URL = "https://my.rizpay.app/api/partners/sandbox/v1"

class NoRedirect(urllib.request.HTTPRedirectHandler):
    def redirect_request(self, req, fp, code, msg, headers, newurl):
        return None

class RizPayError(Exception):
    def __init__(self, code, status=None):
        self.code = code
        self.status = status
        super().__init__("Request not confirmed. Reconcile the stored order before retrying.")

class RizPaySandbox:
    def __init__(self, key=None):
        self.key = key or os.environ.get("RIZPAY_API_KEY", "")
        if not self.key.startswith("sk_test_"):
            raise ValueError("A sandbox key is required.")
        self.http = urllib.request.build_opener(NoRedirect)

    def request(self, path, body=None):
        request = urllib.request.Request(
            BASE_URL + path,
            data=json.dumps(body).encode() if body is not None else None,
            headers={"Authorization": "Bearer " + self.key, "Content-Type": "application/json"},
            method="POST" if body is not None else "GET",
        )
        try:
            with self.http.open(request, timeout=20) as response:
                return json.load(response)
        except urllib.error.HTTPError as error:
            try:
                code = json.load(error).get("error", {}).get("code", "API_ERROR")
                if not isinstance(code, str) or not re.fullmatch(r"[A-Z_]{3,60}", code):
                    code = "API_ERROR"
            except (ValueError, AttributeError):
                code = "API_ERROR"
            raise RizPayError(code, error.code) from None
        except (urllib.error.URLError, TimeoutError, ValueError, OSError):
            raise RizPayError("UNCONFIRMED_RESPONSE") from None

    def list_airtime(self, network="MTN", page=1):
        if network not in ("MTN", "AIRTEL", "GLO", "9MOBILE"):
            raise ValueError("Choose a supported network.")
        if type(page) is not int or page < 1:
            raise ValueError("Invalid page.")
        return self.request("/products/airtimes?" + urllib.parse.urlencode({"network": network, "page": page}))

    def purchase_airtime(self, *, product_id, network, phone_number, amount, external_reference):
        if not re.fullmatch(r"\d{10}[A-Za-z0-9]{6}", external_reference):
            raise ValueError("Use the stored order reference.")
        if not re.fullmatch(r"prd_\d+", product_id):
            raise ValueError("Select a catalogue product.")
        if network not in ("MTN", "AIRTEL", "GLO", "9MOBILE"):
            raise ValueError("Confirm the network.")
        if not re.fullmatch(r"0[789]\d{9}", phone_number):
            raise ValueError("Check the number.")
        if not isinstance(amount, str) or not re.fullmatch(r"\d+\.\d{2}", amount):
            raise ValueError("Use a decimal amount string.")
        return self.request("/purchases", {
            "product_id": product_id, "network": network, "phone_number": phone_number,
            "amount": amount, "external_reference": external_reference,
        })

    def find_order(self, reference):
        if not re.fullmatch(r"\d{10}[A-Za-z0-9]{6}", reference):
            raise ValueError("Invalid reference.")
        return self.request("/account/transactions/" + reference)

    def purchase_status(self, transaction_id):
        if not re.fullmatch(r"txn_[A-Za-z0-9-]+", transaction_id):
            raise ValueError("Invalid transaction.")
        return self.request("/purchases/" + transaction_id)