Production-ready client for an external API with requests

# ======================================================================
# Q: Write a production-ready client for an external API.
#
# What the interviewer looks for: always a timeout (requests waits forever by
# default), a Session for connection pooling, retry only on idempotent methods
# and on 5xx, respecting 429 Retry-After, our own exceptions so the rest of the
# code does not depend on requests, and never calling an external API inside a
# DB transaction.
# ======================================================================

import requests
from requests.adapters import HTTPAdapter
from urllib3.util.retry import Retry

class ExternalAPIError(Exception): ...
class RateLimited(ExternalAPIError):
    def __init__(self, retry_after): self.retry_after = retry_after

class WeatherClient:
    BASE_URL = 'https://api.example.com/v2'

    def __init__(self, api_key, timeout=(3.05, 10)):   # (connect, read)
        self.session = requests.Session()                # connection pooling
        retry = Retry(total=3, backoff_factor=0.5,
                      status_forcelist=[502, 503, 504],
                      allowed_methods=['GET'])          # idempotent only
        self.session.mount('https://', HTTPAdapter(max_retries=retry))
        self.session.headers['Authorization'] = f'Bearer {api_key}'
        self.timeout = timeout

    def forecast(self, city):
        r = self.session.get(f'{self.BASE_URL}/forecast',
                             params={'q': city}, timeout=self.timeout)
        if r.status_code == 429:
            raise RateLimited(int(r.headers.get('Retry-After', 60)))
        if not r.ok:
            raise ExternalAPIError(f'{r.status_code}: {r.text[:200]}')
        return r.json()