HTTPError Explained: Python, JavaScript, .NET and Go

Messages like HTTPError: Not Found, HTTPError: 401, or 404 Client Error: Not Found for url all mean the same thing: the request reached a server, and the server replied with an error status code. To fix it, look up the status code, then check the request your code sent.

Python

requests: requests.exceptions.HTTPError

The requests library does not raise on error statuses by itself. The exception comes from calling raise_for_status(), which raises for any 4xx or 5xx response:

import requests

r = requests.get("https://api.example.com/items/42", timeout=10)
try:
    r.raise_for_status()
except requests.exceptions.HTTPError as e:
    print(e)                     # 404 Client Error: Not Found for url: ...
    print(e.response.status_code, e.response.text)

urllib: urllib.error.HTTPError

The standard library's urllib.request.urlopen() raises HTTPError automatically for error responses. It is also a response object, so you can read the body the server sent:

from urllib.request import urlopen
from urllib.error import HTTPError, URLError

try:
    urlopen("https://example.com/missing")
except HTTPError as e:
    print(e.code, e.reason)      # 404 Not Found
    print(e.read()[:200])        # error body from the server
except URLError as e:
    print("Network problem:", e.reason)   # DNS, refused connection, TLS...

HTTPError means the server answered; URLError means no HTTP answer was received at all.

JavaScript

fetch()

fetch() only rejects on network failures. A 404 or 500 resolves normally, so you must check response.ok yourself - many codebases then throw their own HttpError:

class HttpError extends Error {
  constructor(response) {
    super(`HTTP ${response.status} ${response.statusText}`);
    this.status = response.status;
  }
}

const res = await fetch('/api/items/42');
if (!res.ok) throw new HttpError(res);

Axios

Axios rejects every response outside 2xx by default. The status is in error.response.status; if error.response is undefined, the request never got an answer (network error, CORS, timeout).

Angular: HttpErrorResponse

Angular's HttpClient emits an HttpErrorResponse with status, statusText and error. A status of 0 means no HTTP response was received - usually a CORS rejection, a blocked request, or the server being unreachable.

Node.js: the http-errors package

Express and Koa apps often use http-errors to create errors that carry a status, e.g. createError(404, 'Item not found') or new createError.BadRequest(). An error handler then turns them into HTTP responses.

.NET

HttpClient returns error responses normally; response.EnsureSuccessStatusCode() turns them into an HttpRequestException, which exposes a StatusCode property in .NET 5 and later. Generated clients (such as NSwag) throw an ApiException with messages like "The HTTP status code of the response was not expected (404)".

Go

net/http does not return an error for 4xx/5xx responses - err is only set for transport problems. Always check resp.StatusCode.

How to Fix an HTTPError

1. Read the status code and look it up: 400, 401, 403, 404, 429, 500, 502, 503, 504.
2. Print the response body. APIs usually explain the problem there (missing field, expired token, unknown ID).
3. Check the exact URL and method your code sent - base URL, API version, path parameters, trailing slash.
4. Check authentication headers for 401/403 errors.
5. Retry only when it makes sense: 429, 502, 503 and 504 are often temporary; 400, 401, 403, 404 and 422 will fail again until the request changes.