HTTP status code

It is not always the case that, when a web request is made and sent to a web server, the server will process the request and return the requested data without fail. Sometimes, the server might be completely down or already busy interacting with other clients and therefore unresponsive to a new request; sometimes, the client itself makes bad requests to a server (for example, incorrectly formatted or malicious requests).

As a way to categorize these problems as well as provide the most information as possible during the communication resulting from a web request, HTTP requires servers to respond to each request from its clients an HTTP response status code. A status code is typically a three-digit number that indicates the specific characteristics of the response that the server sends back to a client.

There are in total five large categories of HTTP response status codes, indicated by the first digit of the code. They are as follows:

• 1xx (informational status code): The request was received and the server is processing it. For example, 100 means the request header has been received and the server is waiting for the request body; 102 indicates that the request is currently being processed (this is used for large requests and to prevent clients from timing out).
• 2xx (successful status code): The request was successfully received, understood, and processed by the server. For example, 200 means the request was successfully fulfilled; 202 indicates that the request has been accepted for processing, but the processing itself is not complete.
• 3xx (redirectional status code): Additional actions need to be taken so that the request can be successfully processed. For example, 300 means that there are multiple options regarding how the response from the server should be processed (for example, giving the client multiple video format options when a video file is to be downloaded); 301 indicates that the server has been moved permanently and all requests should be directed to another address (provided in the response from the server).

• 4xx (error status code for the client): The request was incorrectly formatted by the client and could not be processed. For example, 400 means that the client sent in a bad request (for example, syntax error or the size of the request is too large); 404 (arguably the most well-known status code) indicates that the request method is not supported by the server.
• 5xx (error status code for the server): The request, although valid, could not be processed by the server. For example, 500 means there is an internal server error in which an unexpected condition was encountered; 504 (Gateway Timeout) means that the server, which was acting as a gateway or a proxy, did not receive a response from the final server in time.

A lot more can be said about these status codes, but it is already sufficient for us to keep in mind the big five categories previously mentioned when making web requests from Python. If you would like to find more specific information about the above or other status codes, the Internet Assigned Numbers Authority (IANA) maintains the official registry of HTTP status codes.