Resolving POST Request Redirection to GET in Python urllib2

Problem Background and Symptoms

When using Python's urllib2 library for HTTP POST requests, developers often encounter a confusing issue: even when explicitly passing the data parameter to the urlopen() function, server logs indicate that a GET request was received. This typically leads to 404 errors, as the server may only process POST requests. For example, the following code should send a POST request:

import urllib
import urllib2

url = 'http://myserver/post_service'
data = urllib.urlencode({'name': 'joe', 'age': '10'})
content = urllib2.urlopen(url=url, data=data).read()
print(content)

However, the server reports a GET request, causing a 404 error. This phenomenon is not due to a logic error in the code but is related to the HTTP redirection mechanism.

Core Cause Analysis

According to the best answer (Answer 2), the root cause is HTTP 302 redirection. When a server returns a 302 status code, urllib2 automatically follows the redirection, but during this process, the request method changes from POST to GET. This is standard behavior under the HTTP/1.1 protocol (RFC 2616), designed to prevent data from being resubmitted without authorization. Specifically, if the initial request URL is http://myserver/post_service and the server redirects it to http://myserver/post_service/ (adding a trailing slash), the redirected request will use the GET method, losing the original POST data.

Python's urllib2 library strictly adheres to this standard, so it does not preserve the POST method during 302 redirections. This contradicts developer intuition, as the presence of the data parameter usually implies a POST request, but the redirection mechanism takes precedence over parameter settings.

Solutions

Solution 1: Modify the URL to Avoid Redirection

The most straightforward solution is to adjust the URL to match the server's expected format, thereby avoiding triggering a 302 redirection. For example, if the server expects a URL with a trailing slash, use it directly:

url = 'http://myserver/post_service/'  # Add trailing slash
data = urllib.urlencode({'name': 'joe', 'age': '10'})
req = urllib2.Request(url, data)
response = urllib2.urlopen(req)
print(response.read())

This method is simple and effective but relies on knowledge of server behavior. If the server configuration changes, the URL may need to be adjusted again.

Solution 2: Use Custom Request Handlers

When redirection cannot be avoided, custom request handlers can be used to override default behavior. Answer 1 provides a flexible approach that allows explicit setting of the request method (e.g., POST, PUT, DELETE):

import urllib
import urllib2

method = "POST"
handler = urllib2.HTTPHandler()
opener = urllib2.build_opener(handler)
data = urllib.urlencode({'name': 'joe', 'age': '10'})
request = urllib2.Request('http://myserver/post_service', data=data)
request.add_header("Content-Type", "application/x-www-form-urlencoded")
request.get_method = lambda: method  # Override request method

try:
    connection = opener.open(request)
except urllib2.HTTPError as e:
    connection = e

if connection.code == 200:
    data = connection.read()
else:
    # Error handling
    pass

This method forces urllib2 to use the specified method during redirection by setting the request.get_method attribute. It offers greater control and is suitable for complex scenarios, such as RESTful API calls.

Supplementary Insights from Other Answers

Answer 3 provides a basic example using the urllib2.Request object to send a POST request:

req = urllib2.Request(url, data)
response = urllib2.urlopen(req)

However, this is also affected by redirection issues. The answer also recommends the requests library as a modern alternative, which simplifies HTTP request handling and automatically manages many edge cases, including preserving request methods during redirections.

The scoring difference between Answer 1 and Answer 3 (10.0 vs 2.2) reflects the depth of the solutions: Answer 2 directly addresses the root cause (redirection), while Answer 3 only provides basic usage without solving the core problem.

Practical Recommendations and Summary

When handling POST requests, consider the following steps:

1. First, check server logs or use network debugging tools (e.g., Wireshark) to confirm if the request is being redirected.
2. If redirection is the issue, try modifying the URL format to avoid it.
3. For complex scenarios requiring POST method preservation, use custom request handlers.
4. Consider migrating to the requests library, which offers a more intuitive API and better redirection handling.

In summary, the conversion of POST requests to GET in urllib2 stems from the standard behavior of HTTP 302 redirection. By understanding the protocol mechanisms and library implementation details, developers can effectively diagnose and resolve such issues, ensuring reliable network communication.