Cannot read urllib error message once it is read()

ghz 11hours ago ⋅ 2 views

My problem is with error handling of the python urllib error object. I am unable to read the error message while still keeping it intact in the error object, for it to be consumed later.

response = urllib.request.urlopen(request) # request that will raise an error
response.read()
response.read() # is empty now
# Also tried seek(0), that does not work either.

So this how I intend to use it, but when the Exception bubbles up, the.read() second time is empty.

try:
    response = urllib.request.urlopen(request)
except urllib.error.HTTPError as err:
    self.log.exception(err.read())
    raise err

I tried making a deepcopy of the err object,

import copy
try:
    response = urllib.request.urlopen(request)
except urllib.error.HTTPError as err:
    err_obj_copy = copy.deepcopy(err)
    self.log.exception(
        "Method:{}\n"
        "URL:{}\n"
        "Data:{}\n"
        "Details:{}\n"
        "Headers:{}".format(method, url, data, err_obj_copy.read(), headers))
    raise err

but copy is unable to make a deepcopy and throws an error - TypeError: __init__() missing 5 required positional arguments: 'url', 'code', 'msg', 'hdrs', and 'fp'.

How do I read the error message, while still keeping it intact in the object?

I do know how to do it using requests, but I am stuck with legacy code and need to make it work with urllib

Answer

The issue you're encountering arises because when you call .read() on a urllib.error.HTTPError object, it consumes the response body, leaving it empty if you attempt to read it a second time. Unfortunately, this behavior is hard to reverse because the response body is typically consumed when it's read.

The key challenge is to access the error message (or response body) without consuming it and still be able to use it later. Here's a breakdown of the issue and a solution:

Issue

  1. Consuming the Response: The HTTPError object contains the response body in a file-like object (fp), which is consumed when you call .read(). Once read, the content is no longer available unless you explicitly reset the file pointer or retain the content elsewhere.
  2. Deepcopy Limitation: The urllib.error.HTTPError object is not trivially deepcopyable because its internal state is tightly coupled with its file pointer (fp). This causes the TypeError when trying to deepcopy the object.

Solution

To preserve the content for later use, you should save the response body (or error message) into a variable before reading it. This way, you can log the error and still raise the exception without losing the data.

Here's how you can modify your code to preserve the content:

Code Example

import urllib.request
import urllib.error

try:
    request = urllib.request.Request("http://example.com")  # Sample URL
    response = urllib.request.urlopen(request)
    response.read()
except urllib.error.HTTPError as err:
    # Read the response content (body of the error)
    error_content = err.read()  # This will consume the body once
    self.log.exception(
        "Method:{}\n"
        "URL:{}\n"
        "Data:{}\n"
        "Details:{}\n"
        "Headers:{}".format('GET', 'http://example.com', '', error_content, err.headers)
    )
    
    # Reraise the error after logging
    raise err

Explanation:

  1. Reading the Response Body: We capture the error content with err.read() and store it in a variable (error_content). This prevents the body from being consumed multiple times and allows you to access it.
  2. Logging: After reading the error content, you log it (or whatever you need to do with it), while still maintaining the error object intact.
  3. Raising the Exception: After logging, we re-raise the exception (raise err), which allows the exception to propagate as needed.

Notes:

  • Accessing the error content: The HTTPError object contains the error body (in the fp attribute, which is a file-like object). Calling err.read() consumes the body, so if you need to keep it, make sure to store the result in a variable before logging or re-raising the error.
  • Error content handling: The content might be binary, and if you want to process or print it as a string, you might need to decode it using an appropriate encoding (e.g., utf-8), depending on the response.

Alternative: Custom Error Handling (With StringIO)

If you need to reuse the content multiple times (e.g., logging and raising), you could wrap the err.fp in a StringIO buffer or similar approach, allowing you to "rewind" the content. Here's a quick example:

import io

try:
    request = urllib.request.Request("http://example.com")
    response = urllib.request.urlopen(request)
    response.read()
except urllib.error.HTTPError as err:
    # Capture the error content and wrap it in StringIO to allow multiple reads
    error_content = err.read()
    err.fp = io.BytesIO(error_content)  # "Rewind" the content for future reads
    
    # Log the error
    self.log.exception(
        "Method:{}\n"
        "URL:{}\n"
        "Data:{}\n"
        "Details:{}\n"
        "Headers:{}".format('GET', 'http://example.com', '', error_content, err.headers)
    )
    
    # Raise the error
    raise err

In this case, after reading the error body and storing it in a BytesIO buffer, the content is still available for re-reading if necessary. This allows for multiple read() operations without losing the data.

Conclusion

To summarize:

  • Consume error content once: Store it in a variable (error_content) before using read().
  • Custom buffer: If you need multiple reads, use a StringIO or BytesIO buffer to simulate rewinding.
  • Avoid deepcopying HTTPError: It's not necessary and can be error-prone due to the complex internal state of the error object.

This approach should allow you to handle errors properly without losing the response body.

#py