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Sniffing Python urllib2.httperror content and passing it along

In some python code, I am using a library to wrap requests to a web service. The behaviour I intend is that any HTTPErrors have their content output with a logging.error along with the status code, and the error passed along:
def my_request_thing(api, other_stuff):
request = make_request_from(api, other_stuff)
try:
with closing(urllib2.urlopen(request)) as fd:
return fd.read()
except HTTPError as e:
logging.error("Error from server: %s\n%s", e.code, e.read())
raise
This code will log, and pass the error along, with one problem, the exceptions content is exhausted in e.read. This code is intended to be used to most clients to the API substituting things like root paths and http headers...
I may then have another function for more domain specific stuff using this:
function get_my_thing(thing_id, conditions):
try:
return json.loads(my_request_thing(<thing_id + conditions into api and stuff...>))
except HTTPError as e:
if e.code == 404 and "my thing does not exist" in e.read():
return False
else:
raise e
Note here that this also tries to get data with e.read - which is now empty, and may still reraise the error. This will fail to work - there is not data in e.read here.
Is there a good way to reraise this exception such that the content is not exhausted, but so I can sniff out particular exception types and log them all on the way?

As per Karel Kubat comment, why don't you inject the results from e.read() into the exception as a data member upon seeing it for the first time?
For example, derive your own error class from HTTPError with an empty self.content. When catching an exception for the first time, fill self.content from self.read(). Next handlers can inspect e.content.

What is the best way to write try except in Python?

Suppose I have a code snippet as following
r = requests.post(url, data=values, files=files)
Since this is making a network request, a bunch of exceptions can be thrown from this line. For completeness of the argument, I could also have file reads, sending emails, etc. To encounter for such errors I do
try:
r = requests.post(url, data=values, files=files)
if r.status_code != 200:
raise Exception("Could not post to "+ url)
except Exception as e:
logger.error("Error posting to " + url)
There are two problems which I see with this approach.
I have just handled a generic exception and don't know what exact exception would be raised by this line, what is the best way to find it in python.
This makes the code look ugly, which is non pythonic but fine, as long as its robust and handles all the cases.
I am wondering what would be the best way to handle exceptions in python.

The best way to write try-except -- in Python or anywhere else -- is as narrow as possible. It's a common problem to catch more exceptions than you meant to handle!
In particular, at a minimum, I'd re-write your example code as something like:
try:
r = requests.post(url, data=values, files=files)
except Exception as e:
logger.error("Error posting to %r: %s" % (url, e))
raise
else:
if r.status_code != 200:
logger.error("Could not to %r: HTTP code %s" % (url, r.status_code))
raise RuntimeError("HTTP code %s trying to post to %r" % (r.status_code, url))
This embodies several best-practices, such as: detailed error messages, always re-raise exceptions you don't know how to specifically handle (after logging error messages with more details as well as the exception), never raise something as generic as Exception, &c -- and, crucially, catch exceptions only on the narrowest part of code you possibly can, that's what the else: clause in try/except is for!-)
If and when you do expect -- and know how to handle -- specific exceptions, so much the better -- you put other except ThisSpecificProblem as e: clauses before the generic except Exception clause which logs and re-raises. But (from the Zen of Python -- import this at a Python interpreter prompt!) -- "Errors should never pass silently. // Unless explicitly silenced."... and you should only "explicitly silence" errors you fully expect, and fully know how to handle!

I have just handled a generic exception and don't know what exact
exception would be raised by this line, what is the best way to find
it in python.
As always, the answer is to look at the documentation:
In the event of a network problem (e.g. DNS failure, refused
connection, etc), Requests will raise a ConnectionError exception.
In the rare event of an invalid HTTP response, Requests will raise an
HTTPError exception.
If a request times out, a Timeout exception is raised.
If a request exceeds the configured number of maximum redirections, a
TooManyRedirects exception is raised.
All exceptions that Requests explicitly raises inherit from
requests.exceptions.RequestException.
Code that raises exceptions (especially if there are custom exceptions) is documented. You can also have a look at the source if the documentation is not explicit.
Your code is fine, except you should avoid generic except clauses as these can hide other problems with your code. You should except those exceptions that you can predict, and then let the others "rise up" until caught/logged.

Well, answering your first question, what exact exception would be raised by this line, you are one step away.
You already call except Exception as e, but you don't use e anywhere. e contains the information about your exception, so just add a little print statement
print e
And it works:
>>> try:
... x = int(raw_input('Input: '))
... except Exception as e:
... print e
...
Input: 5t
invalid literal for int() with base 10: '5t'
>>>
I don't exactly see what you're asking in the 2nd, you say it is ugly/non-pythonic, but then you say it is fine. Yes, it is fine, and it is also quite pythonic, in my opinion.

You should try avoiding using except Exception as e: as much as possible.
For clarity you can create a custom exception class which takes care of your error code = 200 scenario.
class PostingError(Exception):
pass
And then raise PostingError only. Try catching this error only. By catching all kinds of error, you might be catching wrong information. For example even a memory error might be caught and displayed as a "Error posting to URL".
So this is how it would look like finally
try:
r = requests.post(url, data=values, files=files)
if r.status_code != 200:
raise PostingError("Could not post to "+ url)
except PostingError as e:
logger.error(e)

How to print body of response on urllib2.URLError?

In a script I am creating I am posting a lot of data to a REST API.
The script is quite modularized and at the top level somewhere I am catching a URLError. I need to know what is in the body of the response, because there will be an error message in there.
Is there a method on URLError that I can use?
try:
(calling some function that throws URLError)
except urllib2.URLError, e:
print "Error: " + str(e.body_or_something)

Yes there is. You have an access to the response via e.readlines():
try:
(calling some function that throws URLError)
except urllib2.URLError, e:
print e.readlines()

See the documenet: https://docs.python.org/2/library/urllib2.html#urllib2.URLError
exception urllib2.URLError The handlers raise this exception (or
derived exceptions) when they run into a problem. It is a subclass of
IOError.
reason The reason for this error. It can be a message string or
another exception instance (socket.error for remote URLs, OSError for
local URLs).
exception urllib2.HTTPError Though being an exception (a subclass of
URLError), an HTTPError can also function as a non-exceptional
file-like return value (the same thing that urlopen() returns). This
is useful when handling exotic HTTP errors, such as requests for
authentication.
code An HTTP status code as defined in RFC 2616. This numeric value
corresponds to a value found in the dictionary of codes as found in
BaseHTTPServer.BaseHTTPRequestHandler.responses.
reason The reason for this error. It can be a message string or
another exception instance.
So you can access the response body when the request raise urllib2.HTTPError.
Try this:
try:
(calling some function that throws URLError)
except urllib2.HTTPError as e:
body = e.readlines()
print e.code, e.reason, body
except urllib2.URLError as e:
print e.reason
except:
sys.excepthook(*sys.exc_info())

"2nd hand" Exception Handling in Python

Is there a way to take exception handling output from an imported module and process it from the calling program? For example, I have an imported module that writes an HTTP exception
except urllib2.HTTPError, e:
sys.stderr.write(str(e) + '\n')
If a 404 occurs, then the calling programing only sees the following:
HTTP Error 404: not found
Can this be taken as input without modifying the imported module? I would need to perform different tasks depending on the HTTP Error that is returned.

If you can modify the imported module, raise the error in the except block like so:
except urllib2.HTTPError, e:
sys.stderr.write(str(e) + '\n')
raise e
Then in the calling program, catch the error and inspect it for the error code:
except urllib2.HTTPError, e:
if e.code == 404:
do_something_here()

Python requests exception handling

How to handle exceptions with python library requests?
For example how to check is PC connected to internet?
When I try
try:
requests.get('http://www.google.com')
except ConnectionError:
# handle the exception
it gives me error name ConnectionError is not defined

Assuming you did import requests, you want requests.ConnectionError. ConnectionError is an exception defined by requests. See the API documentation here.
Thus the code should be:
try:
requests.get('http://www.google.com')
except requests.ConnectionError:
# handle the exception
The original link to the Python v2 API documentation from the original answer no longer works.

As per the documentation, I have added the below points:
In the event of a network problem (refused connection e.g internet issue), Requests will raise a ConnectionError exception.
try:
requests.get('http://www.google.com')
except requests.ConnectionError:
# handle ConnectionError the exception
In the event of the rare invalid HTTP response, Requests will raise an HTTPError exception.
Response.raise_for_status() will raise an HTTPError if the HTTP request returned an unsuccessful status code.
try:
r = requests.get('http://www.google.com/nowhere')
r.raise_for_status()
except requests.exceptions.HTTPError as err:
#handle the HTTPError request here
In the event of times out of request, a Timeout exception is raised.
You can tell Requests to stop waiting for a response after a given number of seconds, with a timeout arg.
requests.get('https://github.com/', timeout=0.001)
# timeout is not a time limit on the entire response download; rather,
# an exception is raised if the server has not issued a response for
# timeout seconds
All exceptions that Requests explicitly raises inherit from requests.exceptions.RequestException. So a base handler can look like,
try:
r = requests.get(url)
except requests.exceptions.RequestException as e:
# handle all the errors here
The original link to the Python v2 documentation no longer works, and now points to the new documentation.

Actually, there are much more exceptions that requests.get() can generate than just ConnectionError. Here are some I've seen in production:
from requests import ReadTimeout, ConnectTimeout, HTTPError, Timeout, ConnectionError
try:
r = requests.get(url, timeout=6.0)
except (ConnectTimeout, HTTPError, ReadTimeout, Timeout, ConnectionError):
continue

Include the requests module using import requests .
It is always good to implement exception handling. It does not only help to avoid unexpected exit of script but can also help to log errors and info notification. When using Python requests I prefer to catch exceptions like this:
try:
res = requests.get(adress,timeout=30)
except requests.ConnectionError as e:
print("OOPS!! Connection Error. Make sure you are connected to Internet. Technical Details given below.\n")
print(str(e))
continue
except requests.Timeout as e:
print("OOPS!! Timeout Error")
print(str(e))
continue
except requests.RequestException as e:
print("OOPS!! General Error")
print(str(e))
continue
except KeyboardInterrupt:
print("Someone closed the program")

for clarity, that is
except requests.ConnectionError:
NOT
import requests.ConnectionError
You can also catch a general exception (although this isn't recommended) with
except Exception: