Correct way to try/except using Python requests module?
Have a look at the Requests exception docs. In short:
In the event of a network problem (e.g. DNS failure, refused connection, etc), Requests will raise a
ConnectionErrorexception.In the event of the rare invalid HTTP response, Requests will raise an
HTTPErrorexception.If a request times out, a
Timeoutexception is raised.If a request exceeds the configured number of maximum redirections, a
TooManyRedirectsexception is raised.All exceptions that Requests explicitly raises inherit from
requests.exceptions.RequestException.
To answer your question, what you show will not cover all of your bases. You'll only catch connection-related errors, not ones that time out.
What to do when you catch the exception is really up to the design of your script/program. Is it acceptable to exit? Can you go on and try again? If the error is catastrophic and you can't go on, then yes, you may abort your program by raising SystemExit (a nice way to both print an error and call sys.exit).
You can either catch the base-class exception, which will handle all cases:
try:
r = requests.get(url, params={'s': thing})
except requests.exceptions.RequestException as e: # This is the correct syntax
raise SystemExit(e)
Or you can catch them separately and do different things.
try:
r = requests.get(url, params={'s': thing})
except requests.exceptions.Timeout:
# Maybe set up for a retry, or continue in a retry loop
except requests.exceptions.TooManyRedirects:
# Tell the user their URL was bad and try a different one
except requests.exceptions.RequestException as e:
# catastrophic error. bail.
raise SystemExit(e)
As Christian pointed out:
If you want http errors (e.g. 401 Unauthorized) to raise exceptions, you can call
Response.raise_for_status. That will raise anHTTPError, if the response was an http error.
An example:
try:
r = requests.get('http://www.google.com/nothere')
r.raise_for_status()
except requests.exceptions.HTTPError as err:
raise SystemExit(err)
Will print:
404 Client Error: Not Found for url: http://www.google.com/nothere
One additional suggestion to be explicit. It seems best to go from specific to general down the stack of errors to get the desired error to be caught, so the specific ones don't get masked by the general one.
url='http://www.google.com/blahblah'
try:
r = requests.get(url,timeout=3)
r.raise_for_status()
except requests.exceptions.HTTPError as errh:
print ("Http Error:",errh)
except requests.exceptions.ConnectionError as errc:
print ("Error Connecting:",errc)
except requests.exceptions.Timeout as errt:
print ("Timeout Error:",errt)
except requests.exceptions.RequestException as err:
print ("OOps: Something Else",err)
Http Error: 404 Client Error: Not Found for url: http://www.google.com/blahblah
vs
url='http://www.google.com/blahblah'
try:
r = requests.get(url,timeout=3)
r.raise_for_status()
except requests.exceptions.RequestException as err:
print ("OOps: Something Else",err)
except requests.exceptions.HTTPError as errh:
print ("Http Error:",errh)
except requests.exceptions.ConnectionError as errc:
print ("Error Connecting:",errc)
except requests.exceptions.Timeout as errt:
print ("Timeout Error:",errt)
OOps: Something Else 404 Client Error: Not Found for url: http://www.google.com/blahblah
Here's a generic way to do things which at least means that you don't have to surround each and every requests call with try ... except:
Basic version
# see the docs: if you set no timeout the call never times out! A tuple means "max
# connect time" and "max read time"
DEFAULT_REQUESTS_TIMEOUT = (5, 15) # for example
def log_exception(e, verb, url, kwargs):
# the reason for making this a separate function will become apparent
raw_tb = traceback.extract_stack()
if 'data' in kwargs and len(kwargs['data']) > 500: # anticipate giant data string
kwargs['data'] = f'{kwargs["data"][:500]}...'
msg = f'BaseException raised: {e.__class__.__module__}.{e.__class__.__qualname__}: {e}\n' \
+ f'verb {verb}, url {url}, kwargs {kwargs}\n\n' \
+ 'Stack trace:\n' + ''.join(traceback.format_list(raw_tb[:-2]))
logger.error(msg)
def requests_call(verb, url, **kwargs):
response = None
exception = None
try:
if 'timeout' not in kwargs:
kwargs['timeout'] = DEFAULT_REQUESTS_TIMEOUT
response = requests.request(verb, url, **kwargs)
except BaseException as e:
log_exception(e, verb, url, kwargs)
exception = e
return (response, exception)
NB
- Be aware of
ConnectionErrorwhich is a builtin, nothing to do with the classrequests.ConnectionError*. I assume the latter is more common in this context but have no real idea... - When examining a non-
Nonereturned exception,requests.RequestException, the superclass of all therequestsexceptions (includingrequests.ConnectionError), is not"according to the docs. Maybe it has changed since the accepted answer.**requests.exceptions.RequestException" - Obviously this assumes a logger has been configured. Calling
logger.exceptionin theexceptblock might seem a good idea but that would only give the stack within this method! Instead, get the trace leading up to the call to this method. Then log (with details of the exception, and of the call which caused the problem)
*I looked at the source code: requests.ConnectionError subclasses the single class requests.RequestException, which subclasses the single class IOError (builtin)
**However at the bottom of this page you find "requests.exceptions.RequestException" at the time of writing (2022-02)... but it links to the above page: confusing.
Usage is very simple:
search_response, exception = utilities.requests_call('get',
f'http://localhost:9200/my_index/_search?q={search_string}')
First you check the response: if it's None something funny has happened and you will have an exception which has to be acted on in some way depending on context (and on the exception). In Gui applications (PyQt5) I usually implement a "visual log" to give some output to the user (and also log simultaneously to the log file), but messages added there should be non-technical. So something like this might typically follow:
if search_response == None:
# you might check here for (e.g.) a requests.Timeout, tailoring the message
# accordingly, as the kind of error anyone might be expected to understand
msg = f'No response searching on |{search_string}|. See log'
MainWindow.the().visual_log(msg, log_level=logging.ERROR)
return
response_json = search_response.json()
if search_response.status_code != 200: # NB 201 ("created") may be acceptable sometimes...
msg = f'Bad response searching on |{search_string}|. See log'
MainWindow.the().visual_log(msg, log_level=logging.ERROR)
# usually response_json will give full details about the problem
log_msg = f'search on |{search_string}| bad response\n{json.dumps(response_json, indent=4)}'
logger.error(log_msg)
return
# now examine the keys and values in response_json: these may of course
# indicate an error of some kind even though the response returned OK (status 200)...
Given that the stack trace is logged automatically you often need no more than that...
Advanced version when json object returned
(... potentially sparing a great deal of boilerplate!)
To cross the Ts, when a json object is expected to be returned:
If, as above, an exception gives your non-technical user a message "No response", and a non-200 status "Bad response", I suggest that
- a missing expected key in the response's JSON structure should give rise to a message "Anomalous response"
- an out-of-range or strange value to a message "Unexpected response"
- and the presence of a key such as "error" or "errors", with value
Trueor whatever, to a message "Error response"
These may or may not prevent the code from continuing.
... and in fact to my mind it is worth making the process even more generic. These next functions, for me, typically cut down 20 lines of code using the above requests_call to about 3, and make most of your handling and your log messages standardised. More than a handful of requests calls in your project and the code gets a lot nicer and less bloated:
def log_response_error(response_type, call_name, deliverable, verb, url, **kwargs):
# NB this function can also be used independently
if response_type == 'No': # exception was raised (and logged)
if isinstance(deliverable, requests.Timeout):
MainWindow.the().visual_log(f'Time out of {call_name} before response received!', logging.ERROR)
return
else:
if isinstance(deliverable, BaseException):
# NB if response.json() raises an exception we end up here
log_exception(deliverable, verb, url, kwargs)
else:
# if we get here no exception has been raised, so no stack trace has yet been logged.
# a response has been returned, but is either "Bad" or "Anomalous"
response_json = deliverable.json()
raw_tb = traceback.extract_stack()
if 'data' in kwargs and len(kwargs['data']) > 500: # anticipate giant data string
kwargs['data'] = f'{kwargs["data"][:500]}...'
added_message = ''
if hasattr(deliverable, 'added_message'):
added_message = deliverable.added_message + '\n'
del deliverable.added_message
call_and_response_details = f'{response_type} response\n{added_message}' \
+ f'verb {verb}, url {url}, kwargs {kwargs}\nresponse:\n{json.dumps(response_json, indent=4)}'
logger.error(f'{call_and_response_details}\nStack trace: {"".join(traceback.format_list(raw_tb[:-1]))}')
MainWindow.the().visual_log(f'{response_type} response {call_name}. See log.', logging.ERROR)
def check_keys(req_dict_structure, response_dict_structure, response):
# so this function is about checking the keys in the returned json object...
# NB both structures MUST be dicts
if not isinstance(req_dict_structure, dict):
response.added_message = f'req_dict_structure not dict: {type(req_dict_structure)}\n'
return False
if not isinstance(response_dict_structure, dict):
response.added_message = f'response_dict_structure not dict: {type(response_dict_structure)}\n'
return False
for dict_key in req_dict_structure.keys():
if dict_key not in response_dict_structure:
response.added_message = f'key |{dict_key}| missing\n'
return False
req_value = req_dict_structure[dict_key]
response_value = response_dict_structure[dict_key]
if isinstance(req_value, dict):
# if the response at this point is a list apply the req_value dict to each element:
# failure in just one such element leads to "Anomalous response"...
if isinstance(response_value, list):
for resp_list_element in response_value:
if not check_keys(req_value, resp_list_element, response):
return False
elif not check_keys(req_value, response_value, response): # any other response value must be a dict (tested in next level of recursion)
return False
elif isinstance(req_value, list):
if not isinstance(response_value, list): # if the req_value is a list the reponse must be one
response.added_message = f'key |{dict_key}| not list: {type(response_value)}\n'
return False
# it is OK for the value to be a list, but these must be strings (keys) or dicts
for req_list_element, resp_list_element in zip(req_value, response_value):
if isinstance(req_list_element, dict):
if not check_keys(req_list_element, resp_list_element, response):
return False
if not isinstance(req_list_element, str):
response.added_message = f'req_list_element not string: {type(req_list_element)}\n'
return False
if req_list_element not in response_value:
response.added_message = f'key |{req_list_element}| missing from response list\n'
return False
# put None as a dummy value (otherwise something like {'my_key'} will be seen as a set, not a dict
elif req_value != None:
response.added_message = f'required value of key |{dict_key}| must be None (dummy), dict or list: {type(req_value)}\n'
return False
return True
def process_json_requests_call(verb, url, **kwargs):
# "call_name" is a mandatory kwarg
if 'call_name' not in kwargs:
raise Exception('kwarg "call_name" not supplied!')
call_name = kwargs['call_name']
del kwargs['call_name']
required_keys = {}
if 'required_keys' in kwargs:
required_keys = kwargs['required_keys']
del kwargs['required_keys']
acceptable_statuses = [200]
if 'acceptable_statuses' in kwargs:
acceptable_statuses = kwargs['acceptable_statuses']
del kwargs['acceptable_statuses']
exception_handler = log_response_error
if 'exception_handler' in kwargs:
exception_handler = kwargs['exception_handler']
del kwargs['exception_handler']
response, exception = requests_call(verb, url, **kwargs)
if response == None:
exception_handler('No', call_name, exception, verb, url, **kwargs)
return (False, exception)
try:
response_json = response.json()
except BaseException as e:
logger.error(f'response.status_code {response.status_code} but calling json() raised exception')
# an exception raised at this point can't truthfully lead to a "No response" message... so say "bad"
exception_handler('Bad', call_name, e, verb, url, **kwargs)
return (False, response)
status_ok = response.status_code in acceptable_statuses
if not status_ok:
response.added_message = f'status code was {response.status_code}'
log_response_error('Bad', call_name, response, verb, url, **kwargs)
return (False, response)
check_result = check_keys(required_keys, response_json, response)
if not check_result:
log_response_error('Anomalous', call_name, response, verb, url, **kwargs)
return (check_result, response)
Example call (NB with this version, the "deliverable" is either an exception or a response which delivers a json structure):
success, deliverable = utilities.process_json_requests_call('get',
f'{ES_URL}{INDEX_NAME}/_doc/1',
call_name=f'checking index {INDEX_NAME}',
required_keys={'_source':{'status_text': None}})
if not success: return False
# here, we know the deliverable is a response, not an exception
# we also don't need to check for the keys being present:
# the generic code has checked that all expected keys are present
index_status = deliverable.json()['_source']['status_text']
if index_status != 'successfully completed':
# ... i.e. an example of a 200 response, but an error nonetheless
msg = f'Error response: ES index {INDEX_NAME} does not seem to have been built OK: cannot search'
MainWindow.the().visual_log(msg)
logger.error(f'index |{INDEX_NAME}|: deliverable.json() {json.dumps(deliverable.json(), indent=4)}')
return False
So the "visual log" message seen by the user in the case of missing key "status_text", for example, would be "Anomalous response checking index XYZ. See log." (and the log would give a more detailed technical message, constructed automatically, including the stack trace but also details of the missing key in question).
NB
- mandatory kwarg:
call_name; optional kwargs:required_keys,acceptable_statuses,exception_handler. - the
required_keysdictcan be nested to any depth - finer-grained exception-handling can be accomplished by including a function
exception_handlerinkwargs(though don't forget thatrequests_callwill have logged the call details, the exception type and__str__, and the stack trace). - in the above I also implement a check on key "data" in any
kwargswhich may be logged. This is because a bulk operation (e.g. to populate an index in the case of Elasticsearch) can consist of enormous strings. So curtail to the first 500 characters, for example.
PS Yes, I do know about the elasticsearch Python module (a "thin wrapper" around requests). All the above is for illustration purposes.
Exception object also contains original response e.response, that could be useful if need to see error body in response from the server. For example:
try:
r = requests.post('somerestapi.com/post-here', data={'birthday': '9/9/3999'})
r.raise_for_status()
except requests.exceptions.HTTPError as e:
print (e.response.text)