Develop Reference

Why asyncio raises TimeoutError without any message?

I encountered a small annoyance with this code:
try:
return await asyncio.wait_for(tcp_command(cmd), timeout=timeout)
except (OSError, asyncio.TimeoutError) as err:
print(f"Network problem: {err}")
When the timeout occurs, it prints just "Network problem: ". It is caused by an empty value attached to the raised asyncio.TimeoutError:
# inside wait_for():
raise futures.TimeoutError()
It is easy to hadle the TimeoutError separately, but I find the original construct quite idiomatic and now a core library breaks it. Is there a good reason for it? Is my assumption - that printing an exception should give us a clue what went wrong - correct?

Is there a good reason for it?
Yes, what kind of message you expect from TimeoutError? "Timeout occured"? The exception itself is self-explanatory, no need for such redundancy.
Is my assumption - that printing an exception should give us a clue what went wrong - correct?
Yes and no. Clue? Yes. Full information? No. The exception message is not mandatory. And the type of an exception is an important piece of information as well. And in many cases even more then the message itself.
So first of all: using print is wrong to begin with. Python has a very rich logging support. For example logger.exception(str(exc)) solves your problem because it logs entire traceback in addition to the message. At least by default, it can be customized.
But if you still want to use print then consider logging whole traceback:
import traceback
# traceback.print_exc()
print(traceback.format_exc())
If whole traceback is too big then you can always simply print the exception's class name:
# print(f'[{type(exc).__name__}] {exc}')
print(f'[{type(exc)}] {exc}')
or customize by exception:
try:
return await asyncio.wait_for(tcp_command(cmd), timeout=timeout)
except OSError as err:
print(f"Network problem: {err}")
except asyncio.TimeoutError:
print('Timeout occured')

The expectation that an exception will provide a message that explains the issue is not part of the general exception contract in Python. It is true for system exceptions such as OSError where the program must be able to get to the error message provided by the operating system, as the program is not qualified to guess the message based on a code or an exception subtype.
But more basic language exceptions do not work like that. Take, for example, KeyError raised by dict.__getitem__:
>>> try:
... d[123]
... except KeyError as err:
... print(f"Dict problem: {err}")
...
Dict problem: 123
In this sense, TimeoutError is much more like KeyError than like OSError. When you catch TimeoutError, you know exactly what happened - a timeout. You typically want to do something based on the fact that a timeout happened, rather than just display a message to the user. And even if you did want to provide a message, you'd use one that would make sense for your application, not a generic one provided by Python. This is in contrast to OSError where you often cannot do anything other than display the message coming from the OS and where that message can prove invaluable for investigating the underlying issue.
To sum it up, the problem is that you are catching two fundamentally different exceptions in the same except clause, and that set you up for trouble. I would restructure the code like this:
try:
return await asyncio.wait_for(tcp_command(cmd), timeout=timeout)
except OSError as err:
print(f"Network problem: {err}")
except asyncio.TimeoutError:
print("Operation timed out")

What is the difference between except and except BaseException

What is the difference between those two:
except:
# do something
and
except BaseException as be:
print(be)
I mean in the first case all possible exception are caught, but is this true for the second?
Also can the error message be printed using the first case?

The accepted answer is incorrect incomplete (at least for Python 3.6 and above).
By catching Exception you catch most errors - basically all the errors that any module you use might throw.
By catching BaseException, in addition to all the above exceptions, you also catch exceptions of the types SystemExit, KeyboardInterrupt, and GeneratorExit.
By catching KeyboardInterrupt, for example, you may stop your code from exiting after an initiated exit by the user (like pressing ^C in the console, or stopping launched application on some interpreters). This could be a wanted behavior (for example - to log an exit), but should be used with extreme care!
In the above example, by catching BaseException, you may cause your application to hang when you want it to exit.

Practically speaking, there is no difference between except: and except BaseException:, for any current Python release.
That's because you can't just raise any type of object as an exception. The raise statement explicitly disallows raising anything else:
[...] raise evaluates the first expression as the exception object. It must be either a subclass or an instance of BaseException.
Bold emphasis mine. This has not always been the case however, in older Python releases (2.4 and before) you could use strings as exceptions too.
The advantage then is that you get to have easy access to the caught exception. In order to be able to add as targetname, you must catch a specific class of exceptions, and only BaseException is going to do that.
You can still access the currently active exception by using sys.exc_info() though:
except:
be = sys.exc_info()[1]
Pick what you feel is more readable for your future self and for your colleagues.

Python: Must non-built-in exceptions be imported in order to catch them?

I'm trying to catch some exceptions thrown by the requests library, with the following try-except block:
try:
get = requests.get((requester.batchesUrl)+str(id)+'/', auth=requester.auth)
except (ConnectionRefusedError, ConnectionError, MaxRetryError) as e:
print("CAUGHT ECONNECTION ERROR")
raise type(e)(str(e) + "Additional Info: Method couldn't connect to website, check that your server is running"
).with_traceback(sys.exc_info()[2])
But instead of catching the exceptions and adding "Additional Info:" to the args, I get NameError: global name 'MaxRetryError' is not defined. Now I know MaxRetryError is an exception in urllib3.exceptions.MaxRetryError. Must I import these non-built-in exception in order to catch them? This seems verbose to me considering the number of possible exceptions one is likely to need to watch out for.
Running: Python 3.3, Windows 7.

You must always always always have a name in an accessible scope before you can refer to it without throwing an exception. If that means importing, then so be it.

If you want to handle these three exceptions differently from other exceptions, then yes, you will have to import them. (You already have at least one of them, since it's at the top level of requests, but the others you may not.)
But do you really need to do that? Trying to restrict yourself to the most detailed exception possible can be as bad as just handling everything. Sometimes both extremes are the right thing to do, but think about what you really want to happen.
If you get a RequestException, but it's not a ConnectionError, do you really want that to print a traceback and abort, or do you want to log that CAUGHT ECONNECTION ERROR and reraise it wrapped in your own type? For that matter, what about an OSError that isn't a RequestException?
As a side note, do you really get urllib3.exception.*Errors from requests? I've only seen them wrapped in a requests.exceptions.RequestException. (And from a quick look at the source to HTTPAdapter.send, there's a except MaxRetryError as e: raise ConnectionError(e, request=request), so I think they're supposed to be wrapped, and if you're seeing them unwrapped that may be a bug you need to file.)
As another side note, in Python 3.3, you don't have to hack with exception messages like that anymore; you can chain and wrap exceptions, add arguments, etc. See Exceptions in the docs for more details. But I think what you want here may be to define your own exception type, then do this:
try:
get = requests.get((requester.batchesUrl)+str(id)+'/', auth=requester.auth)
except OSError as e:
print("CAUGHT ECONNECTION ERROR")
raise EConnectionError("Method couldn't connect to website, check that your server is running")
That will put the original exception in the __context__ of your wrapper exception. If you want it in the __cause__ instead, then raise EConnectionError(…) from e. Either way, it'll show up if you format the traceback.

You must import, etc. in order to catch specific exceptions.
Of course you could simply catch everything.
But the real issue may be that you need to understand why you should catch exceptions.
There are really only few cases I can think of.
1) You catch specific exceptions so that you can respond to them effectively -- If you don't know how to respond to them (requiring an understanding of what they mean and how to respond) -- Since these are generally few in number, this is not a real problem in having to import a number of modules.
2) You catch exceptions so that you can continue processing the next transaction, etc. -- though you abandon the current operation, you may still be able to continue processing acceptablely if the transactions are independent.
3) You catch the exception so that you can log detail, etc. Often re-raising the transaction after you have logged it.
4) You have a top-level exception handler so that you can report the problem in a clean manner to the end-user and possibly do some thing like saving work-in-progress before it is lost.
ADDED
You don't just willy nilly catch exceptions so that you can ignore them unless you like pain.

Python Exception Handling - Best Practices

I'm writing a python program the accesses a database. I want to catch three types of exceptions when I make a http request. Timeouts, network errors, and http errors. I'm looking for the best way to deal with this situation. I need to check for these exceptions multiple times in multiple areas of my code, and it will look something like this each time:
try:
//some request
except timeout:
print '\nException: Timeout Error'
except connection error:
print '\nException: Network Error'
except http error, e:
print 'Exception: %s.' % e
Since I have to do this multiple times, at least probably 8 or more, should I make a module to handle these exceptions or no? Also in which of these cases would it be advisable to shut my system down as opposed to just displaying a message?
Thank you.

If you don't want to use decorators, you can also combine all the except statements, and use some function to handle your exception (assuming that your errors are called TimeoutError, ConnectionError, and HttpError...doesn't really matter for clarity) i.e.
try:
# do stuff
except (TimeoutError, ConnectionError, HttpError) as e:
handle_exception(e)

How should I correctly handle exceptions in Python3

I can't understand what sort of exceptions I should handle 'here and now', and what sort of exceptions I should re-raise or just don't handle here, and what to do with them later (on higher tier). For example: I wrote client/server application using python3 with ssl communication. Client is supposed to verify files on any differences on them, and if diff exists then it should send this 'updated' file to server.
class BasicConnection:
#blablabla
def sendMessage(self, sock, url, port, fileToSend, buffSize):
try:
sock.connect((url, port))
while True:
data = fileToSend.read(buffSize)
if not data: break
sock.send(data)
return True
except socket.timeout as toErr:
raise ConnectionError("TimeOutError trying to send File to remote socket: %s:%d"
% (url,port)) from toErr
except socket.error as sErr:
raise ConnectionError("Error trying to send File to remote socket: %s:%d"
% (url,port)) from sErr
except ssl.SSLError as sslErr:
raise ConnectionError("SSLError trying to send File to remote socket: %s:%d"
% (url,port)) from sslErr
finally:
sock.close()
Is it right way to use exceptions in python? The problem is: what if file.read() throws IOError? Should I handle it here, or just do nothing and catch it later? And many other possible exceptions?
Client use this class (BasicConnection) to send updated files to server:
class PClient():
def __init__(self, DATA):
'''DATA = { 'sendTo' : {'host':'','port':''},
'use_ssl' : {'use_ssl':'', 'fileKey':'', 'fileCert':'', 'fileCaCert':''},
'dirToCheck' : '',
'localStorage': '',
'timeToCheck' : '',
'buffSize' : '',
'logFile' : ''} '''
self._DATA = DATA
self._running = False
self.configureLogging()
def configureLogging(self):
#blablabla
def isRun(self):
return self._running
def initPClient(self):
try:
#blablabla
return True
except ConnectionError as conErr:
self._mainLogger.exception(conErr)
return False
except FileCheckingError as fcErr:
self._mainLogger.exception(fcErr)
return False
except IOError as ioErr:
self._mainLogger.exception(ioErr)
return False
except OSError as osErr:
self._mainLogger.exception(osErr)
return False
def startPClient(self):
try:
self._running = True
while self.isRun():
try :
self._mainLogger.debug("Checking differences")
diffFiles = FileChecker().checkDictionary(self._dict)
if len(diffFiles) != 0:
for fileName in diffFiles:
try:
self._mainLogger.info("Sending updated file: %s to remote socket: %s:%d"
% (fileName,self._DATA['sendTo']['host'],self._DATA['sendTo']['port']))
fileToSend = io.open(fileName, "rb")
result = False
result = BasicConnection().sendMessage(self._sock, self._DATA['sendTo']['host'],
self._DATA['sendTo']['port'], fileToSend, self._DATA['buffSize'])
if result:
self._mainLogger.info("Updated file: %s was successfully delivered to remote socket: %s:%d"
% (fileName,self._DATA['sendTo']['host'],self._DATA['sendTo']['port']))
except ConnectionError as conErr:
self._mainLogger.exception(conErr)
except IOError as ioErr:
self._mainLogger.exception(ioErr)
except OSError as osErr:
self._mainLogger.exception(osErr)
self._mainLogger.debug("Updating localStorage %s from %s " %(self._DATA['localStorage'], self._DATA['dirToCheck']))
FileChecker().updateLocalStorage(self._DATA['dirToCheck'],
self._DATA['localStorage'])
self._mainLogger.info("Directory %s were checked" %(self._DATA['dirToCheck']))
time.sleep(self._DATA['timeToCheck'])
except FileCheckingError as fcErr:
self._mainLogger.exception(fcErr)
except IOError as ioErr:
self._mainLogger.exception(ioErr)
except OSError as osErr:
self._mainLogger.exception(osErr)
except KeyboardInterrupt:
self._mainLogger.info("Shutting down...")
self.stopPClient()
except Exception as exc:
self._mainLogger.exception(exc)
self.stopPClient()
raise RuntimeError("Something goes wrong...") from exc
def stopPClient(self):
self._running = False
Is it correct? May be someone spend his own time and just help me to understand pythonic style of handling exceptions? I can't understand what to do with such exceptions as NameError, TypeError, KeyError, ValueError...and so on.......They could be thrown at any statement, at any time... and what to do with them, if I want to logged everything.
And what information should people usually log? If error occurs, what info about it I should log? All traceback, or just relevant message about it or something else?
I hope somebody helps me.
Thanks a lot.

In general, you should "catch" the exceptions that you expect to happen (because they may be caused by user error, or other environmental problems outside of your program's control), especially if you know what your code might be able to do about them. Just giving more details in an error report is a marginal issue, though some programs' specs may require doing that (e.g. a long-running server that's not supposed to crash due to such problems, but rather log a lot of state information, give the user a summary explanation, and just keep working for future queries).
NameError, TypeError, KeyError, ValueError, SyntaxError, AttributeError, and so on, can be thought of as due to errors in the program -- bugs, not problems outside of the programmer's control. If you're releasing a library or framework, so that your code is going to be called by other code outside of your control, then such bugs may quite likely be in that other code; you should normally let the exception propagate to help the other programmer debug their own bugs. If you're releasing an application, you own the bugs, and you must pick the strategy that helps you find them.
If your bugs show up while an end-user is running the program, you should log a lot of state information, and give the user a summary explanation and apologies (perhaps with a request to send you the log info, if you can't automate that -- or, at least, ask permission before you send anything from the user's machine to yours). You may be able to save some of the user's work so far, but often (in a program that's known to be buggy) that may not work anyway.
Most bugs should show up during your own testing of course; in that case, propagating the exception is useful as you can hook it up to a debugger and explore the bug's details.
Sometimes some exceptions like these show up just because "it's easier to ask forgiveness than permission" (EAFP) -- a perfectly acceptable programming technique in Python. In that case of course you should handle them at once. For example:
try:
return mylist[theindex]
except IndexError:
return None
here you might expect that theindex is generally a valid index into mylist, but occasionally outside of mylist's bounds -- and the latter case, by the semantics of the hypothetic app in which this snippet belongs, is not an error, just a little anomaly to be fixed by considering the list to be conceptually extended on both sides with infinite numbers of Nones. It's easier to just try/except than to properly check for positive and negative values of the index (and faster, if being out of bounds is a truly rare occurrence).
Similarly appropriate cases for KeyError and AttributeError happen less frequently, thanks to the getattr builtin and get method of dicts (which let you provide a default value), collections.defaultdict, etc; but lists have no direct equivalent of those, so the try/except is seen more frequently for IndexError.
Trying to catch syntax errors, type errors, value errors, name errors, etc, is a bit rarer and more controversial -- though it would surely be appropriate if the error was diagnosed in a "plug-in", third-party code outside your control which your framework/application is trying to load and execute dynamically (indeed that's the case where you're supplying a library or the like and need to coexist peacefully with code out of your control which might well be buggy). Type and value errors may sometimes occur within an EAFP pattern -- e.g. when you try to overload a function to accept either a string or a number and behave slightly differently in each case, catching such errors may be better than trying to check types -- but the very concept of functions thus overloaded is more often than not quite dubious.
Back to "user and environmental errors", users will inevitably make mistakes when they give you input, indicate a filename that's not actually around (or that you don't have permission to read, or to write if that's what you're supposed to be doing), and so on: all such errors should of course be caught and result in a clear explanation to the user about what's gone wrong, and another chance to get the input right. Networks sometime go down, databases or other external servers may not respond as expected, and so forth -- sometimes it's worth catching such problems and retrying (maybe after a little wait -- maybe with an indication to the user about what's wrong, e.g. they may have accidentally unplugged a cable and you want to give them a chance to fix things and tell you when to try again), sometimes (especially in unattended long-running programs) there's nothing much you can do except an ordered shutdown (and detailed logging of every possibly-relevant aspect of the environment).
So, in brief, the answer to your Q's title is, "it depends";-). I hope I have been of use in listing many of the situations and aspects on which it can depend, and recommending what's generally the most useful attitude to take towards such issues.

To start with, you don't need any _mainLogger.
If you want to catch any exceptions, maybe to log or send them by email or whatever, do that at the highest possible level -- certainly not inside this class.
Also, you definitely don't want to convert every Exception to a RuntimeError. Let it emerge. The stopClient() method has no purpose right now. When it has, we'll look at it..
You could basically wrap the ConnectionError, IOError and OSError together (like, re-raise as something else), but not much more than that...