How should I correctly handle exceptions in Python3

Question

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?

1. 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.

2. 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?

3. I hope somebody helps me. Thanks a lot.

Answer 1

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.