Custom Exception Class,
class CustomFileExcetion(FileNotFoundError):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
print(*args, **kwargs)
Custom Exception Used,
def oppen_file(file_path):
try:
open('xyx')
except CustomFileExcetion as exp:
print(exp)
print('100')
I am trying to make custom exception, but thing is that it does not capture
my exception, it throws exception
Exception Raised,
Traceback (most recent call last):
File "C:/Users/mario/file_exception.py", line 14, in <module>
open('xyx')
FileNotFoundError: [Errno 2] No such file or directory: 'xyx'
But when replace CustomFileException with FileNotFoundError or Exception, it captures exception and output as
[Errno 2] No such file or directory: 'xyx'
100
open raises a FileNotFoundError. That's what it does, you can't change that.
You're extending FileNotFoundError with a new subclass. That subclass is a more specific class than FileNotFoundError. If you're catching that more specific subclass, that won't match the more generic parent class. I.e., except CustomFileExcetion [sic] will not catch FileNotFoundError.
If you want to use your own custom exceptions, you'll have to raise them yourself. E.g. you could make your oppen_file [sic] function to use that custom exception:
def oppen_file(file_path):
try:
open(file_path)
except FileNotFoundError as e:
raise CustomFileExcetion from e
Now every caller of oppen_file can expect to use CustomFileExcetion:
try:
oppen_file(path)
except CustomFileExcetion:
...
Usually you would implement your own custom exceptions for your own business logic; i.e. there's little sense in duplicating/extending/customising a FileNotFoundError, but you may implement entirely new exceptions which are relevant to your specific program, like UserPermissionsInsufficientError or CoordinatesOutOfBoundsError or whatever makes sense in your specific domain.
Your CustomFileExcetion is a sub-class to the FileNotFoundError; your code throws the latter which can only be caught by the same class or its parent-class or super-class. You cannot catch an exception by its sub-class.
To achieve your purpose, you need to raise your custom-exception and then catch it.
Something like #deceze wrote.
Related
some_function() raises an exception while executing, so the program jumps to the except:
try:
some_function()
except:
print("exception happened!")
How do I see what caused the exception to occur?
The other answers all point out that you should not catch generic exceptions, but no one seems to want to tell you why, which is essential to understanding when you can break the "rule". Here is an explanation. Basically, it's so that you don't hide:
the fact that an error occurred
the specifics of the error that occurred (error hiding antipattern)
So as long as you take care to do none of those things, it's OK to catch the generic exception. For instance, you could provide information about the exception to the user another way, like:
Present exceptions as dialogs in a GUI
Transfer exceptions from a worker thread or process to the controlling thread or process in a multithreading or multiprocessing application
So how to catch the generic exception? There are several ways. If you just want the exception object, do it like this:
try:
someFunction()
except Exception as ex:
template = "An exception of type {0} occurred. Arguments:\n{1!r}"
message = template.format(type(ex).__name__, ex.args)
print message
Make sure message is brought to the attention of the user in a hard-to-miss way! Printing it, as shown above, may not be enough if the message is buried in lots of other messages. Failing to get the users attention is tantamount to swallowing all exceptions, and if there's one impression you should have come away with after reading the answers on this page, it's that this is not a good thing. Ending the except block with a raise statement will remedy the problem by transparently reraising the exception that was caught.
The difference between the above and using just except: without any argument is twofold:
A bare except: doesn't give you the exception object to inspect
The exceptions SystemExit, KeyboardInterrupt and GeneratorExit aren't caught by the above code, which is generally what you want. See the exception hierarchy.
If you also want the same stacktrace you get if you do not catch the exception, you can get that like this (still inside the except clause):
import traceback
print traceback.format_exc()
If you use the logging module, you can print the exception to the log (along with a message) like this:
import logging
log = logging.getLogger()
log.exception("Message for you, sir!")
If you want to dig deeper and examine the stack, look at variables etc., use the post_mortem function of the pdb module inside the except block:
import pdb
pdb.post_mortem()
I've found this last method to be invaluable when hunting down bugs.
Get the name of the class that exception object belongs:
e.__class__.__name__
and using print_exc() function will also print stack trace which is essential info for any error message.
Like this:
from traceback import print_exc
class CustomException(Exception): pass
try:
raise CustomException("hi")
except Exception as e:
print ('type is:', e.__class__.__name__)
print_exc()
# print("exception happened!")
You will get output like this:
type is: CustomException
Traceback (most recent call last):
File "exc.py", line 7, in <module>
raise CustomException("hi")
CustomException: hi
And after print and analysis, the code can decide not to handle exception and just execute raise:
from traceback import print_exc
class CustomException(Exception): pass
def calculate():
raise CustomException("hi")
try:
calculate()
except CustomException as e:
# here do some extra steps in case of CustomException
print('custom logic doing cleanup and more')
# then re raise same exception
raise
Output:
custom logic doing cleanup and more
And interpreter prints exception:
Traceback (most recent call last):
File "test.py", line 9, in <module>
calculate()
File "test.py", line 6, in calculate
raise CustomException("hi")
__main__.CustomException: hi
After raise original exception continues to propagate further up the call stack. (Beware of possible pitfall) If you raise new exception it caries new (shorter) stack trace.
from traceback import print_exc
class CustomException(Exception):
def __init__(self, ok):
self.ok = ok
def calculate():
raise CustomException(False)
try:
calculate()
except CustomException as e:
if not e.ok:
# Always use `raise` to rethrow exception
# following is usually mistake, but here we want to stress this point
raise CustomException(e.ok)
print("handling exception")
Output:
Traceback (most recent call last):
File "test.py", line 13, in <module>
raise CustomException(e.message)
__main__.CustomException: hi
Notice how traceback does not include calculate() function from line 9 which is the origin of original exception e.
You usually should not catch all possible exceptions with try: ... except as this is overly broad. Just catch those that are expected to happen for whatever reason. If you really must, for example if you want to find out more about some problem while debugging, you should do
try:
...
except Exception as ex:
print ex # do whatever you want for debugging.
raise # re-raise exception.
Most answers point to except (…) as (…): syntax (rightly so) but at the same time nobody wants to talk about an elephant in the room, where the elephant is sys.exc_info() function.
From the documentation of sys module (emphasis mine):
This function returns a tuple of three values that give information
about the exception that is currently being handled.
(…)
If no exception is being handled anywhere on the stack, a tuple
containing three None values is returned. Otherwise, the values
returned are (type, value, traceback). Their meaning is: type gets the
type of the exception being handled (a subclass of BaseException);
value gets the exception instance (an instance of the exception type);
traceback gets a traceback object (see the Reference Manual) which
encapsulates the call stack at the point where the exception
originally occurred.
I think the sys.exc_info() could be treated as the most direct answer to the original question of How do I know what type of exception occurred?
These answers are fine for debugging, but for programmatically testing the exception, isinstance(e, SomeException) can be handy, as it tests for subclasses of SomeException too, so you can create functionality that applies to hierarchies of exceptions.
Unless somefunction is a very bad coded legacy function, you shouldn't need what you're asking.
Use multiple except clause to handle in different ways different exceptions:
try:
someFunction()
except ValueError:
# do something
except ZeroDivision:
# do something else
The main point is that you shouldn't catch generic exception, but only the ones that you need to. I'm sure that you don't want to shadow unexpected errors or bugs.
In Python 2, the following are useful
except Exception, exc:
# This is how you get the type
excType = exc.__class__.__name__
# Here we are printing out information about the Exception
print 'exception type', excType
print 'exception msg', str(exc)
# It's easy to reraise an exception with more information added to it
msg = 'there was a problem with someFunction'
raise Exception(msg + 'because of %s: %s' % (excType, exc))
Use type class and as statement
try:#code
except Exception as e:
m=type(e)
#m is the class of the exception
strm=str(m)
#strm is the string of m
Hope this will help a little more
import sys
varExcepHandling, varExcepHandlingZer = 2, 0
try:
print(varExcepHandling/varExcepHandlingZer)
except Exception as ex:
print(sys.exc_info())
'sys.exc_info()' will return a tuple, if you only want the exception class name use 'sys.exc_info()[0]'
Note:- if you want to see all the exception classes just write dir(__builtin__)
Here's how I'm handling my exceptions. The idea is to do try solving the issue if that's easy, and later add a more desirable solution if possible. Don't solve the issue in the code that generates the exception, or that code loses track of the original algorithm, which should be written to-the-point. However, pass what data is needed to solve the issue, and return a lambda just in case you can't solve the problem outside of the code that generates it.
path = 'app.p'
def load():
if os.path.exists(path):
try:
with open(path, 'rb') as file:
data = file.read()
inst = pickle.load(data)
except Exception as e:
inst = solve(e, 'load app data', easy=lambda: App(), path=path)()
else:
inst = App()
inst.loadWidgets()
# e.g. A solver could search for app data if desc='load app data'
def solve(e, during, easy, **kwargs):
class_name = e.__class__.__name__
print(class_name + ': ' + str(e))
print('\t during: ' + during)
return easy
For now, since I don't want to think tangentially to my app's purpose, I haven't added any complicated solutions. But in the future, when I know more about possible solutions (since the app is designed more), I could add in a dictionary of solutions indexed by during.
In the example shown, one solution might be to look for app data stored somewhere else, say if the 'app.p' file got deleted by mistake.
For now, since writing the exception handler is not a smart idea (we don't know the best ways to solve it yet, because the app design will evolve), we simply return the easy fix which is to act like we're running the app for the first time (in this case).
To add to Lauritz's answer, I created a decorator/wrapper for exception handling and the wrapper logs which type of exception occurred.
class general_function_handler(object):
def __init__(self, func):
self.func = func
def __get__(self, obj, type=None):
return self.__class__(self.func.__get__(obj, type))
def __call__(self, *args, **kwargs):
try:
retval = self.func(*args, **kwargs)
except Exception, e :
logging.warning('Exception in %s' % self.func)
template = "An exception of type {0} occured. Arguments:\n{1!r}"
message = template.format(type(e).__name__, e.args)
logging.exception(message)
sys.exit(1) # exit on all exceptions for now
return retval
This can be called on a class method or a standalone function with the decorator:
#general_function_handler
See my blog about for the full example: http://ryaneirwin.wordpress.com/2014/05/31/python-decorators-and-exception-handling/
You can start as Lauritz recommended, with:
except Exception as ex:
and then just to print ex like so:
try:
#your try code here
except Exception as ex:
print ex
Your question is: "How can I see exactly what happened in the someFunction() that caused the exception to happen?"
It seems to me that you are not asking about how to handle unforeseen exceptions in production code (as many answers assumed), but how to find out what is causing a particular exception during development.
The easiest way is to use a debugger that can stop where the uncaught exception occurs, preferably not exiting, so that you can inspect the variables. For example, PyDev in the Eclipse open source IDE can do that. To enable that in Eclipse, open the Debug perspective, select Manage Python Exception Breakpoints in the Run menu, and check Suspend on uncaught exceptions.
Use the below for both Exception type and Exception text
import sys
print(str(sys.exc_info()[0]).split(' ')[1].strip('>').strip("'")+"-"+(str(sys.exc_info()[1])))
if you want only exception type: Use -->
import sys
print(str(sys.exc_info()[0]).split(' ')[1].strip('>').strip("'"))
Thanks Rajeshwar
The actual exception can be captured in the following way:
try:
i = 1/0
except Exception as e:
print e
You can learn more about exceptions from The Python Tutorial.
Just refrain from catching the exception and the traceback that Python prints will tell you what exception occurred.
look at this:
RuntimeError: No active exception to reraise
I use raise. with out error like this:
class example:
def __getattribute__(self, attr_name):
raise # I mean: AttributeError: '...' object has no attribute '...'
This is raise statement:
raise_stmt ::= "raise" [expression ["from" expression]]
expression is OPTIONAL.
I check this, but this isn't my answer. if error says "No active exception to reraise", so I can active an error. I do not know what this error means. My question is, what is meant by "active exception" and where it is used? Does it help make the code shorter and more optimized? Is it possible to use it for the task I showed a little higher in the code?
When you use raise keyword barely, Python tries to re-raise the currently occurred exception in the current scope, If there is no exception triggered on, you will get RuntimeError: No active exception to re-raise.
To see which exception is active(being handled), you can use sys.exc_info():
import sys
try:
raise ZeroDivisionError()
except ZeroDivisionError:
type_, value, tb = sys.exc_info()
print(type_) # <class 'ZeroDivisionError'>
In the above except block you can use bare raise keyword, which re-raised the ZeroDivisionError exception for you.
If there is no active exception, the returned value of sys.exc_info() is (None, None, None). So you have to use raise keyword followed by a subclass or an instance of BaseException. This is the case in your question inside __getattribute__ method since there is no active exception.
class Example:
def __getattribute__(self, attr_name):
raise AttributeError(f'Error for "{attr_name}".')
obj = Example()
obj.foo # AttributeError: Error for "foo".
From comments:
Active exception means the exception that is currently triggered on, and is in the workflow, If you don't catch it and let it bubbles up, it will terminate the process.
I can find this for my questions:
what is meant by "active exception" and where it is used?
then using try-except and code goes to except block, error actives.
about usage we can check error and if Should not be handled, can use
raise without arg (Even In Functions). now error raised without
Reference to line. I do not know if there is another way to do this or not, but I was convinced of this method.
Example:
>>> try:
... 10 / 0
... except[ ZeroDivisionError[ as err]]:
... raise
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero
>>> def test():
... raise
...
>>> try:
... 10 / 0
... except[ ZeroDivisionError[ as err]]:
... test()
...
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero
>>>
Does it help make the code shorter and more optimized?
Yes. By using this method, the code and error are shortened. But in this method, there is less control over the exception procedure, which can sometimes be problematic.
Is it possible to use it for the task I showed a little higher in the code?
No. NotImplemented must be returned for the above code to take effect. I have full confidence in this, but there is a possibility that this alone is not enough.
I would like to use raise without printing the traceback on the screen. I know how to do that using try ..catch but doesn't find a way with raise.
Here is an example:
def my_function(self):
resp = self.resp
if resp.status_code == 404:
raise NoSuchElementError('GET'+self.url+'{}'.format(resp.status_code))
elif resp.status_code == 500:
raise ServerErrorError('GET'+self.url+'{}'.format(resp.status_code))
When executing this, if I have a 404, the traceback will print on the screen.
Traceback (most recent call last):
File "test.py", line 32, in <module>
print ins.my_function()
File "api.py", line 820, in my_function
raise NoSuchElementError('GET ' + self.url + ' {} '.format(resp.status_code))
This is an API wrapper and I don't want users to see the traceback but to see the API response codes and error messages instead.
Is there a way to do it ?
I ran into a similar problem where a parent class was using the exception value on raise to pass messages through but where I didn't want to dump the traceback. #lejlot gives a great solution using sys.excepthook but I needed to apply it with a more limited scope. Here's the modification:
import sys
from contextlib import contextmanager
#contextmanager
def except_handler(exc_handler):
"Sets a custom exception handler for the scope of a 'with' block."
sys.excepthook = exc_handler
yield
sys.excepthook = sys.__excepthook__
Then, to use it:
def my_exchandler(type, value, traceback):
print(': '.join([str(type.__name__), str(value)]))
with except_handler(my_exchandler):
raise Exception('Exceptional!')
# -> Exception: Exceptional!
That way, if an exception isn't raised in the block, default exception handling will resume for any subsequent exceptions:
with except_handler(my_exchandler):
pass
raise Exception('Ordinary...')
# -> Traceback (most recent call last):
# -> File "raise_and_suppress_traceback.py", line 22, in <module>
# -> raise Exception('Ordinary...')
# -> Exception: Ordinary...
The problem is not with raising anything, but with what python interpreter does, when your program terminates with an exception (and it simply prints the stack trace). What you should do if you want to avoid it, is to put try except block around everything that you want to "hide" the stack trace, like:
def main():
try:
actual_code()
except Exception as e:
print(e)
The other way around is to modify the exeption handler, sys.excepthook(type, value, traceback), to do your own logic, like
def my_exchandler(type, value, traceback):
print(value)
import sys
sys.excepthook = my_exchandler
you can even condition of exception type and do the particular logic iff it is your type of exception, and otherwise - backoff to the original one.
Modified #Alec answer:
#contextmanager
def disable_exception_traceback():
"""
All traceback information is suppressed and only the exception type and value are printed
"""
default_value = getattr(sys, "tracebacklimit", 1000) # `1000` is a Python's default value
sys.tracebacklimit = 0
yield
sys.tracebacklimit = default_value # revert changes
Usage:
with disable_exception_traceback():
raise AnyYourCustomException()
Use this if you only need to hide a traceback without modifying an exception message. Tested on Python 3.8
UPD: code improved by #DrJohnAStevenson comment
Catch the exception, log it and return something that indicates something went wrong to the consumer (sending a 200 back when a query failed will likely cause problems for your client).
try:
return do_something()
except NoSuchElementError as e:
logger.error(e)
return error_response()
The fake error_response() function could do anything form returning an empty response or an error message. You should still make use of proper HTTP status codes. It sounds like you should be returning a 404 in this instance.
You should handle exceptions gracefully but you shouldn't hide errors from clients completely. In the case of your NoSuchElementError exception it sounds like the client should be informed (the error might be on their end).
You can create a class that takes two values; Type and code for a custom Exception Message. Afterwards, you can just pass the class in a try/except statement.
class ExceptionHandler(Exception):
def __init__(self, exceptionType, code):
self.exceptionType = exceptionType
self.code = code
print(f"Error logged: {self.exceptionType}, Code: {self.code}")
try:
raise(ExceptionHandler(exceptionType=KeyboardInterrupt, code=101))
except Exception:
pass
I have a method that needs some wrapping called joule, so I wrap that joule method inside a wrapper called respond (which you will see shortly):
someclass.respond(somemodule.joule(someArgument, someStrategy), 202)
I have a wrapper called respond:
#classmethod
def respond(cls, method, successStatus):
try:
method, successStatus
except Exception as e:
return {
'status': 500,
'message': str(e)
}
The actual method that gets called and raises an Exception:
def joule(params, strategy):
try:
return strategy(params)
finally:
session.rollback()
conn.execute('UNLOCK TABLES')
For some reason, the re-raised exception does not seem to get caught in the respond wrapper! Can you folks help me understand what am I doing incorrectly here?
If this helps, the exception being thrown by sqlalchemy is (please note that this is a scenario being forcibly created to handle the exception correctly):
ProgrammingError: (ProgrammingError) (1146, u"Table 'matrix.vmop_queue' doesn't exist") 'LOCK TABLES vmop_queue WRITE' ()
You are misunderstanding how exception handling works. Exception handling operates on the stack frame of called functions.
In the example you give, someclass.respond does not actually invoke somemodule.joule, instead wherever the line that you have written in your example, which is some outer context is the place that receives the uncaught exception. Thus someclass.respond can't possibly handle the exception thrown by somemodule.joule.
There are other ways to achieve what you are trying to accomplish, but I would need more context in order to give you a better suggestion.
To make this a bit more concrete, let's say that foo contains the example line you gave:
def foo():
someclass.respond(somemodule.joule(someArgument, someStrategy), 202)
You could add the try block to foo to handle the exception thrown by somemodule.joule. This would look like this:
def foo():
try:
someclass.respond(somemodule.joule(someArgument, someStrategy), 202)
except Exception as e:
pass # do something with the exception here
Alternatively, if the whole purpose for someclass.respond is to handle this exception, then you should move the invocation of somemodule.joule inside of someclass.respond. You could even do this more than one way. You could generically take a function and its arguments, and apply that function to the arguments inside of someclass.respond or you could just directly do the invocation inside of someclass.respond.
Let's take the first approach, since you've said that you don't want to repeat the exception handling. I'll call this new method exception_catcher:
def exception_catcher(func, *args):
try:
return func(*args)
except Exception as e:
pass # do whatever you want to the exception
Now the foo context will look like this:
def foo():
exception_catcher(somemodule.joule, someArgument, someStrategy)
Note that exception_catcher is taking somemodule.joule as an argument, and the remaining arguments will be passed to somemodule.joule from within exception_catcher.
Using custom exceptions in python (v2.7.3): don't get a stack trace when call getSub() whereas getSub(True) invokes one, the difference being that it is caused via an additional try...except which I want to avoid (+feels unnecessary) so why/how can it be avoided?
import sys, traceback
class customException(Exception):
def __init__(self, *args):
super(customException,self).__init__(*args)
print "Stack trace within exception", traceback.extract_tb(sys.exc_info()[2])
errTxt = [a for a in args]
print "error text", errTxt
def getFn():
try:
getSub()
except customException as e:
print "customException was raised"
try:
getSub(True)
except customException as e:
print "customException2 was raised"
def getSub(flag=False):
if flag:
try:
1/0
except:
raise customException('test')
else:
raise customException('test')
getFn()
output:
Stack trace within exception []
error text ['test']
customException was raised
Stack trace within exception [('./test3.py', 25, 'getSub', '1/0')]
error text ['test']
customException2 was raised
To put the above in context I have code (pseudo) along the below lines and it wasn’t until I boiled down the code to the above example that I realised why the traceback access was not always working. The purpose of the custom exception class is to collect an aggregate count of exceptions in order to classify downstream the result of each do_something, e.g. fatal, warning etc. The use of the traceback was to record from WHERE the exception was ‘raised’ and therefore the creation of an exception (1/0 = albeit seems out of place) enables that to work. Wondering about using the inspect module rather than thinking about this within the traceback stack?
__main__
With each item in set:
Try:
do_something(item)
except customException()
clean up = log info etc.
end With
end.__main__
do_something(item)
try:
check_something()
except customException
if exception=typeA.1 raise customException(Type1)
if exception=typeB.2 and item = ‘x’
raise customException(Type2)
check_something()
a = getInfo()
unless getInfo()
raise customException(typeA.1)
try:
b = getOtherInfo()
except customException
raise customException(typeB.2)
…
If I understand you right, you're wondering why your print "Stack trace within exception" line prints an empty list for getSub(), but prints some traceback info for getSub(True).
You have code in your exception class's __init__ to look at the most recent exception via sys.exc_info. When you do raise customException('test'), customException('test') is evaluated first on its own, before it "knows" that it is going to be raised as an exception. So when you do that raise in getSub(), there is no most recent exception.
With getSub(True), there is a most recent exception, because the 1/0 raises an exception before you create your customException. Note that when you do 1/0, the most recent exception is the one from that 1/0; you can see that there's nothing about your customException in that traceback.
An exception object has to be created before the exception is raised. So you can't look at "the current exception" in your exception class's __init__ to get info about the stack trace that will be created when it is raised, because it hasn't been raised yet at that time.
If you want, you could use traceback.extract_stack to get the call stack at the time the exception object is created, but there's no guarantee that that has anything to do with when it will be raised. Just because an instance of your exception class is created doesn't even mean it will ever be raised at all. It's perfectly legal (although generally pointless) for someone to just create an exception object with stuff = customException('blah') but never actually raise the exception.
In any case, it's not clear from your question what you're trying to achieve here. It would help if you explained that.
Because you're catching the exception, there will be no traceback unless you either explicitly re-raise
try:
getSub(True)
except customException as e:
print "customException2 was raised"
raise # add this to re-raise, with original traceback
or print it yourself:
try:
getSub(True)
except customException as e:
print "customException2 was raised"
print traceback.format_exc()