Can I change the behaviour of "raise" or "Exception"? [duplicate] - python

This question already has answers here:
Calling a hook function every time an Exception is raised
(4 answers)
Closed 6 years ago.
My project's code is full of blocks like the following:
try:
execute_some_code()
except Exception:
print(datetime.datetime.now())
raise
simply because, if I get an error message, I'd like to know when it happened. I find it rather silly to repeat this code over and over, and I'd like to factor it away.
I don't want to decorate execute_some_code with something that does the error capturing (because sometimes it's just a block of code rather than a function call, and sometimes I don't need the exact same function to be decorated like that). I also don't want to divert stdout to some different stream that logs everything, because that would affect every other thing that gets sent to stdout as well.
Ideally, I'd like to over-ride the behaviour of either the raise statement (to also print datetime.datetime.now() on every execution) or the Exception class, to pre-pend all of its messages with the time. I can easily sub-class from Exception, but then I'd have to make sure my functions raise an instance of this subclass, and I'd have just as much code duplication as currently.
Is either of these options possible?

You might be able to modify python (I'd have to read code to be sure how complex that'd be), but:
You do not want to replace raise with different behaviour - trying and catching is a very pythonic approach to problem solving, so there's lots of code that works very well by e.g. calling a method and letting that method raise an exception, catching that under normal circumstances. So we can rule that approach out – you really only want to know about the exceptions you care about, not the ones that are normal during operation.
The same goes for triggering some action whenever an Exception instance is created – but:
You might be able to overwrite the global namespace; at least for things that get initialized after you declared your own Exception class. You could then add a message property that includes a timestamp. Don't do that, though – there might be people actually relying on the message to automatically react to Exceptions (bad style, but still not really seldom, sadly).

Related

Python determine whether an exception was thrown (regardless of whether it is caught or not)

I am writing tests for some legacy code that is littered with catch-all constructs like
try:
do_something()
do_something_else()
for x in some_huge_list():
do_more_things()
except Exception:
pass
and I want to tell whether an exception was thrown inside the try block.
I want to avoid introducing changes into the codebase just to support a few tests and I don't want to make the except cases more specific for fear of unintentionally introducing regressions.
Is there a way of extracting information about exceptions that were raised and subsequently handled from the runtime? Or some function with a similar API to eval/exec/apply/call that either records information on every raised exception, lets the user supply an exception handler that gets run first, or lets the user register a callback that gets run on events like an exception being raised or caught.
If there isn't a way to detect whether an exception was thrown without getting under the (C)Python runtime in a really nasty way, what are some good strategies for testing code with catch-all exceptions inside the units you're testing?
Your only realistic option is to instrument the except handlers.
Python does record exception information, which is retrievable with sys.exc_info(), but this information is cleared when a function exits (Python 2) or the try statement is done (Python 3).
A good strategy would be testing observable behaviour. Since exceptions were explicitly excluded from the observable behaviour I do not think you should be testing whether an exception was raised or not.

Python exception base class vs. specific exception

I just wonder, when using try.. except, what's the difference between using the Base class "Except" and using a specific exception like "ImportError" or "IOError" or any other specific exception. Are there pros and cons between one and the other?
Never catch the base exception. Always capture only the specific exceptions that you know how to handle. Everything else should be left alone; otherwise you are potentially hiding important errors.
Of course it has advantages using the correct exception for the corresponding issue. However Python already defined all possible errors for coding problems. But you can make your own exception classes by inheriting from the Exception class. This way you can make more meaningful errors for spesific parts of your code. You can even make the expections print errors like this.
SomeError: 10 should have been 5.
Provides easier debugging of the code.
For more info.

Python - Why is an exception type needed to be put after an 'except'? [duplicate]

This question already has answers here:
Should I always specify an exception type in `except` statements?
(7 answers)
How to properly ignore exceptions
(12 answers)
Closed 8 years ago.
When you use try/except in python, why do you need an exception type to be named after except? Wouldn't it be easier to just catch all exceptions?
try:
#dosomething
except Exception:
#dosomething
Why is the 'Exception' needed after except?
Because you might handle different exceptions different ways.
For example, if you're attempting a network operation, and the network address you're trying to reach can't be resolved, that's likely due to user error, and you'll want to get the user involved, while some other kinds of errors can simply be retried after a short wait.
It's a good practice in exception handling to handle only the narrowest set of exceptions you expect at any one point in the code, and to only catch those exceptions that you're sure you know how to handle. A catch-all exception handler violates this principle.
From a purely syntactic point of view, this is acceptable code:
try:
# Do something
except:
print "Something went wrong."
HOWEVER, it's not a very good idea to do this a lot of the time. By catching all exceptions and not even saving the name, you're losing all information about where the error was. Just seeing Something went wrong. printed out is both useless and frustrating. So even if you don't want to handle each exception individually, you'd want to save the exception information at the very least.
try:
# Do something.
except Exception, e:
print "Encountered error " + str(e) + " during execution. Exiting gracefully."
The above code is something you might do if you absolutely can't let your program exit abruptly, for example.
EDIT: changed the answer to clarify that it's a bad idea, though possible.
Why and how to catch exceptions
Exceptions are really helpful, but they shall be handled in different manner depending on what code
you write.
Core distinction is, if your code is top level one (last resort to handle exceptions) or inner one.
Another aspect is, if some exceptions are excepted or unexpected.
Expected exceptions (like file, you are trying to use is missing) shall be handled, if the code has
a chance to do anything about it.
Unexpected exceptions shall not be handled unless you have to do so in top level code.
Handling exceptions in top level code
If it does not matter, that your code throws a bit ugly stack trace under some circumstances, simply
ingore the unexpected exceptions. This is mostly very efficient, as the code is kept simple, and
stack trace give you good chance to find out, what went wrong.
If you have to make your script "well behaving" - you could catch the exception and print some nice
looking excuse for what went wrong.
Handling exceptions in lower level code (modules, functions)
In your lower level code, you shall catch all expected exceptions, and the rest shall be ignored and
thrown up to higher levels, where is better chance to handle it properly.
If you have no expected exception, simply do not use try .. except block.
Printing some excuses from lower level code is mostly inappropriate (higher level code has no chance
t silence your printouts.
To your question - why except Exception
except with explicitly mentioned type of exception is the only solution to use for expected types
of exceptions. Without mentioning the type (or types), you are catching all and this is bad habit
unless you are in top level code.
As usual, there are exceptions to the recommendations above, but they are occurring less often than one
usually expects.
Different exceptions require different fixing. For example, when I was writing a python irc bot, i would have one exception for invalid access to a string, and that code in the except would try to remedy it. I also had one for bad sockets that would try to deduce why it went bad and fix it. I can't have these under one exception because they are fixed differently

What is the pythonic way to bubble up error conditions

I have been working on a Python project that has grown somewhat large, and has several layers of functions. Due to some boneheaded early decisions I'm finding that I have to go a fix a lot of crashers because the lower level functions are returning a type I did not expect in the higher level functions (usually None).
Before I go through and clean this up, I got to wondering what is the most pythonic way of indicating error conditions and handling them in higher functions?
What I have been doing for the most part is if a function can not complete and return its expected result, I'll return None. This gets a little gross, as you end up having to always check for None in all the functions that call it.
def lowLevel():
## some error occurred
return None
## processing was good, return normal result string
return resultString
def highLevel():
resultFromLow = lowLevel()
if not resultFromLow:
return None
## some processing error occurred
return None
## processing was good, return normal result string
return resultString
I guess another solution might be to throw exceptions. With that you still get a lot of code in the calling functions to handle the exception.
Nothing seems super elegant. What do other people use? In obj-c a common pattern is to return an error parameter by reference, and then the caller checks that.
It really depends on what you want to do about the fact this processing can't be completed. If these are really exceptions to the ordinary control flow, and you need to clean up, bail out, email people etc. then exceptions are what you want to throw. In this case the handling code is just necessary work, and is basically unavoidable, although you can rethrow up the stack and handle the errors in one place to keep it tidier.
If these errors can be tolerated, then one elegant solution is to use the Null Object pattern. Rather than returning None, you return an object that mimics the interface of the real object that would normally be returned, but just doesn't do anything. This allows all code downstream of the failure to continue to operate, oblivious to the fact there was a failure. The main downside of this pattern is that it can make it hard to spot real errors, since your code will not crash, but may not produce anything useful at the end of its run.
A common example of the Null Object pattern in Python is returning an empty list or dict when you're lower level function has come up empty. Any subsequent function using this returned value and iterating through elements will just fall through silently, without the need for error checking. Of course, if you require the list to have at least one element for some reason, then this won't work, and you're back to handling an exceptional situation again.
On the bright side, you have discovered exactly the problem with using a return value to indicate an error condition.
Exceptions are the pythonic way to deal with problems. The question you have to answer (and I suspect you already did) is: Is there a useful default that can be returned by low_level functions? If so, take it and run with it; otherwise, raise an exception (`ValueError', 'TypeError' or even a custom error).
Then, further up the call stack, where you know how to deal with the problem, catch the exception and deal with it. You don't have to catch exceptions immediately -- if high_level calls mid-level calls low_level, it's okay to not have any try/except in mid_level and let high_level deal with it. It may be that all you can do is have a try/except at the top of your program to catch and log all uncaught and undealt-with errors, and that can be okay.
This is not necessarily Pytonic as such but experience has taught me to let exceptions "lie where they lie".
That is to say; Don't unnecessarily hide them or re-raise a different exception.
It's sometimes good practice to let the callee fail rather than trying to capture and hide all kinds of error conditions.
Obviously this topic is and can be a little subjective; but if you don't hide or raise a different exception, then it's much easier to debug your code and much easier for the callee of your functions or api to understand what went wrong.
Note: This answer is not complete -- See comments. Some or all of the answers presented in this Q&A should probably be combined in a nice way presneting the various problems and solutions in a clear and concise manner.

Should I always specify an exception type in `except` statements?

When using PyCharm IDE the use of except: without an exception type triggers a reminder from the IDE that this exception clause is Too broad.
Should I be ignoring this advice? Or is it Pythonic to always specific the exception type?
It's almost always better to specify an explicit exception type. If you use a naked except: clause, you might end up catching exceptions other than the ones you expect to catch - this can hide bugs or make it harder to debug programs when they aren't doing what you expect.
For example, if you're inserting a row into a database, you might want to catch an exception that indicates that the row already exists, so you can do an update.
try:
insert(connection, data)
except:
update(connection, data)
If you specify a bare except:, you would also catch a socket error indicating that the database server has fallen over. It's best to only catch exceptions that you know how to handle - it's often better for the program to fail at the point of the exception than to continue but behave in weird unexpected ways.
One case where you might want to use a bare except: is at the top-level of a program you need to always be running, like a network server. But then, you need to be very careful to log the exceptions, otherwise it'll be impossible to work out what's going wrong. Basically, there should only be at most one place in a program that does this.
A corollary to all of this is that your code should never do raise Exception('some message') because it forces client code to use except: (or except Exception: which is almost as bad). You should define an exception specific to the problem you want to signal (maybe inheriting from some built-in exception subclass like ValueError or TypeError). Or you should raise a specific built-in exception. This enables users of your code to be careful in catching just the exceptions they want to handle.
You should not be ignoring the advice that the interpreter gives you.
From the PEP-8 Style Guide for Python :
When catching exceptions, mention specific exceptions whenever
possible instead of using a bare except: clause.
For example, use:
try:
import platform_specific_module
except ImportError:
platform_specific_module = None
A bare except: clause will catch SystemExit and KeyboardInterrupt exceptions, making it harder to
interrupt a program with Control-C, and can disguise other problems.
If you want to catch all exceptions that signal program errors, use
except Exception: (bare except is equivalent to except
BaseException:).
A good rule of thumb is to limit use of bare 'except' clauses to two
cases:
If the exception handler will be printing out or logging the
traceback; at least the user will be aware that an error has occurred.
If the code needs to do some cleanup work, but then lets the exception
propagate upwards with raise. try...finally can be a better way to
handle this case.
Not specfic to Python this.
The whole point of exceptions is to deal with the problem as close to where it was caused as possible.
So you keep the code that could in exceptional cirumstances could trigger the problem and the resolution "next" to each other.
The thing is you can't know all the exceptions that could be thrown by a piece of code. All you can know is that if it's a say a file not found exception, then you could trap it and to prompt the user to get one that does or cancel the function.
If you put try catch round that, then no matter what problem there was in your file routine (read only, permissions, UAC, not really a pdf, etc), every one will drop in to your file not found catch, and your user is screaming "but it is there, this code is crap"
Now there are a couple of situation where you might catch everything, but they should be chosen consciously.
They are catch, undo some local action (such as creating or locking a resource, (opening a file on disk to write for instance), then you throw the exception again, to be dealt with at a higher level)
The other you is you don't care why it went wrong. Printing for instance. You might have a catch all round that, to say There is some problem with your printer, please sort it out, and not kill the application because of it. Ona similar vain if your code executed a series of separate tasks using some sort of schedule, you wouldnlt want the entire thing to die, because one of the tasks failed.
Note If you do the above, I can't recommend some sort of exception logging, e.g. try catch log end, highly enough.
Always specify the exception type, there are many types you don't want to catch, like SyntaxError, KeyboardInterrupt, MemoryError etc.
You will also catch e.g. Control-C with that, so don't do it unless you "throw" it again. However, in that case you should rather use "finally".
Here are the places where i use except without type
quick and dirty prototyping
That's the main use in my code for unchecked exceptions
top level main() function, where i log every uncaught exception
I always add this, so that production code does not spill stacktraces
between application layers
I have two ways to do it :
First way to do it : when a higher level layer calls a lower level function, it wrap the calls in typed excepts to handle the "top" lower level exceptions. But i add a generic except statement, to detect unhandled lower level exceptions in the lower level functions.
I prefer it this way, i find it easier to detect which exceptions should have been caught appropriately : i "see" the problem better when a lower level exception is logged by a higher level
Second way to do it : each top level functions of lower level layers have their code wrapped in a generic except, to it catches all unhandled exception on that specific layer.
Some coworkers prefer this way, as it keeps lower level exceptions in lower level functions, where they "belong".
Try this:
try:
#code
except ValueError:
pass
I got the answer from this link, if anyone else run into this issue Check it out

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