How to handle multiple exceptions?

Question

I'm a Python learner, trying to handle a few scenarios:

Reading a file.
Formatting Data.
Manipulating/Copying Data.
Writing a file.

So far, I have:

try:
   # Do all
except Exception as err1:
   print err1
   #File Reading error/ File Not Present

except Exception as err2:
   print err2
   # Data Format is incorrect
except Exception as err3:
   print err3
   # Copying Issue
except Exception as err4:
   print err4
   # Permission denied for writing

The idea of implementing in this fashion is to catch the exact error for all different scenarios. I can do it in all separate try/except blocks.

Is this possible? And reasonable?

Yes what your doing is correct. For every exception type, just use the specific error you're going to catch. i.e. except KeyError, ValueError — Eduard
– Eduard, Commented Aug 10, 2016 at 10:40
I take that back, based on your scenario's, you are much better off creating multiple try catch blocks of code. i.e. def check_file(file): try ... except your_ exceptions_here ... — Eduard
– Eduard, Commented Aug 10, 2016 at 10:44
Basically the main idea of a try catch block is to do something, not do all, as multiple errors could be encountered along the way. — Eduard
– Eduard, Commented Aug 10, 2016 at 10:45
If what your trying to achieve is something like try: read_file then you could go with except FileNotFound? ... except IsNotAFile? ..... but if your doing try: read_file then crunch it then yea, segregate them. — Eduard
– Eduard, Commented Aug 10, 2016 at 10:46
@Eduard Daduya : Multiple try/catch blocks is easy. I thought based on customized exceptions, i can handle single try with multiple exception blocks. Can it be done ? — User
– User, Commented Aug 10, 2016 at 10:47

DeepSpace · Accepted Answer · 2016-08-10 12:24:36Z

Your try blocks should be as minimal as possible, so
```
try:
     # do all
except Exception:
     pass
```
is not something you want to do.
The code in your example won't work as you expect it to, because in every except block you are catching the most general exception type Exception. In fact, only the first except block will ever be executed.

What you want to be doing is having multiple try/except blocks, each one responsible for as few things as possible and catching the most specific exception.

For example:

try:
    # opening the file
except FileNotFoundException:
    print('File does not exist')
    exit()

try:
   # writing to file
except PermissionError:
   print('You do not have permission to write to this file')
   exit()

However, sometimes it is appropriate to catch different types of exceptions, in the same except block or in several blocks.

try:
    ssh.connect()
except (ConnectionRefused, TimeoutError):
    pass

or

try:
    ssh.connect()
except ConnectionRefused:
    pass
except TimeoutError:
    pass

Playing devil's advocate: didn't we make the leap from explicit error handling to exceptions that bubble up the stack specifically to get the ability to make larger try blocks and centralize error handling somewhere else?

Community · Accepted Answer · 2017-05-23 12:14:47Z

As DeepSpace stated,

Your try blocks should be as minimal as possible.

If you want to achieve

try:
     # do all
except Exception:
     pass

Then you might as well do something like

def open_file(file):
    retval = False
    try:
        # opening the file succesful?
        retval = True
    except FileNotFoundException:
        print('File does not exist')
    except PermissionError:
        print('You have no permission.')
    return retval

def crunch_file(file):
    retval = False
    try:
        # conversion or whatever logical operation with your file?
        retval = True
    except ValueError:
        print('Probably wrong data type?')
    return retval

if __name__ == "__main__":
    if open_file(file1):
        open(file1)
    if open_file(file2) and crunch_file(file2):
        print('opened and crunched')

Anetair · Accepted Answer · 2016-08-10 10:48:48Z

0

Yes this is possible.

Just say as example:

try:
    ...
except RuntimeError:
    print err1
except NameError:
    print err2

...

Just define the exact Error you want to intercept.

answered Aug 10, 2016 at 10:48

Anetair

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