Handling multiple exceptions in Python

Question

I want to handle different exceptions in such way that there will be common handling for any exception and additional specific handling for subprocess.CalledProcessError. Like this:

try:
    check_output(shlex.split(rsync_cmd))
except CalledProcessException as e:
    # do some stuff
    # do common exception handling
except Exception as e:
    # do the same common handling

I don't want to duplicate code so currently I end up with this code:

try:
    check_output(shlex.split(rsync_cmd))
except Exception as e:
    if isinstance(e, CalledProcessError) and e.returncode == 24:
        # do some stuff
    # do common handling

What's the best practice to run specific code for one exception and to run common code for all exceptions at the same time?

But I don't want to run exactly the same code for both Exceptions. I want to handle it in a different way with one common part. It is a different case. — gevial
– gevial, Commented Mar 17, 2016 at 9:28

PM 2Ring · Accepted Answer · 2016-03-17 10:01:08Z

3

You could use nested try:...except blocks. I don't think it's a satisfactory solution, but here's a simple example.

for i in (1, 0, 'z'):
    try:
        try:
            print(i, end=' ')
            x = 1 / i
            print(x)
        except TypeError as e:
            print(e)
            raise
        except ZeroDivisionError as e:
            print(e)
            raise
    except (TypeError, ZeroDivisionError) as e:
        print('Common stuff', e)

output

1 1.0
0 float division
Common stuff float division
z unsupported operand type(s) for /: 'int' and 'str'
Common stuff unsupported operand type(s) for /: 'int' and 'str'

Tested on Python 2.6.6, with from __future__ import print_function, division

answered Mar 17, 2016 at 10:01

PM 2Ring

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Comments

Eugene Lisitsky · Accepted Answer · 2016-03-17 09:30:29Z

1

You may check error not only against class, but also a tuple of classes. In each branch you may check specific properties.

    try:
           n=0/0
    except (IOError, ImportError) as e:
         print(1)
    except (NameError) as e:
         print(2)

    Traceback (most recent call last):
    File "<input>", line 2, in <module>
         n=0/0
    ZeroDivisionError: division by zero

edited Mar 17, 2016 at 9:30

answered Mar 17, 2016 at 9:27

Eugene Lisitsky

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4 Comments

gevial Over a year ago

Thanks, but I want to have a common code for handling both exceptions with additional code for the first one. This is a different case.

Eugene Lisitsky Over a year ago

What if move to a function common case and call it whenever need?

gevial Over a year ago

I already have a function remove_key() to call it in except: block, but I would like to avoid duplicate function calls. I want to know if there is a better solution than checking exception class like I do now.

Eugene Lisitsky Over a year ago

I do not see any solution which would be much better readable. So-so.

Mp0int · Accepted Answer · 2016-03-17 09:44:06Z

1

Use inheritance in your exceptions.

class GeneralException(Exception)
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return repr(self.value)

class SpecificException(GeneralException)
    def __init__(self, value):
        self.value = value
        super(SpecificException, self).__init__()
    def __str__(self):
        super(SpecificException, self).__str__()
        return repr(self.value)

Base logic is, you trigger SpecificExceptions and they all super call GeneralException to execure related operations

answered Mar 17, 2016 at 9:44

Mp0int

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2 Comments

gevial Over a year ago

That is really great for custom exceptions, but this is not my case. Wrapping CalledProcessError and Exception in my own classes is a bit overkill.

Mp0int Over a year ago

If you avoid Wrapping, then using simple two-step execption handling is the only solution. try to catch CalledProcessError, otherwise, catch any other exception

Ilja Everilä · Accepted Answer · 2016-03-17 09:46:06Z

1

Create a function that handles the common operations. That's what functions are for.

def common_exception_handling(e):
    # do stuff with e
    return  # possibly something

# ...

try:
    check_output(shlex.split(rsync_cmd))
except CalledProcessException as e:
    # do some stuff
    common_exception_handling(e)
except Exception as e:
    common_exception_handling(e)

answered Mar 17, 2016 at 9:46

Ilja Everilä

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4 Comments

gevial Over a year ago

I already have a function, but I would like to avoid duplication of code, even if it is a single line (personal taste). Thanks anyway!

gevial Over a year ago

Maybe you're right ;) But I'm seeking for a best practice, not for a solution (I already have one).

Ilja Everilä Over a year ago

Functions are a best practice.

Antti Haapala Over a year ago

@Gevial there is no other "best practice" for this case.

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