Define with statement that automatically includes try and except

Question

Is it possible to define an own with statement which automatically includes try...except error handling? For example, it would be neat to have a shorthand for this:

with Do_Something():
    try:
        ...
    except Exception as e:
        print(str(e))

...that looks like this:

with Try_Something():
    ...

How can we include the try...except behaviour into the following MWE class?

class Do_Something():
    def __init__(self):
        pass

    def __enter__(self):
        print('Starting...')
        # invoke "try" somewhere here?
        return(self)

    def __exit__(self, except_type, except_value, tb):
        # invoke "except" somewhere here?

The `__exit__()` methods allows exception handling. See the [docs](https://docs.python.org/3/reference/datamodel.html#object.__exit__) for details. — Klaus D., May 14 '21 at 21:34
Does this answer your question? [Handling exceptions inside context managers](https://stackoverflow.com/questions/35483359/handling-exceptions-inside-context-managers) — Corralien, May 14 '21 at 21:38
@Corralien: the link was helpful, but it deals with a far more specific problem. It does not fully answer the question stated here as I do not want to handle errors in a certain way, I want to reproduce the default output of try...except. I.e., the question boils down to how to output the traceback info provided by `__exit__` in a similar way as `str(e)` would do using the argument provided by `except` . — Martin, May 14 '21 at 22:54

score 0 · Answer 1 · answered May 14 '21 at 21:39

0

According to the docs, if you want to suppress exceptions, __exit__ should return True.

The args passed to __exit__ will describe the exception in case you want to make conditional decisions.

answered May 14 '21 at 21:39

Lucas Ng

671
4
11

Martin · Answer 2 · 2021-05-14T22:43:39.503

Looks like with natively comes with try under the hood and propagets the exceptions as arguments to __exit__(...). So this could be a suitable solution:

import traceback
class Try_Something():
    def __init__(self):
        pass

    def __enter__(self):
        print('Starting...')
        # invoke "try" somewhere here? -- No, "with" automatically invokes "try"
        return(self)

    def __exit__(self, except_type, except_value, tb):
        if not except_type:
            print('finished.')
        else:
            print('failed.')
            traceback.print_exc() # prints error similar to str(e) from the question
        return(True)

I did not find a solution that goes without import traceback. Please comment if there is a way to directly print the traceback info using the provided arguments without additional modules.

Define with statement that automatically includes try and except

2 Answers2