[Python-ideas] Potential PEP: with/except

[Chris Angelico]

On Wed, Jan 23, 2019 at 7:11 AM Paul Ferrell <pflarr at gmail.com> wrote:
> As a result, I would like to propose that the syntax for 'with' blocks
> be changed such that they can be accompanied by 'except', 'finally',
> and/or 'else' blocks as per a standard 'try' block. These would handle
> exceptions that occur in the 'with' block, including the execution of
> the applicable __enter__ and __exit__ methods.
>
> Example:
>
> try:
>     with open(path) as myfile:
>       ...   # Do stuff with file
> except (OSError, IOError) as err:
>     logger.error("Failed to read/open file {}: {}".format(path, err)
>
> The above would turn into simply:
>
> with open(path) as myfile:
>     ... # Do stuff with file
> except (OSError, IOError) as err:
>     logger.error(...)

Edge case: The "try/with/except" structure includes the entire 'with'
header inside the try block, including the call to open(). But if the
with block itself is handling the exceptions, the expression
"open(path)" is actually evaluated before the exception handling gets
going. So adding an except clause is NOT the same as just adding
another context manager to the stack.

I'm -0.25 on it, as I don't think overlaying in this way improves
clarity. The current syntax makes it very obvious that the
"open(path)" call is inside the try/except, but the proposed syntax
isn't so clear (and as many people will expect it to be inside as
outside).

ChrisA