[Python-Dev] PEP 389: argparse - new command line parsing module

Steven D'Aprano steve at pearwood.info
Sun Oct 4 03:26:51 CEST 2009

On Sun, 4 Oct 2009 05:35:04 am André Malo wrote:
> * Steven D'Aprano wrote:
> > You don't need a comment warning that you are catching SystemExit
> > because parse_args raises SystemExit, any more than you need a
> > comment saying that you are catching ValueError because some
> > function raises ValueError. The fact that you are catching an
> > exception implies that the function might raise that exception. A
> > comment like:
> >
> > "Catching SystemExit because parse_args() throws SystemExit on
> > parser errors."
> >
> > is up them with comments like this:
> >
> > x += 1  # Add 1 to x.
> It's semantically different. You usually don't catch SystemExit
> directly, because you want your programs to be stopped. 

Exactly -- so why catch it at all? But even if there is a good reason to 
catch it, there's still no reason to subclass SystemExit unless 
argparse wants to distinguish different types of fatal error, and allow 
code to catch some but not all. But frankly, if I'm having a hard time 
thinking of a reason to catch SystemExit, I'm having an even harder 
time thinking why you'd want to (say) catch SystemExitTooManyArguments 
but not SystemExitMissingArgument.

> Additionally, 
> a library exiting your program is badly designed, as it's unexpected.

It's not unexpected for an argument parser. Do you know any applications 
that run when given invalid arguments? As a general rule, what can the 
application do? Guess what you wanted?

> Thatswhy such a comment is useful.

The comment doesn't tell you anything that wasn't obvious from the code. 
It is pointless.

> Here's what I'd do: I'd subclass SystemExit in this case and raise
> the subclass from argparse. 

In the following code snippet:

    ns = argparse.parse_args()
except SystemExit:

is there any confusion between SystemExit raised by parse_args and 
SystemExit raised by other components? *What* other components? If 
SystemExit was raised in that try block, where could it have come from 
other than parse_args?

Do you write comments like these?

    value = mydict[key]
except KeyError:
    # Catch KeyError raised by dict lookup

    n = mylist.index(x)
except ValueError:
    # Catch ValueError raised by mylist.index

Useless comments are worse than no comments, because useless comments 
waste the readers' time and they risk becoming out of sync with the 
code and turning into misleading comments.

> That way all parties here should be satisifed.

No. It wastes the time of the argparse developer, it wastes the time of 
people learning argparse, it wastes the time of people who read the 
code and wonder what's the difference between SystemExit and 
ArgparseSystemExit. (Answer: there is no difference.)

Steven D'Aprano

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