try-except syntax

[Steven D'Aprano]

On Thu, 05 Apr 2018 23:04:18 +0200, ElChino wrote:

> I'm trying to simplify a try-except construct. E.g. how come this:
>    try:
>      _x, pathname, _y = imp.find_module (mod, mod_path)
>      return ("%s" % pathname)

imp.find_module is deprecated and should not be used in new code.

Putting that aside, pathname is already a string. Why are you wasting 
time interpolating it into a bare "%s" string? Just say `return pathname`.


>    except ImportError:
>      pass
>    except RuntimeError:
>      pass
>      return ("<unknown>")

Unnecessary parens surrounding a single value.

Possible indentation error. Surely the return "<unknown>" needs to be 
indented level with the try/except statements?


> Cannot be simplified into this:
>    try:
>      _x, pathname, _y = imp.find_module (mod, mod_path) return ("%s" %
>      pathname)
>    except ImportError:
>    except RuntimeError:
>      pass
>      return ("<unknown>")
> 
> Like a "fall-through" in a C-switch statement.

Because fall-through C switch syntax is an abomination, and fortunately 
the designer of Python has more sense than to allow that awfulness.


The syntax you are looking for is:


    try:
      block
    except (ImportError, RuntimeError):
      block



By the way, RuntimeError is almost never something you want to catch 
(except to log before bailing out). It should represent a fatal coding 
error, not something safe to ignore.


-- 
Steve