Aahz wrote:
On Sun, Mar 15, 2009, Michael Foord wrote:
Aahz wrote:
On Sun, Mar 15, 2009, Michael Foord wrote:
Note that using exceptions for control flow can be bad for other
implementations of Python. For example exceptions on the .NET
framework are very expensive. (Although there are workarounds such
as not really raising the exception - but they're ugly).
Isn't it better practise for exceptions to be used for exceptional
circumstances rather than for control flow?
It seems to me that we as a development community already made a decision
when we switched to StopIteration as the primary mechanism for halting
``for`` loops. (Not that it was really a new decision because parts of
the Python community have always advocated using exceptions for control
flow, but the ``for`` loop enshrines it.) I doubt that using exceptions
for control flow in ``with`` blocks will cause anywhere near so much a
performance degradation.
Well, StopIteration is still an implementation detail that only
occasionally bleeds through to actual programming. It says nothing
about whether using exceptions for non-exceptional circumstances
(control flow) is good practise. Personally I think it makes the
intent of code less easy to understand - in effect the exceptions
*are* being used as a goto.
Let me know how you'd rewrite this more clearly without a control-flow
exception:
try:
for field in curr_fields:
for item in record[field]:
item = item.lower()
for filter in excludes:
if match(item, filter):
raise Excluded
except Excluded:
continue
This is pretty much the canonical example showing why control-flow
exceptions are a Good Thing. They're a *structured* goto.
You didn't include all the code - so impossible to match the exact
semantics. Breaking out of multiple loops with a return is a cleaner way
to handle it IMHO.
def find_excludes():
for field in curr_fields:
for item in record[field]:
item = item.lower()
for filter in excludes:
if match(item, filter):
return
while something:
find_excludes()
Michael
--
http://www.ironpythoninaction.com/
http://www.voidspace.org.uk/blog
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