if / try

[vald]

I wonder if there is any difference in using
if/else and try/except when checking some
return values in functions. For example:

def getValue(self):
try:
return self.values["SOME_VALUE"]
except KeyError:
return None

def getValue2(self):
if self.values.has_key("SOME_VALUE"):
return self.values["SOME_VALUE"]
else:
return None

Is there any preformance gain when using one of
those methods? Which one should be widely used
in applications in such kind of methods that just
check whether something is valid and return value
or None?

Regards,
Kamil

 

[Stephen Horne]

I wonder if there is any difference in using
if/else and try/except when checking some
return values in functions. For example:

def getValue(self):
try:
return self.values["SOME_VALUE"]
except KeyError:
return None

def getValue2(self):
if self.values.has_key("SOME_VALUE"):
return self.values["SOME_VALUE"]
else:
return None

Is there any preformance gain when using one of
those methods? Which one should be widely used
in applications in such kind of methods that just
check whether something is valid and return value
or None?

What I would expect (I don't know for sure) is that for most
applications the 'if' version would be faster.

If the key is almost always present, however, the 'try' version may be
faster.

The reasoning relates to purpose - exceptions are intended to handle
exceptional events. I would expect the language designers to implement
it so that overheads in the no-exception case were minimised, whereas
an 'if' should be handled in a much more balanced way with neither the
True nor False case being prioritised.

In this special case, though, you are probably better off using the
'get' method of the dictionary...

dict.get (key, default) - return default if key not in dictionary
dict.get (key) - return None if key not in dictionary