Core Python Programming (2nd Edition)

etval = None
return retval
Bear in mind that with our change above, nothing about our code changed except that we used one
more local variable. In designing a well-written application programmer interface (API), you may have
kept the return value more flexible. Perhaps you documented that if a proper argument was passed to
safe_float(), then indeed, a floating point number would be returned, but in the case of an error, you
chose to return a string indicating the problem with the input value. We modify our code one more time
to reflect this change:
def safe_float(obj):
try:
retval = float(obj)
except ValueError:
retval = 'could not convert non-number to float'
return retval
The only thing we changed in the example was to return an error string as opposed to just None. We
should take our function out for a test drive to see how well it works so far:
>>> safe_float('12.34')
12.34
>>> safe_float('bad input')
'could not convert non-number to float'
We made a good startnow we can detect invalid string input, but we are still vulnerable to invalid
objects being passed in:
>>> safe_float({'a': 'Dict'})
Traceback (innermost last):
File "", line 3, in ?
retval = float(obj)
TypeError: float() argument must be a string or a number
We will address this final shortcoming momentarily, but before we further modify our example, we
would like to highlight the flexibility of the try-except syntax, especially the except statement, which
comes in a few more flavors.
10.3.3. try Statement with Multiple excepts
Earlier in this chapter, we introduced the following general syntax for except:
except Exception[, reason]:
suite_for_exception_Exception
The except statement in such formats specifically detects exceptions named Exception. You can chain
multiple except statements together to handle different types of exceptions with the same TRy: