Exceptions are handy. But should be as narrow as possible
TL;DR: Be as specific as possible when handling errors.
Problems
Fail fast principle violation
Missing errors
False negatives
Solutions
- Narrow the exception handler as much as possible
Sample Code
Wrong
import calendar, datetime
try:
birthYear= input('Birth year:')
birthMonth= input('Birth month:')
birthDay= input('Birth day:')
#we don't expect the above to fail
print(datetime.date(int(birthYear), int(birthMonth), int(birthDay)))
except ValueError as e:
if str(e) == 'month must be in 1..12':
print('Month ' + str(birthMonth) + ' is out of range. The month must be a number in 1...12')
elif str(e) == 'year {0} is out of range'.format(birthYear):
print('Year ' + str(birthMonth) + ' is out of range. The year must be a number in ' + str(datetime.MINYEAR) + '...' + str(datetime.MAXYEAR))
elif str(e) == 'day is out of range for month':
print('Day ' + str(birthDay) + ' is out of range. The day must be a number in 1...' + str(calendar.monthrange(birthYear, birthMonth)))
Right
import calendar, datetime
birthYear= input('Birth year:')
birthMonth= input('Birth month:')
birthDay= input('Birth day:')
# try scope should be narrow
try:
print(datetime.date(int(birthYear), int(birthMonth), int(birthDay)))
except ValueError as e:
if str(e) == 'month must be in 1..12':
print('Month ' + str(birthMonth) + ' is out of range. The month must be a number in 1...12')
elif str(e) == 'year {0} is out of range'.format(birthYear):
print('Year ' + str(birthMonth) + ' is out of range. The year must be a number in ' + str(datetime.MINYEAR) + '...' + str(datetime.MAXYEAR))
elif str(e) == 'day is out of range for month':
print('Day ' + str(birthDay) + ' is out of range. The day must be a number in 1...' + str(calendar.monthrange(birthYear, birthMonth)))
Detection
[X] Manual
If we have a good enough test suite, we can perform mutation testing to narrow the exception scope as much as possible.
Tags
- Exceptions
Conclusion
We must make exceptions as surgical as possible.
Relations
Credits
Photon from Jakob Braun on Unsplash
The primary duty of an exception handler is to get the error out of the lap of the programmer and into the surprised face of the user.
Verity Stob
This article is part of the CodeSmell Series.
Latest comments (5)
I agree to the idea, but the example violates the 'fail fast' principle.
It should not let you input a month when the year is already unparseable.
Indeed, the very
tryblock contains too much code, which results in an awkward effort to reconstruct which of the three statements went wrong.This could be greatly improved by try/except for each input separately.
I'm not sure if I would agree with that as a general rule. The whole idea of exception handling is to propagate exceptions sometimes even way up the stack. For many cases I don't even catch exceptions and let them propagate into the generic handling code.
Yes.
This is for the case you want to catch it.
If you just want to propagate this code smell does not apply.
Wouldn't any of these also raise an error if the user enters a non-integer character?
Again in
the data above are been cast to
int.So I think you meant,
Yes. You are right.
I am not very fluent in Python.
I try to use as many languages as possible to show code smells are language independent.
Thank you very much for the correction