Photo by Jakob Braun on Unsplash
Code Smell 132 - Exception Try Too Broad
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(birthYear) + ' 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(birthYear) + ' 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.