TL;DR: Be as specific as possible when handling errors.

Problems

• Fail fast principle violation

• Missing errors

• False negatives

Solutions

1. 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

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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

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This article is part of the CodeSmell Series.