Code Smell 132 — Exception Try Too Broad

Exceptions are handy. But should be as narrow as possible

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

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 should exceptions as surgical as possible.

Relations

Code Smell 26 — Exceptions Polluting
It is very nice to have lots of different exceptions. Your code is declarative and robust. Or not?blog.devgenius.io

Code Smell 73 — Exceptions for Expected Cases
blog.devgenius.io

Credits

Photo frome 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

Software Engineering Great Quotes
Sometimes a short thought can bring amazing ideas.blog.devgenius.io

This article is part of the CodeSmell Series.

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