11 Debugging

Question 1

How can you raise your own Exception? (Not ‘except:’)

Answer 1

> > > raise Exception(‘Error Message’):

e.g.:
»> if input < 2:
»> raise Exception(‘Input must be greater than 2!’):

Question 2

How can you display an Error message in a more user-friendly way?

Answer 2

You can save the Excepetion message in a variable (e.g. ‘err’) and convert it to a string

> > > except Exception as err:
print(‘Whatever Message: ‘ + str(err))

Question 3

What is the Traceback, the Callstack and when do they appear?

Answer 3

Traceback:
Error info given when a program crashes, incl. error message, line number that caused the errror and sequence of function calls that led to it.
Call stack:
The sequence mentioned above

They are always displayed when an error is not handled, e.g. by Exceptions

Question 4

What is for user errors?

What is for programmer errors?

Answer 4

User Errors:

• > try & except, Exception
• > You want the code to continue working
• > Designed so, that users can see them

Programmer errors:

• > Assert statements (without try & except!)
• > You want the program to crash and to find faults fast
• > You want to “fail fast” - should only occur while program is still developed (user should never see them)
• > Helps debugging

Question 5

How can you raise an assert statement?

Answer 5

> > > assert a > b, ‘Message’

assert statement + condition (that evaluates to True or False) + ‘,’ + message
If the condition evaluates to False, the AssertionError is raised

Question 6

1. What is logging
2. What are the benefits
3. How can you use it?

Answer 6

1. The logging function is similar to print(): It displays specified log messages
2. They can be disabled all at once, have 5 different levels and give information like timestamp etc. - so you don´t risk deleting valuable code like you do, when you remove print() calls
3. import logging
   logging. basicConfig(level=logging.DEBUG, format=’ %(asctime)s - %(levelname)s - %(message)s’)

And then:
logging.*(‘message’) –> e.g. logging.debug(f’Start of factorial {n}’)

Question 7

How many levels exist in logging and what is it useful for?

Answer 7

5x levels: logging.*(‘message’)
* .debug() lowest level - used for diagnose etc,
.info()
.warning()
.error()
.critical() highest level - used to indicate fatal error

Useful for:
Logging messages can be disabled:
»>logging.disable(logging.*)
Here you can define what levels should be disabled, e.g. everything up to chritical?

Question 8

How can you log to a file instead of to the screen?

Answer 8

Add the information to the logging.basicConfig() function (see: filename=’XY.txt’):

logging.basicConfig(filename=’myProgramLog.txt’, level=logging.DEBUG, format=’
%(asctime)s - %(levelname)s - %(message)s’)