Debugging#

BB1000 Programming in Python KTH

layout: false

Learning outcomes#

  • know how to use assertions

  • know how to use exceptions

  • know how to use logging

  • know how to use a debugger

About bugs#

Best approch to avoiding#

  • TDD

  • Version control

How to find bugs#

  • insert print statements

  • use other builtin tools

Assertions#

>>> assert True
>>> assert False
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

  • evaluates an expression

  • if it is true-ish continue

  • if not stop or handle error

In tests:

  • a fundamental component in testing frameworks

In code:

  • a sanity check

  • I assert that this condition is true, otherwise there is a bug somewhere

  • fail fast and recover

  • catch programming errors

#get_number.py
def get_number(n):
    assert type(n) == int, 'Argument should be an integer'
    print("Number is %d" % n)

get_number("hello")

  • Disable with -O

> python -O program.py

##Exceptions

  • exceptions are errors: the program crashes

  • unless you handle them in some way and continue running

>>> 1/0                                                                          
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

  • Errors can be handled with try...except... 

  • to detect and handle errors you can aniticipate

  • potential problems are put in a try clause

  • if an error happends the code skips to the following except clause

>>> try:
...     1/0
... except ZeroDivisionError:
...     print("Warning:Zero division, continuing")
Warning:Zero division, continuing
>>>

Parents’ approach (ask permission first)

>>> if n != 0:
...     print(1/n)
... else:
...     print('Warning...')
Warning...

Childs’ approach (ask forgiveness after)

>>> try:
...     print(1/n)
... except ZeroDivisionError:
...     print('Warning...')
Warning...

Easier to ask for forgiveness than for permission

###Example

Script which emulates the Linux cat command:

  • take filename argument

  • opens and reads file

  • prints contents to stdout

#cat.py
import sys
try:
    filename = sys.argv[1]
    lines = open(filename).read()
except IndexError:
    print("Usage: %s filename" % sys.argv[0])
    sys.exit()
except FileNotFoundError:
    print("%s: no such file, %s" % (sys.argv[0], filename))
    sys.exit()

print(lines)

python cat.py
Usage: %s filename
python cat.py nofile
cat.py: no such file, nofile
python cat.py somefile
<contents of somefile>

In programs

  • an error condition is indicated by raising an exception

raise Exception('something is wrong here')

  • exceptions can be created with the Exception() call

    • creates an instance of the Exception class

  • many standard exceptions are subclasses to the Exception class

    • ZeroDivisionError (division by zero)

    • FileNotFoundError (file not found)

    • https://docs.python.org/3/library/exceptions.html

Define your own exceptions

class NoBirdError(Exception):
   pass

class ZooCatalog:
    ...
    def add_bird(self, new_bird):
        if type(new_bird) != Bird:
            raise NoBirdError('this is not a bird')

executing

>>> zoo = ZooCatalog()
>>> zoo.add_bird(Reptile())                                                      
Traceback (most recent call last):
	...
    raise NoBirdError('this not a bird')
__main__.NoBirdError: this not a bird

testing

def test_nobird():
    zoo = ZooCatalog()
    with pytest.raises(NoBirdError):
        zoo.add_bird(Reptile())

Logging#

  • a better type of print

>>> import logging
>>> logging.basicConfig(level=logging.DEBUG, format="%(message)")
>>> logging.debug('Hello world')                                                 
Hello world

  • easy to switch between hiding/showing log messages

  • intended for programmer not user

  • catagorize your log messages by importance

  • tracking more info

>>> log_format = ' %(asctime)s %(levelname)s: <%(message)s>'
>>> logging.basicConfig(level=logging.DEBUG, format=log_format)
>>> logging.debug('Hello world')
 2017-05-09 13:42:55,142 DEBUG: <Hello world>

Levels

function

When

DEBUG

logging.debug()

Diagnosing

INFO

logging.info()

Confirming

WARNING

logging.warning()

Potential problem

ERROR

logging.error()

Program failure

CRITICAL

logging.critical()

Fatal error

 
logging.debug('Diagnosic output')
logging.info('Starting ok')
logging.error('Som error has occured')
logging.critical('Unable to recover')

 2017-05-09 14:17:26,822 DEBUG: <Diagnosic output>
 2017-05-09 14:17:26,822 INFO: <Starting ok>
 2017-05-09 14:17:26,822 ERROR: <Some error has occured>
 2017-05-09 14:17:26,822 CRITICAL: <Unable to recover>

  • disable all logging

logging.disable(logging.CRITICAL)

  • logging to a file (keep normal output clean)

logging.basicConfig(level=logging.DEBUG, format="%(message)", filename="my.log")

Debuggers#

  • execute code line by line

  • inspect variables

  • set break points

The pdb module#

>>> import pdb
>>> dir(pdb)
['Pdb', 'Restart', 'TESTCMD', '__all__', '__builtins__', '__cached__',
'__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__',
'_rstr', '_usage', 'bdb', 'cmd', 'code', 'dis', 'find_function',
'getsourcelines', 'glob', 'help', 'inspect', 'lasti2lineno', 'line_prefix',
'linecache', 'main', 'os', 'pm', 'post_mortem', 'pprint', 're', 'run',
'runcall', 'runctx', 'runeval', 'set_trace', 'signal', 'sys', 'test',
'traceback']

Breakpoints#

>>> def add(a, b):
...    breakpoint()  
...    # import pdb; pdb.set_trace() for python<3.7
...    c  = a - b
...    return c

pdb commands#

Some of the most used commands are common gdb

  • step (s) : execute the next line (for function calls step into that function)

  • next (n) : execute the next line (for function calls complete the function all and stop at the next line of current function)

  • break (b): set break point

  • continue (c): continue execution

(Pdb) help
Documented commands (type help <topic>):
========================================
EOF    bt         cont      enable  jump  pp       run      unt   
a      c          continue  exit    l     q        s        until 
alias  cl         d         h       list  quit     step     up    
args   clear      debug     help    n     r        tbreak   w     
b      commands   disable   ignore  next  restart  u        whatis
break  condition  down      j       p     return   unalias  where 
...