Debugging Python Code(logging/pdb)

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Prashant Lakhera

Logging is a great way to debug our code, normal way to do that is to add print() in your code but Python provides logging module which help us to debug what happening in our code and in which order. In order to enable logging add the below entry in your code

import logging
# This is setup code to enable logging in Python
logging.basicConfig(level=logging.DEBUG, format='%(asctime)s -%(levelname)s - %(message)s')

To illustrate logging let’s take a simple example where I am going to add first five number

>>> 1+2+3+4+5
15
Now let’s do it with the help of Python Code

sum = 1
for i in range(1,6):
    sum = sum + i

print(sum)

Output is 16 which is wrong it should be 15 so there is some sort of bug in this code

16

After enable logging

import logging
logging.basicConfig(level=logging.DEBUG, format='%(asctime)s -%(levelname)s - %(message)s')
logging.debug("Start of Program")
sum = 1
for i in range(1,6):
    logging.debug("Value of i is (%s)" %(i))
    sum = sum + i
    logging.debug("Value of i is (%s), Value of sum is (%s)" % (i,sum))

print(sum)
logging.debug("End of Program")

Output look like this,as you can see here even at first iteration for i = 1 sum =2 which is wrong so sum declaration is wrong and it should be set to zero(sum=0)

2017–04–17 16:51:20,810 -DEBUG — Start of Program
2017–04–17 16:51:20,810 -DEBUG — Value of i is (1)
2017–04–17 16:51:20,810 -DEBUG — Value of i is (1), Value of sum is (2)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (2)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (2), Value of sum is (4)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (3)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (3), Value of sum is (7)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (4)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (4), Value of sum is (11)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (5)
2017–04–17 16:51:20,811 -DEBUG — Value of i is (5), Value of sum is (16)
2017–04–17 16:51:20,811 -DEBUG — End of Program

So the program should look like

sum = 0
for i in range(1,6):
    sum = sum + i

print(sum)

Output

15

In order to disable logging we need to add this at the start of program

logging.disable(logging.CRITICAL)

Doing this is much easier when comparing with adding print statement and the reason for that it’s difficult to remove print when we are done with debugging(by mistake we might remove the print which outputting the program output)

There are five log levels going from lowest to highest

#Different logging level
>>> logging.DEBUG
10
>>> logging.INFO
20
>>> logging.WARNING
30
>>> logging.ERROR
40
>>> logging.CRITICAL
50

Instead of logging all these messages to console we can redirect it to a file using filename directive

logging.basicConfig(filename='debugfile',level=logging.DEBUG, format='%(asctime)s -%(levelname)s - %(message)s')
cat debugfile
2017–04–17 21:57:20,101 -DEBUG — Start of Program
2017–04–17 21:57:20,101 -DEBUG — Value of i is (1)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (1), Value of sum is (1)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (2)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (2), Value of sum is (3)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (3)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (3), Value of sum is (6)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (4)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (4), Value of sum is (10)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (5)
2017–04–17 21:57:20,101 -DEBUG — Value of i is (5), Value of sum is (15)
2017–04–17 21:57:20,101 -DEBUG — End of Program

Python Debugger(pdb)

Advantage of pdb is it allow a program to pause execution and give user chance to run the code and inspect the variable and step through code

Let look at this simple program,it’s similar to what illustrated with logging example

import pdb
val = [1,2,3,4,5]
for i in val:
    sum = 0
    sum = sum + i

print(sum)

Output of this program is wrong it’s suppose to be 15

5

So let’s try to debug it via pdb

import pdb

val = [1,2,3,4,5]


for i in val:
    sum = 0
    sum = sum + i
    pdb.set_trace()

print(sum)

Under pdb

To get the list of command supported by pdb(type h → help)

Coming back to our problem

-> for i in val:
(Pdb) i # Checking the value of the i
1
(Pdb) sum # Checking the value of the sum
1
(Pdb) c #c is the abbreviation of continue
-> for i in val:
(Pdb) i
2
(Pdb) sum
2
(Pdb)

So here we found out that value of sum is not incrementing

To fix this problem we declared sum outside loop

import pdb

val = [1,2,3,4,5]

sum = 0
for i in val:
    sum = sum + i

print(sum)

Now output is equal to

15