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Exception Handling and Debugging⚓︎


Exception Handling⚓︎

  • We have seen plenty of errors in previous chapters when something goes wrong or some input was given erroneously
  • For example:
$ ./user_input_int.py 
Enter an integer number: abc
Traceback (most recent call last):
  File "./user_input_int.py", line 6, in <module>
    usr_num = int(usr_ip)
ValueError: invalid literal for int() with base 10: 'abc'
  • In such cases, it might be preferred to inform the user on the error and give a chance to correct it
  • Python provides the try-except construct to achieve this
#!/usr/bin/python3

while True:
    try:
        usr_num = int(input("Enter an integer number: "))
        break
    except ValueError:
        print("Not an integer, try again")

print("Square of entered number is: {}".format(usr_num * usr_num))
  • except can be used for particular error (in this case ValueError)
$ ./user_input_exception.py
Enter an integer number: a
Not an integer, try again
Enter an integer number: 1.2
Not an integer, try again
Enter an integer number: 3
Square of entered number is: 9

Further Reading


Syntax check⚓︎

  • Python's command line options can be used for variety of purposes
  • Syntax checking is one of them
$ python3 -m py_compile syntax_error.py 
  File "syntax_error.py", line 3
    print "Have a nice day"
                          ^
SyntaxError: Missing parentheses in call to 'print'
  • Useful to quickly catch syntax errors like missing : for if for with etc and () for print statements
  • While this example might be trivial, real world program might have thousands of lines and tougher to find typos
  • Python docs - cmdline


pdb⚓︎

  • Invoking debugger is another use of cmdline
  • Use it instead of using print all over a program when something goes wrong, plus one can use breakpoints and other features specific to debugging programs
$ python3 -m pdb if_elif_else.py 
> /home/learnbyexample/python_programs/if_elif_else.py(3)<module>()
-> num = 45
(Pdb) p num
*** NameError: name 'num' is not defined
(Pdb) n
> /home/learnbyexample/python_programs/if_elif_else.py(6)<module>()
-> if num > 25:
(Pdb) p num
45
(Pdb) l
  1     #!/usr/bin/python3
  2     
  3     num = 45
  4     
  5     # only if
  6  -> if num > 25:
  7         print("Hurray! {} is greater than 25".format(num))
  8     
  9     # if-else
 10     if num % 2 == 0:
 11         print("{} is an even number".format(num))
(Pdb) q
  • l prints code around the current statement the debugger is at, useful to visualize the progress of debug effort
  • s execute current line, steps inside function calls
  • n execute current line and treats function as single execution step
  • c continue execution until next breakpoint
  • p variable print value of variable
  • h list of commands
    • h c help on c command
  • q quit the debugger

Further Reading


Importing program⚓︎

  • One can also import a program directly in Interpreter to test functions
  • if __name__ == "__main__": construct
    • code inside that construct will be executed when program is intended to be run - ex: executing the program from command line
    • code inside that construct will NOT be executed when program is intended to be imported as module - ex: to use programs's functions
  • A good practice is to put all code outside of functions inside a main function and call main() inside the if __name__ == "__main__": construct
  • Note that __name__ and __main__ have two underscore characters as prefix and suffix
#!/usr/bin/python3

# ----- function without arguments -----
def greeting():
    print("-----------------------------")
    print("         Hello World         ")
    print("-----------------------------")

# ----- function with arguments -----
def sum_two_numbers(num1, num2):
    sum = num1 + num2
    print("{} + {} = {}".format(num1, num2, sum))

# ----- function with return value -----
def num_square(num):
    return num * num

# ----- main -----
def main():
    greeting()
    sum_two_numbers(3, 4)

    my_num = 3
    print(num_square(2))
    print(num_square(my_num))

if __name__ == "__main__":
    main()
  • When run as a program
$ ./functions_main.py
-----------------------------
         Hello World         
-----------------------------
3 + 4 = 7
4
9
  • When importing
>>> import functions_main

>>> functions_main.greeting()
-----------------------------
         Hello World         
-----------------------------

>>> functions_main.num_square()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: num_square() missing 1 required positional argument: 'num'

>>> functions_main.num_square(5)
25

>>> functions_main.main()
-----------------------------
         Hello World         
-----------------------------
3 + 4 = 7
4
9

Further Reading