Debugger

July 18, 2023

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The pdb module provides an interactive debugger for Python programs. It supports breakpoints and stepping through the source code. To demonstrate using the debugger and setting a breakpoint, first consider the example shown below.

def main():
    """
    Iterate through a range of numbers, double the number, and determine if
    the number is divisible by 3.
    """

    for number in range(10):

        number_x2 = number * 2

        print('number   is', number)
        print('number×2 is', number_x2)

        if number != 0 and number % 3 == 0:
            print('number is divisible by 3')


if __name__ == '__main__':
    main()

number   is 0
number×2 is 0
number   is 1
number×2 is 2
number   is 2
number×2 is 4
number   is 3
number×2 is 6
number is divisible by 3
number   is 4
number×2 is 8
number   is 5
number×2 is 10
number   is 6
number×2 is 12
number is divisible by 3
number   is 7
number×2 is 14
number   is 8
number×2 is 16
number   is 9
number×2 is 18
number is divisible by 3

Using pdb and set_trace()

Import pdb then call pdb.set_trace() at the location where you want to break into the debugger as shown in the code below. The (Pdb) is the debugger's prompt which indicates you are in debug mode. While in debug mode, you can inspect variables and perform actions such as move to the next line with n or continue execution with c. Enter q to quit debug mode.

import pdb


def main():
    """
    Set trace if number divisible by 3.
    """

    for number in range(10):

        number_x2 = number * 2

        print('number   is', number)
        print('number×2 is', number_x2)

        if number != 0 and number % 3 == 0:
            pdb.set_trace()
            print('number is divisible by 3')


if __name__ == '__main__':
    main()

number   is 0
number×2 is 0
number   is 1
number×2 is 2
number   is 2
number×2 is 4
number   is 3
number×2 is 6
> /Desktop/pythonic/python-projects/debugger/pdb_example.py(18)main()
-> print('number is divisible by 3')
(Pdb) number_x2
6

Using breakpoint()

The breakpoint() function introduced in Python 3.7 can be used instead of the import pdb; pdb.set_trace() as shown in the next example.

def main():
    """
    Call breakpoint if number divisible by 3.
    """

    for number in range(10):

        number_x2 = number * 2

        print('number   is', number)
        print('number×2 is', number_x2)

        if number != 0 and number % 3 == 0:
            breakpoint()
            print('number is divisible by 3')


if __name__ == '__main__':
    main()

number   is 0
number×2 is 0
number   is 1
number×2 is 2
number   is 2
number×2 is 4
number   is 3
number×2 is 6
> /Desktop/pythonic/python-projects/debugger/breakpoint_example.py(16)main()
     15             breakpoint()
---> 16             print('number is divisible by 3')
     17

ipdb>

Notice the debug prompt ipdb> for this example looks different than the debug prompt for the previous example. The advantage of using breakpoint() is that it can call other debuggers by setting the PYTHONBREAKPOINT environment variable. In this example, the ipdb debugger is used because it is defined in the .zshrc file as shown below.

# Use ipdb as the Python breakpoint() debugger
export PYTHONBREAKPOINT=ipdb.set_trace