#79 - Using Python's pdb debugger

Python has a library called pdb that can be imported into the project and it is very straight-forward to use. 

It only requires the programmer to insert the breaking point with the code pdb.set_trace() on the desired location to debug, and the program will enters in debugging mode when it reaches this point of execution. Another option that is available on the more recent Python versions (3.7) is to use the command breakpoint() that imports the pdb lib and is similar to the pdb.set_trace().

For debugging from the command prompt> python -m pdb debug_ex.py to enter in debug mode from the terminal. 

Some of the commands are very similar to other commonly used debuggers in other programming languages, such as gdb for C++. Very frequently used commands are for example: n(ext), s(tep), c(ontinue), l(ist), p(rint) variable, q(uit), cl(ear breakpoints), b(set a breakpoint), r(eturn), h(elp).

Here below a simple example: debug_ex.py

import pdb

def Multiply(input_list):

    output_list=[]

    pdb.set_trace()

    for i in range(len(input_list)):

        output_list.append(input_list[i]*10)

    return output_list

def main():

    my_list=[1,2,3]

    print(Multiply(my_list))

if __name__ == '__main__':

    main()

Executing from the terminal:

$ python debug_ex.py

> c:\...\debug_ex.py(6)Multiply()

-> for i in range(len(input_list)):

(Pdb) n

> c:\...\debug_ex.py(7)Multiply()

-> output_list.append(input_list[i]*10)

(Pdb) n

> c:\...\debug_ex.py(6)Multiply()

-> for i in range(len(input_list)):

(Pdb) n

> c:\...\debug_ex.py(7)Multiply()

-> output_list.append(input_list[i]*10)

(Pdb) i

1

(Pdb) s

> c:\...\debug_ex.py(6)Multiply()

-> for i in range(len(input_list)):

(Pdb) s

> c:\...\debug_ex.py(7)Multiply()

-> output_list.append(input_list[i]*10)

(Pdb) s

> c:\...\debug_ex.py(6)Multiply()

-> for i in range(len(input_list)):

(Pdb) s

> c:\...\debug_ex.py(8)Multiply()

-> return output_list

(Pdb) s

--Return--

> c:\...\debug_ex.py(8)Multiply()->[10, 20, 30]

-> return output_list

(Pdb) s

[10, 20, 30]

--Return--