If the program is crashing with an exception, running the program as python3 -i can be a useful tool for simply looking around. The -i option starts an interactive shell as soon as a program terminates. From there, the environment can be explored. Code #1: Given the code

# abc.py
def func(n):
    return n + 10

func('Hello')

Code #2: Running python3 -i produces the following

bash % python3 -i sample.py
Traceback (most recent call last):
    File "sample.py", line 6, in <module>
        func('Hello')
    File "sample.py", line 4, in func
        return n + 10
TypeError: Can't convert 'int' object to str implicitly

func(10)

Output :

20

A further step is to launch the Python debugger after a crash as shown in the code below. Code #3 :

import pdb
pdb.pm()

Output :

> sample.py(4)func()
-> return n + 10
(Pdb) w
  sample.py(6)()
-> func('Hello')
> sample.py(4)func()
-> return n + 10
(Pdb) print n
'Hello'
(Pdb) q

If the code is deeply buried in an environment where it is difficult to obtain an interactive shell (e.g., in a server), catch errors and produce tracebacks as shown in the code below. Code #4:

import traceback
import sys

try:
    func(arg)
except:
    print('**** AN ERROR OCCURRED ****')
    traceback.print_exc(file = sys.stderr)

If the program isn’t crashing, but it’s producing wrong answers or working wrong, there is often nothing wrong with just injecting a few print() calls in places of interest. However, there are related techniques of interest. First, the traceback.print_stack() function will create a stack track of your program immediately at that point as shown in the code below. Code #5 :

def sample(n):
    if n > 0:
        sample(n-1)
    else:
        traceback.print_stack(file = sys.stderr)

sample(5)

Output :

File "", line 1, in <module>
File "", line 3, in sample
File "", line 3, in sample
File "", line 3, in sample
File "", line 3, in sample
File "", line 3, in sample
File "", line 5, in sample

Alternatively, the debugger at any point in the program cab also be launched manually using pdb.set_trace() as - Code #6:

import pdb
def func(arg):
    ...
    pdb.set_trace()
    ...

This can be a useful technique for poking around in the internals of a large program and answering questions about the control flow or arguments to functions.