🐛 Using Debuggers

A debugger lets you pause your program and inspect its state interactively. Python's built-in debugger is pdb (Python DeBugger).

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✅ Starting the Debugger

# Method 1: breakpoint() (Python 3.7+, recommended)
def calculate(x, y):
    breakpoint()  # Execution pauses here
    return x + y

# Method 2: import pdb
import pdb
def calculate(x, y):
    pdb.set_trace()  # Same effect
    return x + y

# Method 3: Run script with debugger
# python -m pdb script.py

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✅ Essential pdb Commands

Command	Short	Description
help	h	Show help
next	n	Execute next line (step over)
step	s	Step into function call
continue	c	Continue until next breakpoint
return	r	Continue until function returns
quit	q	Exit debugger
print expr	p expr	Print expression value
pp expr		Pretty-print expression
list	l	Show current code context
where	w	Show call stack
up	u	Move up the call stack
down	d	Move down the call stack

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✅ Inspecting State

(Pdb) p variable_name      # Print a variable
(Pdb) pp complex_dict      # Pretty-print
(Pdb) p type(x)            # Check type
(Pdb) p len(items)         # Evaluate expressions
(Pdb) p locals()           # See all local variables
(Pdb) p dir(obj)           # See object attributes

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✅ Navigation Commands

(Pdb) n        # Next line (don't enter functions)
(Pdb) s        # Step into function
(Pdb) r        # Run until current function returns
(Pdb) c        # Continue to next breakpoint
(Pdb) l        # List source code around current line
(Pdb) l 1, 20  # List lines 1-20
(Pdb) w        # Where am I? (show stack)

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✅ Breakpoints

(Pdb) b 42              # Break at line 42
(Pdb) b func_name       # Break when func_name is called
(Pdb) b file.py:42      # Break at line 42 in file.py
(Pdb) b 42, x > 10      # Conditional breakpoint
(Pdb) cl 1              # Clear breakpoint 1
(Pdb) bl                # List all breakpoints

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✅ Example Debug Session

def find_bug(numbers):
    breakpoint()
    total = 0
    for n in numbers:
        total += n
    return total / len(numbers)  # Bug: ZeroDivisionError if empty!

find_bug([])

> find_bug([])
(Pdb) p numbers          # Check input
[]
(Pdb) n                  # Execute next line
(Pdb) p total            # Check state
0
(Pdb) n                  # Step through loop
(Pdb) n                  # Loop doesn't execute (empty list)
(Pdb) p len(numbers)     # Aha! Length is 0
0
(Pdb) c                  # Continue - see the ZeroDivisionError

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✅ Post-Mortem Debugging

Debug after an exception:

# In interactive Python or script
import pdb

try:
    buggy_function()
except:
    pdb.post_mortem()  # Debug at point of failure

Or run with: python -m pdb -c continue script.py

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🔍 Key Takeaways

• Use breakpoint() to pause execution.
• n (next) and s (step) for navigation.
• p and pp to inspect variables.
• c to continue, q to quit.
• Debuggers are powerful for complex, stateful bugs.

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