51. Python's pdb Debugger

Here are 10 Python code snippets demonstrating how to use Python's built-in debugger, pdb, for step-by-step code analysis:

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1. Basic Usage of pdb.set_trace()

Inserting a breakpoint in the code to start the debugger.

import pdb

def calculate_sum(a, b):
    result = a + b
    pdb.set_trace()  # Start debugger here
    return result

print(calculate_sum(3, 4))

When pdb.set_trace() is hit, the debugger will pause execution, and you can inspect variables and step through the code.

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2. Navigating Code with Commands in pdb

Using n (next), s (step), and c (continue) to control the execution flow.

import pdb

def calculate_sum(a, b):
    result = a + b
    pdb.set_trace()  # Start debugger
    print(f"Result: {result}")
    return result

calculate_sum(3, 4)

When the debugger is triggered, use:

• n: Move to the next line.

• s: Step into functions.

• c: Continue execution.

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3. Inspecting Variable Values in Debugger

Inspecting variables using pdb commands.

import pdb

def greet(name):
    message = f"Hello, {name}!"
    pdb.set_trace()  # Start debugger
    print(message)

greet("Alice")

When the debugger stops, use p name to inspect the value of the variable name.

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4. Setting Breakpoints with pdb.set_trace()

Setting multiple breakpoints in a function.

import pdb

def calculate_difference(a, b):
    result = a - b
    pdb.set_trace()  # Breakpoint 1
    print(f"Intermediate result: {result}")
    return result

calculate_difference(10, 5)

You can add multiple breakpoints, and at each one, you can examine the execution flow and variable values.

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5. Using pdb for Conditional Breakpoints

Setting a breakpoint that only activates under a condition.

import pdb

def find_max(a, b):
    if a > b:
        result = a
    else:
        result = b
    pdb.set_trace() if result == 10 else None  # Conditional breakpoint
    return result

find_max(3, 10)

The debugger will only be triggered if result equals 10.

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6. Listing the Source Code with l (list)

Using the l command to list the current source code around the breakpoint.

import pdb

def multiply(a, b):
    result = a * b
    pdb.set_trace()  # Trigger debugger
    return result

multiply(3, 5)

When the debugger stops, you can use l to list the current source code around the breakpoint.

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7. Exiting the Debugger with q (quit)

Exiting the debugger session using the q command.

import pdb

def subtract(a, b):
    result = a - b
    pdb.set_trace()  # Trigger debugger
    return result

subtract(9, 3)

In the debugger, type q to quit the debugger and stop the execution.

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8. Using pdb with Exception Handling

Debugging inside a try-except block.

import pdb

def divide(a, b):
    try:
        result = a / b
        pdb.set_trace()  # Trigger debugger
    except ZeroDivisionError as e:
        print("Cannot divide by zero!")
    return result

divide(10, 0)

The debugger will trigger before the exception handling takes place, allowing you to inspect the exception context.

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9. Running the Python Script with Debugger from Command Line

Using the -m pdb command to run the script in debugging mode.

python -m pdb myscript.py

This will start the Python script inside the pdb debugger. You can set breakpoints, step through, and inspect variables interactively.

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10. Tracing Function Calls with pdb

Using pdb to trace the function calls.

import pdb

def add(a, b):
    return a + b

def multiply(a, b):
    return a * b

def calculate(a, b):
    result = add(a, b)
    pdb.set_trace()  # Start debugger
    result = multiply(a, b)
    return result

calculate(2, 3)

Here, you can trace the execution, going through both the add and multiply function calls, inspecting variables as needed.

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These snippets demonstrate different ways to use the pdb debugger for step-by-step code analysis, inspecting variables, and controlling the execution flow. You can use these techniques to troubleshoot, understand the code better, and refine your program.