# Debugging A structured overview of the concepts, tools, and techniques used to find and resolve errors in computer programs. This process is a critical phase of the . ## Bug and Debugging - **Bug**: An unanticipated error in a program, which is generally logical in nature. Simple bugs can be found by manually tracing the code (*dry run*), but complex bugs in large programs require specialized tools. - **Debugging**: The process of finding and fixing bugs in a program. For complex code, manual tracing is tedious, making debugging tools essential. ### Debugger in an IDE (Integrated Development Environment) Most IDEs, like Python's IDLE, include a built-in graphical debugger to help programmers inspect their code's execution. - **Activation**: In Python IDLE, navigate to the `Debug` menu and select `Debugger`. This will display a 'Debug On' message in the shell. - **Debug Control Window**: When a program is run with the debugger active, a 'Debug Control' window appears, allowing for monitoring of variables and program flow. - **Controls and Features**: | Button | Function | | :--- | :--- | | **Go** | Runs the program until the next *breakpoint* is reached or the program ends. | | **Step** | Executes the code one line at a time. This is the most common button for detailed inspection. | | **Over** | Executes a function call without stepping through the function's internal code. The debugger "steps over" the function, executing it in one go. | | **Out** | When inside a function, this executes the remaining lines of the function and returns to the line where the function was called. | - **Information Panes**: * **Stack**: Highlights the function currently being executed. * **Source**: Shows the source code file being debugged. * **Globals**: Displays all global variables and their current values. * **Locals**: Displays all local variables (within the current function or scope) and their values. - **Breakpoint**: A specific line in the code marked as a stopping point for the debugger. When the program executes, it will pause at each breakpoint, allowing the programmer to inspect the state of the variables. * **Setting a Breakpoint**: Right-click on a line of code in the editor and select 'Set Breakpoint'. The line will be highlighted (usually in yellow). * **Clearing a Breakpoint**: Right-click on a highlighted line and select 'Clear Breakpoint'. ### Python Debugger (pdb) Python includes a built-in, command-line debugger module called `pdb`. 1. **Running from the Command Line**: You can start a script with the debugger attached directly from your terminal. ```bash > Python -m pdb your_script.py ``` The `-m` switch tells Python to run the `pdb` module as a script. You can then use commands like `step` or `next` to execute the code line-by-line. 2. **Setting Breakpoints in `pdb`**: * Use the `break ` command to set a breakpoint at a specific line. * Use the `continue` command to run the code until the next breakpoint is hit. * The `list` command shows the source code around the current line. 3. **In-Code Debugging**: * **Import `pdb`**: Add `import pdb` to the top of your Python script. * **Set Trace**: Place the line `pdb.set_trace()` in your code where you want to start debugging. When the program runs, it will pause at this line and open the `pdb` command prompt in your terminal. ### Other Debugging Techniques - **Debugging via `print()` statements**: This is a simple yet powerful technique. By strategically placing `print()` statements in your code, you can display the values of variables at different stages of execution to trace their changes and identify where a bug occurs. *Example Code:* ```python x = 5 y = 3 temp = 0 print('Values of x:', x, 'and y:', y) print("Let's swap the value of x & y") temp = x print('Values of x:', x, 'and y:', y, 'and temp:', temp) y = temp print('Values of x:', x, 'and y:', y, 'and temp:', temp) x = y # This is the bug, should be x = y print('Values of x:', x, 'and y:', y, 'and temp:', temp) ``` *Output revealing the bug:* | Output | | :--- | | `Values of x: 5 and y: 3` | | `Let's Swap the value of x & y` | | `Values of x: 5 and y: 3 and temp: 5` | | `Values of x: 5 and y: 5 and temp: 5` | | `Values of x: 5 and y: 5 and temp: 5` | The output shows that after `x = y`, both `x` and `y` become 5, which is incorrect for a swap. - **`assert` Statement**: The `assert` keyword tests a condition. * If the condition is `True`, the program continues normally. * If the condition is `False`, the program halts and raises an `AssertionError`. * This is useful for checking assumptions about the state of your program and quickly locating logical errors. *Example:* ```python x = 5 x = 5 * -1 # A potential error is introduced assert x > 0 # This will fail print(x) # Output: # AssertionError ```