# Python Debugging (pdb)

The module `pdb` defines an interactive source code debugger for Python programs. It supports setting (conditional) breakpoints and single stepping at the source line level.

## Starting the Debugger

### Using `pdb.set_trace()`

You can insert `pdb.set_trace()` anywhere in your code to pause execution and enter the debugger.

```python
import pdb

def add(a, b):
    pdb.set_trace() # Execution pauses here
    return a + b

print(add(1, 2))
```

### Using `breakpoint()` (Python 3.7+)

In Python 3.7 and newer, there is a built-in function `breakpoint()` which calls `sys.breakpointhook()`, effectively doing the same thing as `pdb.set_trace()` by default, but is cleaner and more flexible.

```python
def add(a, b):
    breakpoint()
    return a + b
```

### From the Command Line

You can also run a script under the debugger without modifying the source code:

```bash
python -m pdb myscript.py
```

## Common Debugger Commands

Once inside the debugger, you can use the following commands:

*   **`n` (next)**: Continue execution until the next line in the current function is reached or it returns.
*   **`s` (step)**: Execute the current line, stop at the first possible occasion (either in a function that is called or on the next line in the current function).
*   **`c` (continue)**: Continue execution, only stop when a breakpoint is encountered.
*   **`p <expression>` (print)**: Evaluate the expression in the current context and print its value.
*   **`l` (list)**: List source code for the current file.
*   **`ll` (long list)**: List the whole source code for the current function or frame.
*   **`w` (where)**: Print a stack trace, with the most recent frame at the bottom.
*   **`q` (quit)**: Quit the debugger.

## Post-Mortem Debugging

If your program crashes, you can use `pdb` to inspect the state at the moment of the crash.

```python
import pdb

try:
    1 / 0
except:
    pdb.post_mortem()
```

[[programming/python/python]]