python-debugpy

Debug Python: pdb REPL + debugpy remote (DAP).

# Python Debugger (pdb + debugpy)

## Overview

Three tools, picked by situation:

| Tool | When |
|---|---|
| **`breakpoint()` + pdb** | Local, interactive, simplest. Add `breakpoint()` in the source, run normally, get a REPL at that line. |
| **`python -m pdb`** | Launch an existing script under pdb with no source edits. Useful for quick poking. |
| **`debugpy`** | Remote / headless / "attach to already-running process." Talks DAP, scriptable from terminal, works for long-lived processes (gateway, daemon, PTY children). |

**Start with `breakpoint()`.** It's the cheapest thing that works.

## When to Use

- A test fails and the traceback doesn't reveal why a value is wrong
- You need to step through a function and watch a collection mutate
- A long-running process (hermes gateway, tui_gateway) misbehaves and you can't restart it
- Post-mortem: an exception fired in prod-ish code and you want to inspect locals at the crash site
- A subprocess / child (Python `_SlashWorker`, PTY bridge worker) is the actual bug site

**Don't use for:** things `print()` / `logging.debug` solve in under a minute, or things `pytest -vv --tb=long --showlocals` already reveals.

## pdb Quick Reference

Inside any pdb prompt (`(Pdb)`):

| Command | Action |
|---|---|
| `h` / `h cmd` | help |
| `n` | next line (step over) |
| `s` | step into |
| `r` | return from current function |
| `c` | continue |
| `unt N` | continue until line N |
| `j N` | jump to line N (same function only) |
| `l` / `ll` | list source around current line / full function |
| `w` | where (stack trace) |
| `u` / `d` | move up / down in the stack |
| `a` | print args of the current function |
| `p expr` / `pp expr` | print / pretty-print expression |
| `display expr` | auto-print expr on every stop |
| `b file:line` | set breakpoint |
| `b func` | break on function entry |
| `b file:line, cond` | conditional breakpoint |
| `cl N` | clear breakpoint N |
| `tbreak file:line` | one-shot breakpoint |
| `!stmt` | execute arbitrary Python (assignments included) |
| `interact` | drop into full Python REPL in current scope (Ctrl+D to exit) |
| `q` | quit |

The `interact` command is the most powerful — you can import anything, inspect complex objects, even call methods that mutate state. Locals are read-only by default; use `!x = 42` from the `(Pdb)` prompt to mutate.

## Recipe 1: Local breakpoint

Easiest. Edit the file:

```python
def compute(x, y):
    result = some_helper(x)
    breakpoint()           # <-- drops into pdb here
    return result + y
```

Run the code normally. You land at the `breakpoint()` line with full access to locals.

**Don't forget to remove `breakpoint()` before committing.** Use `git diff` or a pre-commit grep:
```bash
rg -n 'breakpoint\(\)' --type py
```

## Recipe 2: Launch a script under pdb (no source edits)

```bash
python -m pdb path/to/script.py arg1 arg2
# Lands at first line of script
(Pdb) b path/to/script.py:42
(Pdb) c
```

## Recipe 3: Debug a pytest test

The hermes test runner and pytest both support this:

```bash
# Drop to pdb on failure (or on any raised exception):
scripts/run_tests.sh tests/path/to/test_file.py::test_name --pdb

# Drop to pdb at the START of the test:
scripts/run_tests.sh tests/path/to/test_file.py::test_name --trace

# Show locals in tracebacks without pdb:
scripts/run_tests.sh tests/path/to/test_file.py --showlocals --tb=long
```

Note: `scripts/run_tests.sh` uses xdist (`-n 4`) by default, and pdb does NOT work under xdist. Add `-p no:xdist` or run a single test with `-n 0`:

```bash
scripts/run_tests.sh tests/foo_test.py::test_bar --pdb -p no:xdist
# or
source .venv/bin/activate
python -m pytest tests/foo_test.py::test_bar --pdb
```

This bypasses the hermetic-env guarantees — fine for debugging, but re-run under the wrapper to confirm before pushing.

## Recipe 4: Post-mortem on any exception

```python
import pdb, sys
try:
    run_the_thing()
except Exception:
    pdb.post_mortem(sys.exc_info()[2])
```

Or wrap a whole script:

```bash
python -m pdb -c continue script.py
# When it crashes, pdb catches it and you're in the frame of the exception
```

Or set a global hook in a repl/jupyter:

```python
import sys
def excepthook(etype, value, tb):
    import pdb; pdb.post_mortem(tb)
sys.excepthook = excepthook
```

## Recipe 5: Remote debug with debugpy (attach to running process)

For long-lived processes: Hermes gateway, tui_gateway, a daemon, a process that's already misbehaving and can't be restarted clean.

### Setup

```bash
source /home/bb/hermes-agent/.venv/bin/activate
pip install debugpy
```

### Pattern A: Source-edit — process waits for debugger at launch

Add near the top of the entry point (or inside the function you want to debug):

```python
import debugpy
debugpy.listen(("127.0.0.1", 5678))
print("debugpy listening on 5678, waiting for client...", flush=True)
debugpy.wait_for_client()
debugpy.breakpoint()       # optional: pause immediately once attached
```

Start the process; it blocks on `wait_for_client()`.

### Pattern B: No source edit — launch with `-m debugpy`

```bash
python -m debugpy --listen 127.0.0.1:5678 --wait-for-client your_script.py arg1
```

Equivalent for module entry:

```bash
python -m debugpy --listen 127.0.0.1:5678 --wait-for-client -m your.module
```

### Pattern C: Attach to an already-running process

Needs the PID and debugpy preinstalled in the target's environment:

```bash
python -m debugpy --listen 127.0.0.1:5678 --pid <pid>
# debugpy injects itself into the process. Then attach a client as below.
```

Some kernels/security configs block the ptrace-based injection (`/proc/sys/kernel/yama/ptrace_scope`). Fix with:
```bash
echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
```

### Connecting a client from the terminal

The easiest terminal-side DAP client is VS Code CLI or a small script. From inside Hermes you have two practical options:

**Option 1: `debugpy`'s own CLI REPL** — not an official feature, but a tiny DAP client script:

```python
# /tmp/dap_client.py
import socket, json, itertools, time, sys

HOST, PORT = "127.0.0.1", 5678
s = socket.create_connection((HOST, PORT))
seq = itertools.count(1)

def send(msg):
    msg["seq"] = next(seq)
    body = json.dumps(msg).encode()
    s.sendall(f"Content-Length: {len(body)}\r\n\r\n".encode() + body)

def recv():
    header = b""
    while b"\r\n\r\n" not in header:
        header += s.recv(1)
    length = int(header.decode().split("Content-Length:")[1].split("\r\n")[0].strip())
    body = b""
    while len(body) < length:
        body += s.recv(length - len(body))
    return json.loads(body)

send({"type": "request", "command": "initialize", "arguments": {"adapterID": "python"}})
print(recv())
send({"type": "request", "command": "attach", "arguments": {}})
print(recv())
send({"type": "request", "command": "setBreakpoints",
      "arguments": {"source": {"path": sys.argv[1]},
                    "breakpoints": [{"line": int(sys.argv[2])}]}})
print(recv())
send({"type": "request", "command": "configurationDone"})
# ... loop reading events and sending continue/stepIn/etc.
```

This is fine for one-off automation but painful as an interactive UX.

**Option 2: Attach from VS Code / Cursor / Zed** — if the user has one open, they can add a `launch.json`:

```json
{
  "name": "Attach to Hermes",
  "type": "debugpy",
  "request": "attach",
  "connect": { "host": "127.0.0.1", "port": 5678 },
  "justMyCode": false,
  "pathMappings": [
    { "localRoot": "${workspaceFolder}", "remoteRoot": "/home/bb/hermes-agent" }
  ]
}
```

**Option 3: Ditch DAP, use `remote-pdb`** — usually what you actually want from a terminal agent:

```bash
pip install remote-pdb
```

In your code:
```python
from remote_pdb import set_trace
set_trace(host="127.0.0.1", port=4444)   # blocks until connection
```

Then from the terminal:
```bash
nc 127.0.0.1 4444
# You get a (Pdb) prompt exactly as if debugging locally.
```

`remote-pdb` is the cleanest agent-friendly choice when `d