Python Automated Debugging

Python Automated Debugging

Overview

Run the debugger. Don't just plan to run it.

This skill uses debug_session.py to run non-interactive pdb sessions. You specify commands upfront, execute, analyze output, then iterate. Works on macOS/Linux only.

When to Use

• Test has failed 2+ times despite your fixes
• Code looks correct but behaves wrong
• You need to see runtime state, not just read code
• Print statements haven't revealed the issue

The Cycle

digraph debug_cycle {
    rankdir=LR;
    "Form Hypothesis" -> "Write pdb commands";
    "Write pdb commands" -> "RUN debug_session.py";
    "RUN debug_session.py" -> "Analyze output";
    "Analyze output" -> "Hypothesis confirmed?" [style=invis];
    "Hypothesis confirmed?" [shape=diamond];
    "Hypothesis confirmed?" -> "Fix bug" [label="yes"];
    "Hypothesis confirmed?" -> "Form Hypothesis" [label="no - new hypothesis"];
}

You MUST actually execute the tool. Planning what you would run is not debugging.

Tool Usage

Location: debug_session.py in this skill's directory.

# Basic usage
python debug_session.py <script.py> "cmd1" "cmd2" ... "q"

# Debug a module
python debug_session.py -m <module> "cmd1" "cmd2" "q"

# Save output
python debug_session.py <script.py> "cmd1" "cmd2" "q" --output debug.log

Always end with q to quit cleanly and avoid hanging.

Example Sessions

Investigate a caching bug:

python debug_session.py test_file.py \
    "b process_item" \
    "c" \
    "p multiplier" \
    "p self.cache" \
    "c" \
    "p multiplier" \
    "p self.cache" \
    "q"

Hypothesis: cache isn't updating. Commands compare state across two calls.

Trace unexpected None return:

python debug_session.py buggy.py \
    "b compute_result" \
    "c" \
    "a" \
    "n" \
    "n" \
    "p intermediate" \
    "p result" \
    "w" \
    "q"

Hypothesis: intermediate calculation fails. Commands trace through function.

Debug exception origin:

python debug_session.py crash.py \
    "c" \
    "w" \
    "pp locals()" \
    "l" \
    "q"

Let it crash, then inspect stack and locals at crash point.

Iteration Is Expected

Your first debug run rarely gives complete answers. Common pattern:

1. Run 1: Set breakpoint, print obvious variables → discover which variable is wrong
2. Run 2: Trace where that variable gets set → discover it's cached
3. Run 3: Inspect cache behavior across calls → find the bug

Each run narrows the search space. Don't repeat the same debug session - each iteration should test a new hypothesis or investigate what you learned from the previous run.

Common Mistakes

Red Flags - You're Not Actually Debugging

• "I would run..." → Run it now
• "The commands I'd use are..." → Execute them
• "Based on reading the code..." → You need runtime data, not code reading
• Repeating the same session hoping for different results → New hypothesis needed