pdf-debug-unbuffered

PDF Generation Debug Technique

When Python PDF libraries (reportlab, fpdf, etc.) fail with generic "unknown error" messages, the actual cause is often hidden due to output buffering or error swallowing. This skill provides a technique to expose the real traceback.

Core Technique

Run the Python script with the -u flag (unbuffered output) and pipe stderr through head to capture the actual error:

python -u your_script.py 2>&1 | head -100

Or more specifically, to focus on stderr:

python -u your_script.py 2>&1 | head -50

Why This Works

1. -u flag: Forces Python to run in unbuffered mode, ensuring output (including errors) is flushed immediately rather than held in buffers that may be lost on crash
2. 2>&1: Redirects stderr to stdout so both streams are captured together
3. head: Limits output to show the most relevant error messages at the top

Step-by-Step Debugging

Step 1: Identify the failing script

Locate the Python script that generates the PDF and is producing generic errors.

Step 2: Run with unbuffered output

python -u generate_pdf.py 2>&1 | head -100

Step 3: Analyze the traceback

Look for:

• Import errors (missing dependencies)
• File path issues (invalid paths, permission errors)
• Font registration problems (common with reportlab)
• Memory or resource constraints

Step 4: Fix the underlying issue

Address the specific error revealed in the traceback.

Step 5: Re-run normally

Once fixed, run the script without the debug flags to confirm it works:

python generate_pdf.py

Common Issues Revealed

Symptom	Likely Cause
"unknown error" on PDF save	File path doesn't exist or no write permissions
Generic failure during build	Missing font files or font registration issues
Silent crash	ImportError for missing dependencies
Incomplete PDF	Script terminated early due to unhandled exception

Alternative Approaches

If the above doesn't work, try:

# Capture full stderr to a file
python -u script.py 2> error.log

# Use Python's traceback module explicitly
python -c "import traceback; exec(open('script.py').read())" 2>&1

# Run with verbose import tracing
python -v -u script.py 2>&1 | head -200

Example: Debugging reportlab

# Before: Generic error
python create_checklist.py
# Output: "Error: unknown"

# After: Actual traceback  
python -u create_checklist.py 2>&1 | head -50
# Output: "FileNotFoundError: [Errno 2] No such file or directory: '/fonts/Helvetica.ttf'"

Notes

• This technique applies to any Python script with opaque error messages, not just PDF generation
• The -u flag is available in Python 2.7+ and all Python 3 versions
• For production debugging, consider logging to a file instead of using head