Lesson Learned: F-String Syntax Error Crash (Dec 13, 2025)

ID: ll_024 Date: December 13, 2025 Severity: CRITICAL Category: Python Syntax, CI/CD, Weekend Trading Impact: Weekend crypto trading broken for 6 days

Executive Summary

A Python 3.12+ incompatible f-string syntax on line 796 of autonomous_trader.py caused the script to crash immediately (0 seconds execution), breaking weekend crypto trading for nearly a week.

The Bug

# BROKEN - Python 3.12+ doesn't allow backslashes in f-string expressions:
logger.info(f"Allocation: {[(s['symbol'], f\"{s['strength']*100:.0f}%\") for s in signals]}")

Why It Failed

  1. Python 3.12 introduced stricter f-string parsing
  2. Backslash escapes (\") inside f-string expressions are now forbidden
  3. The script couldn’t even be parsed - python -m py_compile fails
  4. GitHub Actions showed “0 seconds” execution time (instant crash)

The Fix

# FIXED - Pre-compute the string outside the f-string:
allocation_str = [(s['symbol'], f"{s['strength']*100:.0f}%") for s in signals]
logger.info(f"Allocation: {allocation_str}")

Detection Timeline

Time What Happened
~Dec 7 Bug introduced (Precious Metals Strategy PR)
Dec 7-13 Weekend crypto trading silently failing
Dec 13 CEO noticed no weekend trades
Dec 13 CTO investigated, found syntax error
Dec 13 Fixed in PR #598, workflow now succeeds

Prevention Rules

Rule 1: Run py_compile Before Commit

# Add to pre-commit hook:
python3 -m py_compile scripts/autonomous_trader.py

Rule 2: Test Python 3.11+ Compatibility

# Avoid nested f-strings with escapes:
# BAD:  f"outer {f\"inner\"}"
# GOOD: inner = f"inner"; f"outer {inner}"

Rule 3: Check Workflow Execution Times

If a GitHub Actions step completes in 0 seconds, it’s a crash:

  • Import error → ~0.5s with error message
  • Syntax error → 0.0s instant crash
  • Runtime error → Variable timing

Rule 4: Add Syntax Check to CI

- name: Verify Python syntax
  run: python3 -m py_compile scripts/autonomous_trader.py

Verification Test

def test_ll_024_no_syntax_errors():
    """Verify all critical scripts have valid Python syntax."""
    import subprocess
    import sys

    scripts = [
        "scripts/autonomous_trader.py",
        "scripts/pre_market_health_check.py",
    ]

    for script in scripts:
        result = subprocess.run(
            [sys.executable, "-m", "py_compile", script],
            capture_output=True,
        )
        assert result.returncode == 0, f"Syntax error in {script}: {result.stderr}"
  • PR #597: Added PYTHONPATH (red herring - wasn’t the actual issue)
  • PR #598: Fixed the actual syntax error
  • ll_023: Session start checklist violation (how this was discovered)

Tags

#python-syntax #f-string #python-312 #ci-cd #weekend-trading #critical #lessons-learned

Change Log

  • 2025-12-13: Discovered and fixed f-string syntax error in autonomous_trader.py