App: Code Review — Automated Review

A3.0 What This Teaches

• Using a system prompt to enforce structured output across specific review categories
• Reviewing for bugs, security, error handling, type hints, and style
• Writing review results to a .review.txt file with pathlib
• Reviewing multiple files in one run by looping over sys.argv

A3.1 Review Categories

• Bugs / logic errors - off-by-one, wrong operator, unreachable code
• Security issues - shell injection, unsafe eval(), hardcoded credentials
• Missing error handling - uncaught exceptions that will crash in production
• Missing type hints - function signatures without int, str, etc.
• Style / readability - PEP 8 violations, unclear names, overly long lines
• Performance - quadratic loops, unnecessary copies, repeated lookups

A3.2 The System Prompt

# review_system.py - the review format.
SYSTEM = (
    "You are a strict Python code reviewer. "
    "Review the code for:\n"
    "BUGS (logic errors)\n"
    "SECURITY (injection, unsafe operations)\n"
    "ERROR HANDLING (missing try/except)\n"
    "TYPE HINTS (missing annotations)\n"
    "STYLE (PEP 8 violations)\n"
    "For each category, list findings as bullet points. "
    "If no issues, write 'None found.'"
)

A3.3 Complete Implementation

# review.py - automated code review using Claude.
# Usage: python review.py <file1.py> [file2.py ...]

import sys
from pathlib import Path
import anthropic

SYSTEM = (
    "You are a strict Python code reviewer. "
    "Review the code for:\n"
    "BUGS (logic errors)\n"
    "SECURITY (injection, unsafe operations)\n"
    "ERROR HANDLING (missing try/except)\n"
    "TYPE HINTS (missing annotations)\n"
    "STYLE (PEP 8 violations)\n"
    "For each category, list findings as bullet points. "
    "If no issues, write 'None found.'"
)

def review_file(client: anthropic.Anthropic, path: str) -> str:
    try:
        code = Path(path).read_text(encoding="utf-8")
    except FileNotFoundError:
        print(f"Error: file not found: {path}")
        sys.exit(1)

    response = client.messages.create(
        model="claude-sonnet-4-6",
        max_tokens=1024,
        system=SYSTEM,
        messages=[{"role": "user", "content": f"Review this Python code:\n\n```python\n{code}\n```"}],
    )
    return response.content[0].text

def main():
    if len(sys.argv) < 2:
        print("Usage: python review.py <file1.py> [file2.py ...]")
        sys.exit(1)

    client = anthropic.Anthropic()

    for path in sys.argv[1:]:
        print(f"\n=== Review: {path} ===")
        review = review_file(client, path)
        print(review)

        out_path = Path(path).with_suffix(".review.txt")
        out_path.write_text(review, encoding="utf-8")
        print(f"\nSaved to {out_path}")

if __name__ == "__main__":
    main()

A3.4 Writing the Review to a File

# save_review.py - write review output alongside the source file.
from pathlib import Path

def save_review(source_path: str, review_text: str) -> Path:
    out = Path(source_path).stem + ".review.txt"
    out_path = Path(source_path).parent / out
    out_path.write_text(review_text, encoding="utf-8")
    return out_path

A3.5 Sample Review Output

$ python review.py loader.py

=== Review: loader.py ===

BUGS
- None found.

SECURITY
- None found.

ERROR HANDLING
- load_data() calls open() without a try/except; a missing file will crash the caller
  with an unhandled FileNotFoundError.

TYPE HINTS
- load_data(filename) has no parameter type annotation (expected str).
- Return type is not annotated (expected list[str]).

STYLE
- Line 14 exceeds 79 characters (PEP 8 E501).

Saved to loader.review.txt

A3.6 Reviewing Multiple Files

# multi_review.py - loop over every argument.
for path in sys.argv[1:]:
    review = review_file(client, path)
    print(f"\n=== {path} ===\n{review}")
    Path(path).with_suffix(".review.txt").write_text(review, encoding="utf-8")

A3.7 Exercise

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("files", nargs="+")
parser.add_argument("--summary", action="store_true")
args = parser.parse_args()

reviews = []
for path in args.files:
    review = review_file(client, path)
    reviews.append(f"--- {path} ---\n{review}")
    print(review)

if args.summary:
    combined = "\n\n".join(reviews)
    summary_response = client.messages.create(
        model="claude-sonnet-4-6",
        max_tokens=256,
        messages=[{
            "role": "user",
            "content": (
                "Write a one-paragraph executive summary of the most critical issues "
                f"found across these code reviews:\n\n{combined}"
            ),
        }],
    )
    print("\n=== Executive Summary ===")
    print(summary_response.content[0].text)

A3.8 Common Mistakes

Key Terms