Claude Python Debugger Prompt
You are a senior Python developer and debugging expert.
Category
💻 Coding
Difficulty
Beginner
Models
4
Last Updated
2026-06-28
Works with
📄 Example output
⚠️ Common Mistakes
❓ FAQ
⚙️ Fill in your variables
📋 Prompt
You are a senior Python developer and debugging expert.
Error message: [error message]
Expected behaviour: [expected behaviour]
BUGGY CODE:
[code causing error]
Task:
1. DIAGNOSIS: What is the exact cause of this error?
2. EXPLANATION: Why does this happen in Python specifically?
3. FIXED CODE: The corrected version with inline comments explaining every change
4. PREVENTION: How to avoid this class of error in future Python code
5. RELATED BUGS: 2–3 similar bugs to check for in the same codebase
Bonus: Provide a defensive version of the fixed code with:
- Input validation
- Proper try/except with specific exception types (not bare except)
- Type hints (Python 3.10+)
- Logging instead of print statements
Error message: [error message]
Expected behaviour: [expected behaviour]
BUGGY CODE:
[code causing error]
Task:
1. DIAGNOSIS: What is the exact cause of this error?
2. EXPLANATION: Why does this happen in Python specifically?
3. FIXED CODE: The corrected version with inline comments explaining every change
4. PREVENTION: How to avoid this class of error in future Python code
5. RELATED BUGS: 2–3 similar bugs to check for in the same codebase
Bonus: Provide a defensive version of the fixed code with:
- Input validation
- Proper try/except with specific exception types (not bare except)
- Type hints (Python 3.10+)
- Logging instead of print statements
DIAGNOSIS:
You're getting a TypeError: 'NoneType' object is not subscriptable because get_user_data() is returning None when the user doesn't exist, and the code tries to access the result with [0] without checking if data was actually returned.
Line 7: user_name = get_user_data(user_id)[0]
FIXED CODE:
```python
import logging
from typing import Optional
logger = logging.getLogger(__name__)
def get_user_name(user_id: int) -> Optional[str]:
"""Retrieve username by ID. Returns None if user not found."""
if not isinstance(user_id, int) or user_id <= 0:
raise ValueError(f'Invalid user_id: {user_id}')
try:
user_data = get_user_data(user_id)
if user_data is None: # ← THE FIX: check before subscripting
logger.warning(f'No user found for id={user_id}')
return None
return user_data[0]
except (IndexError, KeyError) as e:
logger.error(f'Failed to parse user data for id={user_id}: {e}')
return None
```
RELATED BUGS TO CHECK:
1. Any other place get_user_data() result is used without None check
2. get_user_data() itself — does it handle database connection errors?
3. Are there similar patterns with other database fetch functions?
You're getting a TypeError: 'NoneType' object is not subscriptable because get_user_data() is returning None when the user doesn't exist, and the code tries to access the result with [0] without checking if data was actually returned.
Line 7: user_name = get_user_data(user_id)[0]
FIXED CODE:
```python
import logging
from typing import Optional
logger = logging.getLogger(__name__)
def get_user_name(user_id: int) -> Optional[str]:
"""Retrieve username by ID. Returns None if user not found."""
if not isinstance(user_id, int) or user_id <= 0:
raise ValueError(f'Invalid user_id: {user_id}')
try:
user_data = get_user_data(user_id)
if user_data is None: # ← THE FIX: check before subscripting
logger.warning(f'No user found for id={user_id}')
return None
return user_data[0]
except (IndexError, KeyError) as e:
logger.error(f'Failed to parse user data for id={user_id}: {e}')
return None
```
RELATED BUGS TO CHECK:
1. Any other place get_user_data() result is used without None check
2. get_user_data() itself — does it handle database connection errors?
3. Are there similar patterns with other database fetch functions?
🏆
💡 Pro Tips
Best model for this prompt
DeepSeek
DeepSeek V3 / R1
Always check what a function returns on failure — Python functions that return None on error are a common source of NoneType errors
Use isinstance() checks before assuming a variable's type, especially from external data sources
Read the full traceback from bottom to top — the last line is usually the exact line, but the cause is often higher up
Add assert statements during development (remove in production) to catch assumptions early
Catching bare except: — this catches SystemExit and KeyboardInterrupt and hides real errors
Printing the error instead of logging it — print statements disappear in production
Fixing the symptom without finding the root cause — NoneType errors are usually symptoms of a missing data check upstream
Using mutable default arguments: def func(data=[]) — this is a Python-specific trap that bites almost every developer once
- Which AI model is best for Python debugging?DeepSeek R1 and Claude are both excellent for Python debugging. DeepSeek R1's reasoning approach is particularly good for complex logic bugs. Claude follows complex multi-step debugging instructions precisely.
- Can this help with async Python bugs?Yes — paste your async code and specify the error. Add context about whether you're using asyncio, FastAPI, aiohttp, or another async framework for more targeted debugging.
- What if I don't have an error message?Replace [error message] with a description of the unexpected behaviour ('the function returns an empty list instead of user data' is enough). Include what you expected vs. what actually happened.
- Can this debug Django or Flask code?Yes. Specify the framework in the prompt and paste the relevant view function, model, or middleware code. Framework-specific patterns (ORM queries, middleware chains) are well-understood by Claude and DeepSeek.