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skills/n8n-code-python/ERROR_PATTERNS.md

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+# Error Patterns - Python Code Node
+
+Common Python Code node errors and how to fix them.
+
+---
+
+## Error Overview
+
+**Top 5 Python Code Node Errors**:
+
+1. **ModuleNotFoundError** - Trying to import external libraries (Python-specific)
+2. **Empty Code / Missing Return** - No code or return statement
+3. **KeyError** - Dictionary access without .get()
+4. **IndexError** - List access without bounds checking
+5. **Incorrect Return Format** - Wrong data structure returned
+
+These 5 errors cover the majority of Python Code node failures.
+
+---
+
+## Error #1: ModuleNotFoundError (MOST CRITICAL)
+
+**Frequency**: Very common in Python Code nodes
+
+**What it is**: Attempting to import external libraries that aren't available.
+
+### The Problem
+
+```python
+# ❌ WRONG: External libraries not available
+import requests  # ModuleNotFoundError: No module named 'requests'
+import pandas    # ModuleNotFoundError: No module named 'pandas'
+import numpy     # ModuleNotFoundError: No module named 'numpy'
+import bs4       # ModuleNotFoundError: No module named 'bs4'
+import pymongo   # ModuleNotFoundError: No module named 'pymongo'
+import psycopg2  # ModuleNotFoundError: No module named 'psycopg2'
+
+# This code will FAIL - these libraries are not installed!
+response = requests.get("https://api.example.com/data")
+```
+
+### The Solution
+
+**Option 1: Use JavaScript Instead** (Recommended for 95% of cases)
+
+```javascript
+// ✅ JavaScript Code node with $helpers.httpRequest()
+const response = await $helpers.httpRequest({
+  method: 'GET',
+  url: 'https://api.example.com/data'
+});
+
+return [{json: response}];
+```
+
+**Option 2: Use n8n HTTP Request Node**
+
+```python
+# ✅ Add HTTP Request node BEFORE Python Code node
+# Access the response in Python Code node
+
+response = _input.first()["json"]
+
+return [{
+    "json": {
+        "status": response.get("status"),
+        "data": response.get("body"),
+        "processed": True
+    }
+}]
+```
+
+**Option 3: Use Standard Library Only**
+
+```python
+# ✅ Use urllib from standard library (limited functionality)
+from urllib.request import urlopen
+from urllib.parse import urlencode
+import json
+
+# Simple GET request (no headers, no auth)
+url = "https://api.example.com/data"
+with urlopen(url) as response:
+    data = json.loads(response.read())
+
+return [{"json": data}]
+```
+
+### Common Library Replacements
+
+| Need | ❌ External Library | ✅ Alternative |
+|------|-------------------|----------------|
+| HTTP requests | `requests` | Use HTTP Request node or JavaScript |
+| Data analysis | `pandas` | Use Python list comprehensions |
+| Database | `psycopg2`, `pymongo` | Use n8n database nodes |
+| Web scraping | `beautifulsoup4` | Use HTML Extract node |
+| Excel | `openpyxl` | Use Spreadsheet File node |
+| Image processing | `pillow` | Use external API or node |
+
+### Available Standard Library Modules
+
+```python
+# ✅ THESE WORK - Standard library only
+import json          # JSON parsing
+import datetime      # Date/time operations
+import re            # Regular expressions
+import base64        # Base64 encoding
+import hashlib       # Hashing (MD5, SHA256)
+import urllib.parse  # URL parsing and encoding
+import math          # Math functions
+import random        # Random numbers
+import statistics    # Statistical functions
+import collections   # defaultdict, Counter, etc.
+```
+
+---
+
+## Error #2: Empty Code / Missing Return
+
+**Frequency**: Common across all Code nodes
+
+**What it is**: Code node has no code or no return statement.
+
+### The Problem
+
+```python
+# ❌ WRONG: Empty code
+# (nothing here)
+
+# ❌ WRONG: Code but no return
+items = _input.all()
+processed = [item for item in items if item["json"].get("active")]
+# Forgot to return!
+
+# ❌ WRONG: Return in wrong scope
+if _input.all():
+    return [{"json": {"result": "success"}}]
+# Return is inside if block - may not execute!
+```
+
+### The Solution
+
+```python
+# ✅ CORRECT: Always return
+all_items = _input.all()
+
+if not all_items:
+    # Return empty array or error
+    return [{"json": {"error": "No items"}}]
+
+# Process items
+processed = [item for item in all_items if item["json"].get("active")]
+
+# Always return at the end
+return processed if processed else [{"json": {"message": "No active items"}}]
+```
+
+### Best Practice
+
+```python
+# ✅ GOOD: Return at end of function (unconditional)
+def process_items():
+    items = _input.all()
+
+    if not items:
+        return [{"json": {"error": "Empty input"}}]
+
+    # Process
+    result = []
+    for item in items:
+        result.append({"json": item["json"]})
+
+    return result
+
+# Call function and return result
+return process_items()
+```
+
+---
+
+## Error #3: KeyError
+
+**Frequency**: Very common in Python Code nodes
+
+**What it is**: Accessing dictionary key that doesn't exist.
+
+### The Problem
+
+```python
+# ❌ WRONG: Direct key access
+item = _input.first()["json"]
+
+name = item["name"]        # KeyError if "name" doesn't exist!
+email = item["email"]      # KeyError if "email" doesn't exist!
+age = item["age"]          # KeyError if "age" doesn't exist!
+
+return [{
+    "json": {
+        "name": name,
+        "email": email,
+        "age": age
+    }
+}]
+```
+
+### Error Message
+
+```
+KeyError: 'name'
+```
+
+### The Solution
+
+```python
+# ✅ CORRECT: Use .get() with defaults
+item = _input.first()["json"]
+
+name = item.get("name", "Unknown")
+email = item.get("email", "no-email@example.com")
+age = item.get("age", 0)
+
+return [{
+    "json": {
+        "name": name,
+        "email": email,
+        "age": age
+    }
+}]
+```
+
+### Nested Dictionary Access
+
+```python
+# ❌ WRONG: Nested key access
+webhook = _input.first()["json"]
+name = webhook["body"]["user"]["name"]  # Multiple KeyErrors possible!
+
+# ✅ CORRECT: Safe nested access
+webhook = _input.first()["json"]
+body = webhook.get("body", {})
+user = body.get("user", {})
+name = user.get("name", "Unknown")
+
+# ✅ ALSO CORRECT: Chained .get()
+name = (
+    webhook
+    .get("body", {})
+    .get("user", {})
+    .get("name", "Unknown")
+)
+
+return [{"json": {"name": name}}]
+```
+
+### Webhook Body Access (Critical!)
+
+```python
+# ❌ WRONG: Forgetting webhook data is under "body"
+webhook = _input.first()["json"]
+name = webhook["name"]        # KeyError!
+email = webhook["email"]      # KeyError!
+
+# ✅ CORRECT: Access via ["body"]
+webhook = _input.first()["json"]
+body = webhook.get("body", {})
+name = body.get("name", "Unknown")
+email = body.get("email", "no-email")
+
+return [{
+    "json": {
+        "name": name,
+        "email": email
+    }
+}]
+```
+
+---
+
+## Error #4: IndexError
+
+**Frequency**: Common when processing arrays/lists
+
+**What it is**: Accessing list index that doesn't exist.
+
+### The Problem
+
+```python
+# ❌ WRONG: Assuming items exist
+all_items = _input.all()
+first_item = all_items[0]        # IndexError if list is empty!
+second_item = all_items[1]       # IndexError if only 1 item!
+
+return [{
+    "json": {
+        "first": first_item["json"],
+        "second": second_item["json"]
+    }
+}]
+```
+
+### Error Message
+
+```
+IndexError: list index out of range
+```
+
+### The Solution
+
+```python
+# ✅ CORRECT: Check length first
+all_items = _input.all()
+
+if len(all_items) >= 2:
+    first_item = all_items[0]["json"]
+    second_item = all_items[1]["json"]
+
+    return [{
+        "json": {
+            "first": first_item,
+            "second": second_item
+        }
+    }]
+else:
+    return [{
+        "json": {
+            "error": f"Expected 2+ items, got {len(all_items)}"
+        }
+    }]
+```
+
+### Safe First Item Access
+
+```python
+# ✅ CORRECT: Use _input.first() instead of [0]
+# This is safer than manual indexing
+first_item = _input.first()["json"]
+
+return [{"json": first_item}]
+
+# ✅ ALSO CORRECT: Check before accessing
+all_items = _input.all()
+if all_items:
+    first_item = all_items[0]["json"]
+else:
+    first_item = {}
+
+return [{"json": first_item}]
+```
+
+### Slice Instead of Index
+
+```python
+# ✅ CORRECT: Use slicing (never raises IndexError)
+all_items = _input.all()
+
+# Get first 5 items (won't fail if fewer than 5)
+first_five = all_items[:5]
+
+# Get items after first (won't fail if empty)
+rest = all_items[1:]
+
+return [{"json": item["json"]} for item in first_five]
+```
+
+---
+
+## Error #5: Incorrect Return Format
+
+**Frequency**: Common for new users
+
+**What it is**: Returning data in wrong format (n8n expects array of objects with "json" key).
+
+### The Problem
+
+```python
+# ❌ WRONG: Returning plain dictionary
+return {"name": "Alice", "age": 30}
+
+# ❌ WRONG: Returning array without "json" wrapper
+return [{"name": "Alice"}, {"name": "Bob"}]
+
+# ❌ WRONG: Returning None
+return None
+
+# ❌ WRONG: Returning string
+return "success"
+
+# ❌ WRONG: Returning single item (not array)
+return {"json": {"name": "Alice"}}
+```
+
+### The Solution
+
+```python
+# ✅ CORRECT: Array of objects with "json" key
+return [{"json": {"name": "Alice", "age": 30}}]
+
+# ✅ CORRECT: Multiple items
+return [
+    {"json": {"name": "Alice"}},
+    {"json": {"name": "Bob"}}
+]
+
+# ✅ CORRECT: Transform items
+all_items = _input.all()
+return [
+    {"json": item["json"]}
+    for item in all_items
+]
+
+# ✅ CORRECT: Empty array (valid)
+return []
+
+# ✅ CORRECT: Single item still needs array wrapper
+return [{"json": {"result": "success"}}]
+```
+
+### Common Scenarios
+
+**Scenario 1: Aggregation (Return Single Result)**
+
+```python
+# Calculate total
+all_items = _input.all()
+total = sum(item["json"].get("amount", 0) for item in all_items)
+
+# ✅ CORRECT: Wrap in array with "json"
+return [{
+    "json": {
+        "total": total,
+        "count": len(all_items)
+    }
+}]
+```
+
+**Scenario 2: Filtering (Return Multiple Results)**
+
+```python
+# Filter active items
+all_items = _input.all()
+active = [item for item in all_items if item["json"].get("active")]
+
+# ✅ CORRECT: Already in correct format
+return active
+
+# ✅ ALSO CORRECT: If transforming
+return [
+    {"json": {**item["json"], "filtered": True}}
+    for item in active
+]
+```
+
+**Scenario 3: No Results**
+
+```python
+# ✅ CORRECT: Return empty array
+return []
+
+# ✅ ALSO CORRECT: Return error message
+return [{"json": {"error": "No results found"}}]
+```
+
+---
+
+## Bonus Error: AttributeError
+
+**What it is**: Using _input.item in wrong mode.
+
+### The Problem
+
+```python
+# ❌ WRONG: Using _input.item in "All Items" mode
+current = _input.item        # None in "All Items" mode
+data = current["json"]       # AttributeError: 'NoneType' object has no attribute '__getitem__'
+```
+
+### The Solution
+
+```python
+# ✅ CORRECT: Check mode or use appropriate method
+# In "All Items" mode, use:
+all_items = _input.all()
+
+# In "Each Item" mode, use:
+current_item = _input.item
+
+# ✅ SAFE: Check if item exists
+current = _input.item
+if current:
+    data = current["json"]
+    return [{"json": data}]
+else:
+    # Running in "All Items" mode
+    return _input.all()
+```
+
+---
+
+## Error Prevention Checklist
+
+Before running your Python Code node, verify:
+
+- [ ] **No external imports**: Only standard library (json, datetime, re, etc.)
+- [ ] **Code returns data**: Every code path ends with `return`
+- [ ] **Correct format**: Returns `[{"json": {...}}]` (array with "json" key)
+- [ ] **Safe dictionary access**: Uses `.get()` instead of `[]` for dictionaries
+- [ ] **Safe list access**: Checks length before indexing or uses slicing
+- [ ] **Webhook body access**: Accesses webhook data via `_json["body"]`
+- [ ] **No None returns**: Returns empty array `[]` instead of `None`
+- [ ] **Mode awareness**: Uses `_input.all()`, `_input.first()`, or `_input.item` appropriately
+
+---
+
+## Quick Fix Reference
+
+| Error | Quick Fix |
+|-------|-----------|
+| `ModuleNotFoundError` | Use JavaScript or HTTP Request node instead |
+| `KeyError: 'field'` | Change `data["field"]` to `data.get("field", default)` |
+| `IndexError: list index out of range` | Check `if len(items) > 0:` before `items[0]` |
+| Empty output | Add `return [{"json": {...}}]` at end |
+| `AttributeError: 'NoneType'` | Check mode setting or verify `_input.item` exists |
+| Wrong format error | Wrap result: `return [{"json": result}]` |
+| Webhook KeyError | Access via `_json.get("body", {})` |
+
+---
+
+## Testing Your Code
+
+### Test Pattern 1: Handle Empty Input
+
+```python
+# ✅ Always test with empty input
+all_items = _input.all()
+
+if not all_items:
+    return [{"json": {"message": "No items to process"}}]
+
+# Continue with processing
+# ...
+```
+
+### Test Pattern 2: Test with Missing Fields
+
+```python
+# ✅ Use .get() with defaults
+item = _input.first()["json"]
+
+# These won't fail even if fields missing
+name = item.get("name", "Unknown")
+email = item.get("email", "no-email")
+age = item.get("age", 0)
+
+return [{"json": {"name": name, "email": email, "age": age}}]
+```
+
+### Test Pattern 3: Test Both Modes
+
+```python
+# ✅ Code that works in both modes
+try:
+    # Try "Each Item" mode first
+    current = _input.item
+    if current:
+        return [{"json": current["json"]}]
+except:
+    pass
+
+# Fall back to "All Items" mode
+all_items = _input.all()
+return all_items if all_items else [{"json": {"message": "No data"}}]
+```
+
+---
+
+## Summary
+
+**Top 5 Errors to Avoid**:
+1. **ModuleNotFoundError** - Use JavaScript or n8n nodes instead
+2. **Missing return** - Always end with `return [{"json": {...}}]`
+3. **KeyError** - Use `.get()` for dictionary access
+4. **IndexError** - Check length before indexing
+5. **Wrong format** - Return `[{"json": {...}}]`, not plain objects
+
+**Golden Rules**:
+- NO external libraries (use JavaScript instead)
+- ALWAYS use `.get()` for dictionaries
+- ALWAYS return `[{"json": {...}}]` format
+- CHECK lengths before list access
+- ACCESS webhook data via `["body"]`
+
+**Remember**:
+- JavaScript is recommended for 95% of use cases
+- Python has limitations (no requests, pandas, numpy)
+- Use n8n nodes for complex operations
+
+**See Also**:
+- [SKILL.md](SKILL.md) - Python Code overview
+- [DATA_ACCESS.md](DATA_ACCESS.md) - Data access patterns
+- [STANDARD_LIBRARY.md](STANDARD_LIBRARY.md) - Available modules
+- [COMMON_PATTERNS.md](COMMON_PATTERNS.md) - Production patterns