python-gotchas

## Quick Reference

| Gotcha | Problem | Fix |
|--------|---------|-----|
| Mutable default | `def f(x=[])` | Use `None`, create in function |
| Iterate + mutate | Skips items | Iterate over copy `items[:]` |
| `is` vs `==` | Identity vs value | Use `is` only for `None` |
| Late binding | `lambda: i` captures var | `lambda i=i: i` |
| Float precision | `0.1 + 0.2 != 0.3` | `math.isclose()` |
| Dict mutation | RuntimeError | `list(d.keys())` |
| Class attribute | Shared mutable | Init in `__init__` |

| Falsy Values | Examples |
|--------------|----------|
| Boolean | `False` |
| None | `None` |
| Numbers | `0`, `0.0`, `0j` |
| Empty collections | `""`, `[]`, `{}`, `set()` |

| Scope Rule | Order |
|------------|-------|
| LEGB | Local → Enclosing → Global → Built-in |
| `global` | Access module-level variable |
| `nonlocal` | Access enclosing function variable |

## When to Use This Skill

Use for **debugging and prevention**:
- Understanding why code behaves unexpectedly
- Avoiding common Python pitfalls
- Reviewing code for subtle bugs
- Learning Python's evaluation rules
- Fixing mutable default arguments

**Related skills:**
- For fundamentals: see `python-fundamentals-313`
- For testing: see `python-testing`
- For type hints: see `python-type-hints`

---

# Python Common Gotchas and Pitfalls

## Overview

Python has several well-known pitfalls that trip up developers of all experience levels. Understanding these gotchas prevents subtle bugs and unexpected behavior.

## 1. Mutable Default Arguments

### The Problem

```python
# BAD: Mutable default argument
def add_item(item, items=[]):
    items.append(item)
    return items

# Unexpected behavior!
print(add_item("a"))  # ['a']
print(add_item("b"))  # ['a', 'b'] - NOT ['b']!
print(add_item("c"))  # ['a', 'b', 'c']
```

### Why It Happens

Default arguments are evaluated **once** when the function is defined, not each time it's called. The same list object is reused across all calls.

### The Fix

```python
# GO