What is the __missing__ method in dict subclasses?

Question

Accepted Answer

**__missing__** handles **KeyError** when accessing missing dictionary keys.

```python
class DefaultDict(dict):
    def __missing__(self, key):
        self[key] = f"default_{key}"
        return self[key]

d = DefaultDict()
print(d["existing"])  # KeyError (not in dict)

d["name"] = "Alice"
print(d["name"])      # "Alice"
print(d["missing"])   # "default_missing"
print(d)              # {"name": "Alice", "missing": "default_missing"}

# Counter-like implementation
from collections import Counter

class AutoCounter(dict):
    def __missing__(self, key):
        return 0

counter = AutoCounter()
counter["a"] += 1  # Works! 0 + 1 = 1
counter["b"] += 5
print(counter)  # {"a": 1, "b": 5}

# Cached function results
class CachedDict(dict):
    def __init__(self, func):
        self.func = func

def __missing__(self, key):
        result = self.func(key)
        self[key] = result
        return result

square_cache = CachedDict(lambda x: x ** 2)
print(square_cache[5])   # 25 (computed)
print(square_cache[5])   # 25 (cached)
```

**vs `__getattr__`:**
- `__missing__` for dict key access.
- `__getattr__` for attribute access.

Python: What is the missing method in dict subclasses?