What is the ClassVar type hint?

Question

Accepted Answer

**ClassVar** marks an attribute as **class-level**, not instance-level.

```python
from typing import ClassVar

class User:
    count: ClassVar[int] = 0  # Shared across all instances
    name: str  # Instance attribute

def __init__(self, name):
        self.name = name
        User.count += 1

u1 = User("Alice")
u2 = User("Bob")

print(User.count)  # 2
print(u1.count)    # 2 (access via instance)
print(u1.name)     # "Alice"

# Type checker understands:
User.count = 5     # OK — class variable
# u1.count = 5    # Warning: setting class var on instance

# With Pydantic
from pydantic import BaseModel

class Config(BaseModel):
    debug: ClassVar[bool] = False  # Not serialized
    name: str  # Serialized

c = Config(name="test")
print(c.dict())  # {"name": "test"} — no debug!

# In Django
from django.db import models

class Article(models.Model):
    MAX_TITLE_LENGTH: ClassVar[int] = 200
    title = models.CharField(max_length=MAX_TITLE_LENGTH)
```

**Benefits:**
- Clear intent.
- Type checker validation.
- Not included in serialization.

Python: What is the ClassVar type hint?