Field
- Python course
- Advanced
- Lesson 56/83
- 5 min
Learn validation rules with Field(...).
Field
Field(...) adds validation rules to a field. With gt, lt, ge, le you set bounds; with min_length and max_length you set length.
from pydantic import BaseModel, Field
class User(BaseModel):
name: str = Field(min_length=2)
age: int = Field(gt=0, lt=200)gt=0 means the value must be greater than zero; lt=200 means less than 200. If a rule is broken, Pydantic raises.
Bounds and length
For numeric fields, use gt (greater than), lt (less than), ge (greater or equal), le (less or equal). For string fields, min_length and max_length limit length.
age: int = Field(ge=18, le=65) # between 18-65
name: str = Field(min_length=2, max_length=50) # 2-50 chars
tags: list = Field(min_length=1) # at least one itemThese rules prevent bad data from entering the system; they are especially useful in APIs.
Practice User model with Field rules
Start with from pydantic import BaseModel, Field. Define class User(BaseModel):. Add fields name: str = Field(min_length=2) and age: int = Field(gt=0, lt=200). Create an instance with u = User(name="Ayşe", age=30). Print the values with print(u.name, u.age).
Starter code
# bring in BaseModel and Field from pydantic
# define User class (derive from BaseModel)
# add a min length of 2 to name, a bound of zero and 200 to age
# create a User instance (Ayşe, 30)
# print the valuesSample solution
from pydantic import BaseModel, Field
class User(BaseModel):
name: str = Field(min_length=2)
age: int = Field(gt=0, lt=200)
u = User(name="Ayşe", age=30)
print(u.name, u.age)Expected output
Ayşe 30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the Field call
from pydantic import BaseModel, Field
class User(BaseModel):
age: int = ____(gt=0, lt=200)from pydantic import BaseModel, Field
class User(BaseModel):
age: int = Field(gt=0, lt=200)Explanation: Field(gt=0, lt=200) sets bounds.
2 What does the Field(gt=0) rule mean?
- The value must be greater than zero Correct answer
- The value must be negative
- The value must equal zero
- The value cannot be zero
Explanation: gt=0 means the value must be greater than zero.
3 Match the Field concepts
| Term | Meaning |
|---|---|
gt=0 |
Greater than zero |
lt=200 |
Less than 200 |
min_length=2 |
At least 2 chars |
max_length=50 |
At most 50 chars |