Field

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 item

These rules prevent bad data from entering the system; they are especially useful in APIs.

Exercise 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).

Show example 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)

Test yourself

Complete the Field call

from pydantic import BaseModel, Field

class User(BaseModel):
    age: int = ____(gt=0, lt=200)

Show answer

from pydantic import BaseModel, Field

class User(BaseModel):
    age: int = Field(gt=0, lt=200)

What does the Field(gt=0) rule mean?
- The value must be greater than zero
- The value must be negative
- The value must equal zero
- The value cannot be zero
Show answer

The value must be greater than zero — gt=0 means the value must be greater than zero.
Match the Field concepts

gt=0

Greater than zero
lt=200

Less than 200
min_length=2

At least 2 chars
max_length=50

At most 50 chars

Next lesson: validator

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