Field validation rules in FastAPI - Step-by-Step Execution

Concept Flow - Field validation rules

Define Pydantic Model with Fields

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Add Validation Rules (e.g., min_length, gt)

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Receive Input Data

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Pydantic Validates Input

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Process Data

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Client Fixes Input

Input data is checked against rules defined in Pydantic models. If valid, data is processed; if not, errors are returned.

Execution Sample

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

class User(BaseModel):
    name: str = Field(min_length=3)
    age: int = Field(gt=0)

app = FastAPI()

Defines a User model with validation rules: name must be at least 3 characters, age must be greater than 0.

Execution Table

Step	Input Data	Validation Check	Result	Action
1	{"name": "Al", "age": 25}	name min_length=3	Fail	Return error: name too short
2	{"name": "Alice", "age": -1}	age gt=0	Fail	Return error: age must be > 0
3	{"name": "Bob", "age": 30}	name min_length=3 and age gt=0	Pass	Process data successfully
4	{"name": "Eve", "age": 0}	age gt=0	Fail	Return error: age must be > 0

💡 Pydantic validates all fields, collects all errors if any fail, and returns them; passes if all rules are met.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
name	undefined	"Al"	"Alice"	"Bob"	"Eve"
age	undefined	25	-1	30	0
validation_result	none	fail	fail	pass	fail

Key Moments - 3 Insights

Why does the validation fail when name is "Al"?

Because the name field requires at least 3 characters (see execution_table step 1), "Al" has only 2 characters.

What happens if age is 0?

Validation fails since age must be greater than 0 (execution_table step 4), so 0 is not accepted.

Does validation check all fields even if one fails?

Yes, Pydantic validates all fields and collects all errors even if some fail (see exit_note).

Visual Quiz - 3 Questions

Look at the execution table, what is the validation result at step 3?

A Pass

B Error

C Fail

D Skipped

💡 Hint

Check the 'Validation Check' and 'Result' columns at step 3 in the execution_table.

At which step does the age field fail validation?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look for 'age gt=0' failure in the execution_table rows.

If the name field had min_length=2 instead of 3, which step would pass validation?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Consider the name length in step 1 and the min_length rule in the execution_table.

Concept Snapshot

Field Validation Rules in FastAPI:
- Use Pydantic models to define fields.
- Add rules like min_length, gt (greater than).
- Input data is checked automatically.
- Errors returned if rules fail.
- Valid data proceeds to processing.

Full Transcript

In FastAPI, field validation rules are set using Pydantic models. Each field can have constraints like minimum length for strings or greater-than for numbers. When input data arrives, FastAPI uses these rules to check if the data is valid. If any rule fails, FastAPI returns an error immediately, telling the client what is wrong. If all rules pass, the data is accepted and processed. This helps keep data clean and safe without extra code.