Numeric validation (gt, lt, ge, le) in FastAPI - Step-by-Step Execution

Concept Flow - Numeric validation (gt, lt, ge, le)

Receive input data

↓

Check numeric constraints

↓

Validation passes?

No→Return error message

Yes↓

Process valid data

↓

Return success

Input data is checked against numeric rules (greater than, less than, etc.). If it passes, processing continues; if not, an error is returned.

Execution Sample

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

class Item(BaseModel):
    price: float = Field(..., gt=0, le=100)

app = FastAPI()

Defines a FastAPI model where 'price' must be greater than 0 and less or equal to 100.

Execution Table

Step	Input price	Validation Check	Result	Action
1	50	50 > 0 and 50 <= 100	True	Accept input, proceed
2	0	0 > 0 and 0 <= 100	False	Reject input, return error
3	101	101 > 0 and 101 <= 100	False	Reject input, return error
4	100	100 > 0 and 100 <= 100	True	Accept input, proceed
5	-5	-5 > 0 and -5 <= 100	False	Reject input, return error

💡 Validation stops when input fails numeric constraints or all checks pass.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
price	undefined	50	0	101	100	-5
validation_result	undefined	True	False	False	True	False

Key Moments - 3 Insights

Why does input 0 fail validation when the rule is gt=0?

Because 'gt=0' means the value must be strictly greater than 0, not equal. Row 2 in execution_table shows 0 > 0 is False.

What happens if input is exactly 100 with le=100?

It passes because 'le=100' means less than or equal to 100. Row 4 confirms 100 <= 100 is True.

Why is negative input rejected even if le=100?

Because it must also satisfy gt=0. Negative numbers fail the greater than 0 check, as shown in row 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the validation_result at step 3 when price is 101?

A False

B True

C Error

D Undefined

💡 Hint

Check the 'validation_result' column for step 3 in execution_table.

At which step does the input price exactly meet the upper limit of the validation?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look for price equal to 100 in execution_table.

If the 'gt' rule changed from 0 to -10, which step's validation_result would change from False to True?

A Step 2

B Step 5

C Step 3

D Step 1

💡 Hint

Check which input is negative and currently fails validation in execution_table.

Concept Snapshot

FastAPI numeric validation uses Field with gt, lt, ge, le.
'gt' means greater than, 'lt' less than.
'ge' means greater or equal, 'le' less or equal.
Input failing these returns validation error.
Use to ensure numbers fit expected ranges.

Full Transcript

This visual execution shows how FastAPI validates numeric inputs using gt, lt, ge, and le rules. The input value is checked step-by-step against these constraints. If the value passes all checks, it is accepted; otherwise, an error is returned. For example, a price must be greater than 0 and less or equal to 100. Inputs like 50 and 100 pass, while 0, 101, and -5 fail. This helps ensure data is within expected numeric ranges before processing.