Numeric validation (gt, lt, ge, le) in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does gt mean in FastAPI numeric validation?

gt means 'greater than'. It ensures the number is strictly more than the given value.

beginner

How do you enforce a number to be less than or equal to 10 in FastAPI?

Use le=10 in the parameter declaration to allow numbers up to 10, including 10.

intermediate

What is the difference between ge and gt in FastAPI validation?

ge means 'greater than or equal to', allowing the number to be equal to the limit.
gt means 'greater than', excluding the limit itself.

beginner

Show a FastAPI example that validates an integer query parameter to be between 1 and 5 inclusive.

<pre>from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(q: int = Query(..., ge=1, le=5)):
    return {"q": q}</pre>

beginner

What happens if a client sends a number outside the allowed range in FastAPI numeric validation?

FastAPI returns a clear error response with status code 422 and a message explaining the validation failure.

In FastAPI, which keyword ensures a number is strictly less than 10?

A le=10

B gt=10

C lt=10

D ge=10

lt=10 means less than 10, excluding 10 itself.

What does ge=5 mean in FastAPI validation?

A Number must be greater than or equal to 5

B Number must be less than or equal to 5

C Number must be less than 5

D Number must be greater than 5

ge=5 means the number can be 5 or more.

Which FastAPI validation keyword would you use to allow numbers up to and including 100?

A le=100

B lt=100

C gt=100

D ge=100

le=100 means less than or equal to 100.

If you want a number strictly greater than 0, which validation keyword do you use?

A ge=0

B le=0

C lt=0

D gt=0

gt=0 means the number must be more than 0, not equal.

What status code does FastAPI return when numeric validation fails?

A 404

B 422

C 200

D 500

FastAPI returns 422 Unprocessable Entity for validation errors.

Explain how to use FastAPI numeric validation keywords to restrict a query parameter between 10 and 20 inclusive.

Think about how to set minimum and maximum limits including the limits themselves.

Describe the difference between gt and ge in FastAPI numeric validation and when you might use each.

Consider if the boundary number should be allowed or not.

Practice

1. In FastAPI, which parameter would you use in a Pydantic Field to ensure a number is strictly greater than 10?

easy

A. gt=10

B. lt=10

C. ge=10

D. le=10

Solution

1. Step 1: Understand the meaning of gt

   The gt parameter means "greater than" and excludes the value itself.

2. Step 2: Match the requirement

   Since the question asks for strictly greater than 10, gt=10 is the correct choice.

3. Final Answer:

   gt=10 -> Option A

4. Quick Check:

   Strictly greater than 10 = gt=10 [OK]

Hint: Use gt for strictly greater than, ge for greater or equal [OK]

Common Mistakes:

• Confusing gt with ge and using ge=10 instead
• Using lt or le which mean less than
• Thinking gt includes the number 10

2. Which of the following is the correct syntax to validate a number field in FastAPI to be less than or equal to 100 using Pydantic?

easy

A. Field(ge=100)

B. Field(lt=100)

C. Field(le=100)

D. Field(gt=100)

Solution

1. Step 1: Identify the parameter for less than or equal

   The parameter le stands for "less than or equal to".

2. Step 2: Check the syntax

   The correct syntax is Field(le=100) to enforce the value to be at most 100.

3. Final Answer:

   Field(le=100) -> Option C

4. Quick Check:

   Less than or equal to 100 = le=100 [OK]

Hint: Use le for less or equal, lt for strictly less [OK]

Common Mistakes:

• Using lt=100 which excludes 100
• Using ge or gt which are for greater values
• Writing Field(le100) without equals sign

3. Given this FastAPI model snippet:

from pydantic import BaseModel, Field

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

What happens if you send a request with price=0?

medium

A. Accepted because price is a float

B. Accepted because price is less than or equal to 100

C. Validation error because price must be less than or equal to 0

D. Validation error because price must be greater than 0

Solution

1. Step 1: Analyze the validation rules

   The field has gt=0 (greater than 0) and le=100 (less or equal to 100).

2. Step 2: Check the input value

   Input price is 0, which is not greater than 0, so it violates the gt=0 rule.

3. Final Answer:

   Validation error because price must be greater than 0 -> Option D

4. Quick Check:

   Price 0 fails gt=0 validation [OK]

Hint: gt excludes the number itself; 0 is not > 0 [OK]

Common Mistakes:

• Thinking 0 passes because it is less than 100
• Confusing gt with ge and allowing 0
• Ignoring validation errors in FastAPI

4. You wrote this FastAPI model:

from pydantic import BaseModel, Field

class User(BaseModel):
    age: int = Field(ge=18, lt=65)

But when you send age=65, it passes validation. Why?

medium

A. Because lt=65 means less than 65, so 65 should fail but it passes because you used le=65 instead of lt=65

B. Because lt=65 means less than 65, so 65 should fail but it passes because you used ge incorrectly

C. Because lt=65 means less than 65, so 65 should fail but it passes because the validation is not enforced on int

D. Because lt=65 means age must be less than 65, so 65 should fail but it passes due to a bug

Solution

1. Step 1: Understand the meaning of lt and le

   lt=65 means strictly less than 65, so 65 should fail validation.

2. Step 2: Identify the cause of passing validation

   If 65 passes, likely the code uses le=65 (less or equal) instead of lt=65, allowing 65.

3. Final Answer:

   Because lt=65 means less than 65, so 65 should fail but it passes because you used le=65 instead of lt=65 -> Option A

4. Quick Check:

   lt excludes value; le includes it [OK]

Hint: lt excludes value; le includes it. Check which you used [OK]

Common Mistakes:

• Confusing lt and le parameters
• Assuming validation is not enforced on integers
• Thinking ge affects upper bound

5. You want to create a FastAPI model for a product rating that must be between 1 and 5 inclusive. Which is the correct way to define this field using Pydantic validation?

hard

A. rating: int = Field(gt=1, lt=5)

B. rating: int = Field(ge=1, le=5)

C. rating: int = Field(gt=1, le=5)

D. rating: int = Field(ge=1, lt=5)

Solution

1. Step 1: Understand inclusive range requirements

   Inclusive means the value can be equal to the limits, so use ge (greater or equal) and le (less or equal).

2. Step 2: Match parameters to the range 1 to 5 inclusive

   Use ge=1 and le=5 to allow 1 and 5 as valid values.

3. Final Answer:

   rating: int = Field(ge=1, le=5) -> Option B

4. Quick Check:

   Inclusive range uses ge and le [OK]

Hint: Use ge and le for inclusive ranges, gt and lt for exclusive [OK]

Common Mistakes:

• Using gt or lt excludes boundary values
• Mixing gt with le or ge with lt incorrectly
• Forgetting to include both bounds