Effortless Query Parameter Validation in FastAPI

The validation are performed to ensure the Data Integrity, with FastAPI, it is even simple to put validation in the decorator method. We will see this in follow up example.

The parameter validation are of two type:

  1. Query Parameter Validation
  2. Path Parameter Validation

In this post we will cover different kind of validation we can perform with Query Parameter:

  1. Required Parameter Validation:
  2. Default Value with Query Parameter
  3. Min and Max validation on Query Parameter
  4. Less than and greater than Query Validation

Required Parameter Validation:

async def read_jupiter(jup_value: str = Query(..., description="Query Para is required field")):
    return {"jup": jup_value}

Here we have used “…” which refers that this field is mandatory and the value need to be passed in this parameter mandatorily.

Default Value with Query Parameter

async def read_count(count: int = Query(5,title="Less Than Validation", description="less then 10", le=10)):
    return {"jup": count}

Here in the Query construct, if you see the first parameter is passed as 5, that is nothing but the default parameter, if you don’t pass anything while calling this api, by default it will take 5 as parameter.

Min and Max validation on Query Parameter

async def read_min_max_input(count: int = Query(5,title="Less Than Validation", description="less than 100", ge=0, le=100)):
    return {"jup": count}

Here in the Query construct, we have used ge and le, which corresponds to “greater than equal to” and “less than equal it” as 0 and 100. So it will accept the value which is ranging from 1 to 100.

Min Length and Max Length Query Parameter

async def read_min_max_str_input(name: str = Query(...,title="min and max length validation",
                                                   description="min and max validation", min_length=3, max_length=10)):
    return {"name": name}

Here the validation is performed on string datatype where it checks the length of the string and find if the length of the string is less than 3 or more than 10 through validation parameter min_length and max_length.

Checking the validation on swagger UI:

Required Parameter Validation on SwiggerUI:

Here if you see the jup value parameter comes with red *, which signifies this is compulsory field and need a value:

Default Value with Query Parameter with SwaggerUI:

Here if you see the default value is set to 5, if we don’t pass anything, by default it will take 5 as parameter.

Min and Max validation on Query Parameter with SwaggerUI:

Here it shows the default value is 5, min value can be entered is 0 and max is 100, lets tryout

Since we passed parameter value out of range i.e. more than 100, we got the validation error.

Min Length and Max Length Query Parameter

In this post we saw some validation on query parameter, in the next post we will see more validation with Path Parameter.

Happy Coding!


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