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FastAPI: Building High-Performing APIs with Path Parameters and Request Body

The modern world is built on APIs (Application Programming Interfaces), which help systems and applications communicate with one another. FastAPI is a popular Python web framework that allows the creation of high-performing APIs. FastAPI offers a simple yet powerful solution for building APIs using its robust features.

In this article, we will explore how FastAPI handles Path Parameters and 404 Not Found errors, two crucial aspects of API development that are often overlooked. Path Parameters:

Path Parameters are variables used in the URL to fetch specific data from an API.

As the name suggests, Path Parameters represent a part of the URL’s path. With FastAPI, declaring Path Parameters is simple and can be achieved by including curly braces {} in your path string.

For example, to declare a Path Parameter in FastAPI, you can use a Python formatted string, which is inside the function definition. Path Parameters With Types:

Type declaration of path Parameters can help editor support for better typing, and parsing data at the same time.

FastAPI automatically parses and converts strings to the declared type. For instance, an integer Path Parameter can be declared with int type, and FastAPI will convert the string to int before handling it.

As a result, data type conversion and validation are simplified. Data Conversion:

FastAPI handles automatic request parsing, reducing the complexity of code logic.

With FastAPI, an HTTP request with a Path Parameter is parsed, validated, and converted to the appropriate data type. For example, if the Path Parameter is a string and the API expects an integer, FastAPI will automatically convert the string to an integer.

Data Validation:

FastAPI ensures that the data received in an HTTP request with Path Parameters is valid by validating it. If the data is not in the correct format, an HTTP error is returned, making the API more robust.

In addition, FastAPI can handle the conversion of data types such as float to int. Documentation:

FastAPI is designed to help developers create clean code with interactive API documentation.

FastAPI provides two documentation frameworks- Swagger UI and ReDoc. FastAPI’s documentation framework makes the API documentation interactive and presents it in an easy-to-understand format with live APIs that help to verify the request/response.

Data Handling with Pydantic:

Pydantic is a library used to power data validation and settings management, which can be integrated into the FastAPI for sophisticated data validation. It validates the incoming data against a data schema, saving time and reducing code complexity.

With Pydantic, you can declare specific data types for API request/response data, making your APIs more predictable. Order Matters: Put Fixed Paths First:

Order plays an essential role in how FastAPI handles requests for the Path Parameters.

FastAPI checks the paths and the HTTP operations in the order they are registered. Therefore, fixed paths should come first in the path operations to handle requests for Path Parameters correctly.

Not Found:

404 Not Found is a common error that occurs with an API when the requested resource is not available. FastAPI’s error handling mechanism automatically generates a 404 HTTP response status code when an incorrect endpoint URL or Path Parameter is requested, informing the user that the resource is not found.


FastAPI is an efficient and effective Python web framework that optimizes the work of building APIs. Handling Path Parameters and 404 Not Found errors are essential in designing user-friendly APIs. Path Parameters provide a simple way of accessing data and converting data types; they can also validate incoming data. FastAPI’s built-in support for Pydantic adds an extra layer of validation to incoming data, simplifying code logic.

FastAPI also efficiently handles 404 HTTP errors, providing a smooth user experience. With a clear understanding of these features, developers can create robust, user-friendly, and efficient APIs.

Path Operation Configuration:

FastAPI’s Path Operation functions allow developers to define HTTP endpoints, which the API will represent.

Path Operation functions are an essential part of building a web application with FastAPI. They define the actions that an API performs.

Some of the key features of Path Operations include request methods and HTTP verbs. Request Methods:

FastAPI supports all HTTP request methods(GET, POST, PUT, DELETE, etc.) when defining Path Operation functions.

It’s crucial to choose the appropriate HTTP request method depending on the type of action being performed. For example, a GET request method is utilized to retrieve data from the server, while POST implies adding data.

Similarly, PUT is used to update data on the server, and DELETE, as the name suggests, is used to delete data from the server database. Decorating Path Operation Functions:

FastAPI’s Path Operations can be decorated to inject dependencies into the function.

Dependency Injection is a design pattern that allows dependencies to be injected into an object through its constructor or setter methods. Decorators incorporate the Dependency Injection feature in FastAPI.

By using decorator factories, the Path Operation’s functions are modified to use additional dependencies that are not explicitly provided. Some common decorators built into FastAPI include `@app.get`, ``, `@app.put`, and `@app.delete.`

Extra Data:

Sometimes developers need to pass in extra data that is not part of the endpoints’ predetermined parameters.

FastAPI allows developers to pass extra data to their Path Operation functions as part of the request. This is a useful feature that enables developers to run operations on servers that are specific to their needs.

This extra data can be passed in through the additional parameters of the request method. Query Parameters:

Query Parameters are one key feature of web services that allow developers to send specific information in the HTTP request URL.

These parameters help in filtering, sorting, and limiting data on the server-side. FastAPI offers several ways of handling Query Parameters.

Required Query Parameters:

Most times, developers require the exact amount of information from clients to perform a request. With FastAPI, developers can make Query Parameters required by defining them in the Path Operations’ function definition.

Required Query Parameters enforce the need for the client to provide all the necessary data before proceeding with a request. The lack of required data results in a 422 Unprocessable Entity error, indicating that the server could not process the request.

Multiple Path and Query Parameters:

FastAPI supports multiple Path and Query parameters for each endpoint. Multiple Path parameters are declared by defining them in the URL’s path definition, separated by slashes.

Multiple Query parameters are defined by using an ampersand `&` symbol that separates each query parameter. Optional Parameters:

Optional Parameters are query parameters that are not mandatory for the request to function.

They are often given default parameters to enable the API to function without them in the request scope. With FastAPI, developers can set default values for optional parameters in Path Operations’ function definition.

These default values will be used when a request with no entered optional parameters is made. Conclusion:

FastAPI provides a powerful and flexible system for building APIs with Path Operations & Query Parameters features.

FastAPI supports all common HTTP verbs, enabling developers to design flexible and efficient APIs. Path Operation objects can be decorated to utilize Dependency Injection facilities, passing additional dependencies into Path Operations. Extra data is also a feature that can be used to pass additional client-specific information to the server.

Query parameters are crucial features of APIs and come in required and optional forms. With FastAPI, developers can efficiently handle these parameters to configure API behavior accurately.

Request Body:

Request Body is a feature of FastAPI that facilitates the handling of incoming data sent by a request to the API. It proves useful when the request contains more data than can be provided by the URL path parameters or query parameters.

The Request Body provides a way for clients to pass complex data structures to the server, such as JSON or XML formats. Body Parameters:

In FastAPI, developers can define request bodies by creating a Pydantic model with the required application/request-specific schema.

This data model captures the expected incoming data’s structure, making it easier to serialize and deserialize the data. The model defines the types of data required for the request.

Body parameters are useful when dealing with complex JSON or XML data, such as nested lists or dictionaries. Body + Path Parameters:

FastAPI enables developers to use the body parameter feature in combination with path parameters.

Path parameters help to provide a unique URL endpoint and any data required. Body parameters come in handy when the data required to process a request cannot be provided entirely with only the endpoint.

By using both path and body parameters, developers can create a highly flexible API that is robust and delivers the required data. Body + Query Parameters:

FastAPI also supports the use of Body parameters in conjunction with Query parameters.

Query parameters provide a simple way of filtering, limiting and even sorting resources for the server-side processing in APIs. Query parameters come in handy when clients want to limit the scope of data returned by an API. Sometimes, clients may need to use body parameters and Query parameters.

This scenario is enabled by FastAPI, as it allows developers to use both body and Query parameters in a single API request. Response Model:

FastAPI’s Response Model is a critical feature for returning the expected data to clients.

The Response Model facilitates the completion and response of an API request, providing clients with the expected data and building confidence in the API’s functionality.

Data Model:

Pydantic is used in FastAPI to define data models to capture the expected data structure for the API response.

The data model serves as a blueprint for the expected output data, providing a structure that can be easily serialized and deserialized into readable JSON and XML formats.

Singular Values in Body:

FastAPI’s Response Model handles singular values by allowing developers to return the allow_single_value field when defining their return models.

This field allows developers to define the expected data structure for single value responses from an API request that contains a body parameter.

Multiple Models in Body:

FastAPI allows developers to define more than one model in the request body if they are expecting multiple types of data or data structures in a single API request.

Developers can easily define multiple data models using Pydantic. By doing so, the server-side processing in an API request can handle multiple data structures, simplifying the development process for complex APIs.


FastAPI provides a robust Request Body feature that enables developers to handle complex data structures in API requests efficiently.

Body parameters are particularly useful when dealing with complex data types such as JSON, XML, or Binary data. Path and Query parameters can be used in combination with Body parameters to provide developers with a flexible API system capable of handling unique client requests and filtering data appropriately.

FastAPI’s Response Model features enable developers to hit the expected response structure that is easy to understand, serialize, and deserialize. By using Pydantic data models, developers can define data structures that enable flexible and scalable API data processing.

FastAPI’s allowance for multiple models within the body of an API request adds another layer of flexibility when handling complex data structures and types. In this article, we explored various features of FastAPI that make it a powerful tool for API development.

We looked at Path Operation Configuration, Query Parameters, Request Body, and Response Model. FastAPI’s Path Operation Configuration allows developers to define HTTP endpoints with support for all HTTP request methods.

The Query Parameters feature allows for flexible filtering, sorting, and limiting of data on the server-side. The Request Body handles complex data sent from API requests.

The Response Model handles API responses and facilitates the completion and response of API requests. A takeaway from this article is that FastAPI is a flexible and efficient tool for building robust APIs that meet clients’ specific needs.

It is suitable for developers working in various industries, such as finance, healthcare, and education, who need to develop high-performing APIs with complex data handling features.

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