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FastAPI: Troubleshooting the ASGI App Error for High-Performance Web Development

Troubleshooting FastAPI: How to Fix “Error Loading ASGI App”

FastAPI is a popular Python framework used for developing high-performance web applications. It is built on top of the ASGI (Asynchronous Server Gateway Interface) standard, which makes it ideal for creating fast APIs. However, while working with FastAPI, you may encounter the “Error loading ASGI app” issue.

This error can occur due to a variety of reasons, including incorrect directory structure, missing dependencies, circular imports, and more. In this article, we will discuss some of the common causes of this error and provide solutions to fix it.

Specifying the Path When Running the Command

When you encounter the “Error loading ASGI app” issue with FastAPI, the first thing you should check is whether the correct path has been specified when running the command. This is a common mistake that can lead to the error.

To fix this issue, navigate to the correct directory where the file is located and run the command from there. For example, if your file is located in the “api” folder, you need to navigate to that folder and run the command from there.

You can do this by opening your terminal and typing “cd api” followed by “uvicorn main:app –reload” to run the server. Change Your Terminal to the Directory That Contains

If you are still encountering the “Error loading ASGI app” issue after specifying the correct path, the problem may be caused by the terminal not being in the correct directory.

Ensure that your terminal is open in the directory that contains the file before running the command. To do this, open your terminal and use the “cd” command to navigate to the directory that contains the file.

If the file is located in the “app” folder, for instance, type “cd app” to change the directory to the correct folder.

Make Sure You Have FastAPI and Uvicorn Installed

Another common cause of the “Error loading ASGI app” issue is that you may not have installed FastAPI and/or Uvicorn properly. To fix this issue, use pip to install both frameworks by running the following command: “

pip install fastapi uvicorn”.

This will install all dependencies needed for FastAPI and Uvicorn to run smoothly.

Make Sure You Have Saved the Changes in Your IDE

If you are using an Integrated Development Environment (IDE) to work with FastAPI, it is crucial to ensure that you have saved all the changes made to the file. It is also essential to ensure that your IDE is configured to compile Python files correctly.

Always check the “Save” button to ensure your changes are saved correctly before attempting to run your server. Don’t Use a Slash Between Your Directory Name and the File Name

Another common mistake that can cause the “Error loading ASGI app” issue is using a slash between the directory name and the file name.

When specifying the path to the file, make sure to use a dot instead of a slash. For example, instead of typing “path/to/,” you should type “” as this is the correct way to reference the file.

Make Sure You Don’t Have Circular Imports

Circular imports occur when two or more modules are interdependent, making it impossible for them to be imported correctly. This is a common problem that can lead to the “Error loading ASGI app” issue in FastAPI.

To prevent circular imports, always ensure that the module hierarchy is correct, and all dependents are imported first. In some cases, you may need to separate modules into different directories to avoid circular imports.

Example File

Here’s an example of what a file for a FastAPI application might look like:


from fastapi import FastAPI

app = FastAPI()


async def root():

return {“message”: “Hello World”}


This code creates a new FastAPI instance and defines a single endpoint at the root of the application. When accessed with an HTTP GET request, it will return a JSON payload with a “message” field containing the string “Hello World”.


In conclusion, the “Error loading ASGI app” issue can occur for a variety of reasons when working with FastAPI. However, with the help of the tips provided in this article, you can identify the source of the problem and fix it quickly.

Remember to always check your command-line arguments and directory structure, ensure that you have installed all dependencies correctly, and avoid circular dependencies in your modules. By following these best practices, you can create stable and fast APIs with FastAPI.

Prerequisites for Developing FastAPI Applications

FastAPI is a Python framework built for high-performance web applications. It is designed to be simple, fast, and easy to use, making it a popular choice for developers who want to create robust APIs. However, before you start using FastAPI, there are some prerequisites you need to have in place.

In this article, we’ll go through the necessary software you need to install and some tips to help you get started.

Software Prerequisites

Before you can start developing with FastAPI, there are a few software prerequisites you need to have in place. 1.

Python 3.6+

FastAPI is built on top of Python 3.6+, so you’ll need to have Python 3.6 or above installed on your machine. You can download the latest version of Python from the official website:



Pip is the standard package manager for Python. It is used to install, upgrade, and manage Python packages.

Pip comes preinstalled with Python 3.4 and above, so if you have a newer version of Python installed, you should already have pip installed. If you’re not sure if you have pip installed, you can check by running the following command in your terminal:


pip –version


3. FastAPI

To use FastAPI, you’ll need to install it using pip.

You can do this by running the following command in your terminal:


pip install fastapi


This will install FastAPI and its dependencies on your machine. 4.


Uvicorn is a high-performance ASGI server that works well with FastAPI. You can install Uvicorn by running the following command:


pip install uvicorn


5. An IDE or Text Editor

While it’s possible to develop FastAPI applications using a plain text editor, it’s much easier to use an integrated development environment (IDE) or text editor with features like syntax highlighting to help you write code more efficiently.

Some popular choices for Python development include PyCharm, Visual Studio Code, and Sublime Text.

Components of the Command

Once you have all the necessary software installed, you can start developing your FastAPI application. To run FastAPI, you need to use the Uvicorn server.

The command to run Uvicorn and start the server has a few components:


uvicorn [module]:app –reload


1. Uvicorn

Uvicorn is the name of the server software.

2. [module]:app

This refers to the module name and the application object of your FastAPI application.

The [module] part should be replaced by the name of the Python file that contains your FastAPI code (e.g., main). The “:app” part refers to the name of the application object that you create in your main Python file.

3. –reload

This tells Uvicorn to watch for changes to your code and automatically reload your application when it detects changes.

This is especially useful during development, as it means you don’t need to manually restart the server every time you make a change to your code. Using these components, you can create a command tailored to your application.

For example, if your FastAPI code is in a file called and your application object is named app, your command to start the server would look like this:


uvicorn main:app –reload



In conclusion, before you start developing with FastAPI, you need to have a few software prerequisites in place, including Python, pip, FastAPI, Uvicorn, and a text editor or IDE. Once you have all the necessary software installed, you can start using FastAPI to develop high-performance web applications.

By understanding the components of the command to start the server, you can customize the command for your specific application and start developing with FastAPI today. In conclusion, to develop high-performance web applications using FastAPI, you need to have Python 3.6+, pip, FastAPI, Uvicorn, and a text editor or IDE installed on your machine.

With these prerequisites in place, you can create an ASGI application with FastAPI. The command to run the server has three essential components, which helps you customize it according to your specific application requirements.

The main takeaways include ensuring you specify the path correctly, avoiding circular dependencies, and understanding the command components. By adhering to these best practices, you can build robust APIs with FastAPI and improve your web development skills.

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