I had the same problem and solved it adding package name before main, in your case trying:

uvicorn src.main:app --reload

may solve the problem

This worked for me look at the docs for fastAPI. I am very thankful I ran accross that as the Python script needs to be named main.py not app.py

The command uvicorn main:app refers to:

1. main: the file main.py (the Python "module").
2. app: the object created inside of main.py with the line app = FastAPI()
3. --reload: make the server restart after code changes. Only use for
   development.

One reason this might be happening is that you are using:

uvicorn src/main:app --reload    

instead of the correct syntax:

uvicorn src.main:app --reload 

Notice the . instead of the /
Currently auto-completion in the terminal suggests the wrong format.

That’s assuming that:

(1) your structure is something like this:

project_folder/
├── some_folder
├── src
│   └── main.py
└── tests
    ├── test_xx.py
    └── test_yy.py

(2) your FastAPI() object is indeed assigned to an object named app in main.py:

app = FastAPI()

(3) you are running the uvicorn command from the project_folder, e.g.:

(venv) <username>@<pcname>:~/PycharmProjects/project_folder$ uvicorn src.main:app --reload

It seems it is important that you name your file main.py otherwise it won’t work.

Edit: Actually I was running Jupyter Notebook on port 8888 so that port was already occupied. If you have to run Jupyter notebook, run it after running the API server, the notebook will automatically run on 8889.
Alternatively, you can run the API server on a different port.

Hit the Save button in VS Code etc because sometimes it will throw this error if you have not saved the file. Happened to me.

Use this folder structure and configuration where main is in parent directory

launch.json

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Python: FastAPI",
            "type": "python",
            "request": "launch",
            "module": "uvicorn",
            "args": [
                "main:app"
            ],
            "jinja": true
        }
    ]
}

main.py

from fastapi import FastAPI
app = FastAPI(
    title="test",
    description="test",
    version="0.0.1",
)
if __name__ == "__main__":
import uvicorn

uvicorn.run(
    "main:app",
    host="0.0.0.0",
    reload=True,
    port=3001,
)

Another reason for this error: there is a circular reference between files.

Example:

• main.py:

  from tools import validator
  
  # ...
  
  class GlobalException(Exception):
      # ...
  

• /tools/validator.py:

  from main import GlobalException
  

Also another reason for this error:

There’s already a file named exactly like your running folder, then it will make a conflict to the system.

Example you have a file inside a folder named test_app/main.py, then you should run uvicorn test_app.main:app --reload to run the app.

But, if you have a file named test_app.py (which is the same name as the folder above), it will throw the error when you run the app.

So the solution is to rename the folder or rename the file so they won’t be conflict to each other.

• Step-1 make sure your main file is located in the same directory of your terminal/command prompt, use ls command

(virtual-env) shayon@shayon-X556UQK:~/Documents/python-fast-api$ ls
main.py  __pycache__  README.md  requirements.txt  virtual-env

• Step-2 check the directory of your terminal using pwd command

(virtual-env) shayon@shayon-X556UQK:~/Documents/python-fast-api$ pwd
/home/shayon/Documents/python-fast-api

• Step-3 check the name of your object – here app is the object of FastAPI so we need to use the command uvicorn main:app --reload, if you keep the object name app1 you need to run uvicorn main:app1 --reload

(virtual-env) shayon@shayon-X556UQK:~/Documents/python-fast-api$ cat main.py

main.py

from fastapi import FastAPI
app = FastAPI()


@app.get("/")
def index():
    return {"Hello": "World"}

• Step-4 In my case, I made a mistake making command of uvicorn main.py:app --reload and this was the reason for my error. The command should be uvicorn main:app --reload (without dot py extension)

Hope this help – Enjoy 🙂

If you are using Pycharm , the reason could be the source directory not being set correctly. For example, I was trying to configure Python remote debugger using docker-compose and my folder structure was like this:

my_app_name
├── docker-compose.yml
├── src
│   └── main.py
└── tests
    ├── test_file1.py
    └── test_file2.py

I had the src folder inside the root folder, which contained the main.py file. In PyCharm project settings, by default your source folder is set to root.But if you have a nested structure like the case above, you have to go to Preferences->Project->Project Structure->Sources and add your src folder under Source Folders.

For me it was a space problem, I had to delete some files and folders in my system.

I also encountered the same issue,
being a total beginner to FastAPI, the solution given by Yagiz Degirmenci
helped to come up with a simple idea for beginners like me, i.e

cd directory of code
uvicorn fast-api:main --reload 

PS. fast-api is the name of my file (i.e fast-api.py)

I fixed this by using the following in my main:

import uvicorn

app = FastAPI()

@app.get("/")
def index():
    return {"index": "root"}

if __name__ == '__main__':

    uvicorn.run(f"{Path(__file__).stem}:app", host="127.0.0.1", port=8888, reload=True)

and then from the terminal, I typed uvicorn main:app --reload as my main.py is in my root folder.

don’t give the same name for your python file like uvicorn.py,Folder same name, package name

for me , it’s such stupid : I have main,py instead of main.py, and guess what, I uninstall my whole conda env and reinstall pure python3 before I found this…