tiangolo.fastapi/docs/tutorial/bigger-applications.md

If you are building an application or a web API, it's rarely the case that you can put everything on a single file.

FastAPI provides a convenience tool to structure your application while keeping all the flexibility.

!!! info If you come from Flask, this would be the equivalent of Flask's Blueprints.

An example file structure

Let's say you have a file structure like this:

.
├── app
│   ├── __init__.py
│   ├── main.py
│   └── routers
│       ├── __init__.py
│       ├── items.py
│       └── users.py

!!! tip There are two __init__.py files: one in each directory or subdirectory.

This is what allows importing code from one file into another.

For example, in `app/main.py` you could have a line like:

```
from app.routers import items
```

• The app directory contains everything.
• This app directory has an empty file app/__init__.py.
  • So, the app directory is a "Python package" (a collection of "Python modules").
• The app directory also has a app/main.py file.
  • As it is inside a Python package directory (because there's a file __init__.py), it is a "module" of that package: app.main.
• There's a subdirectory app/routers/.
• The subdirectory app/routers also has an empty file __init__.py.
  • So, it is a "Python subpackage".
• The file app/routers/items.py is beside the app/routers/__init__.py.
  • So, it's a submodule: app.routers.items.
• The file app/routers/users.py is beside the app/routers/__init__.py.
  • So, it's a submodule: app.routers.users.

APIRouter

Let's say the file dedicated to handling just users is the submodule at /app/routers/users.py.

You want to have the path operations related to your users separated from the rest of the code, to keep it organized.

But it's still part of the same FastAPI application/web API (it's part of the same "Python Package").

You can create the path operations for that module using APIRouter.

Import APIRouter

You import it and create an "instance" the same way you would with the class FastAPI:

{!./src/bigger_applications/app/routers/users.py!}

Path operations with APIRouter

And then you use it to declare your path operations.

Use it the same way you would use the FastAPI class:

{!./src/bigger_applications/app/routers/users.py!}

You can think of APIRouter as a "mini FastAPI" class.

All the same options are supported.

All the same parameters, responses, dependencies, tags, etc.

!!! tip In this example, the variable is called router, but you can name it however you want.

We are going to include this APIrouter in the main FastAPI app, but first, let's add another APIRouter.

Another module with APIRouter

Let's say you also have the endpoints dedicated to handling "Items" from your application in the module at app/routers/items.py.

You have path operations for:

• /items/
• /items/{item_id}

It's all the same structure as with app/routers/users.py.

But let's say that this time we are more lazy.

And we don't want to have to explicitly type /items/ and tags=["items"] in every path operation (we will be able to do it later):

{!./src/bigger_applications/app/routers/items.py!}

Add some custom tags and responses

We are not adding the prefix /items/ nor the tags=["items"] to add them later.

But we can add custom tags and responses that will be applied to a specific path operation:

{!./src/bigger_applications/app/routers/items.py!}

The main FastAPI

Now, let's see the module at app/main.py.

Here's where you import and use the class FastAPI.

This will be the main file in your application that ties everything together.

Import FastAPI

You import and create a FastAPI class as normally:

{!./src/bigger_applications/app/main.py!}

Import the APIRouter

But this time we are not adding path operations directly with the FastAPI app.

We import the other submodules that have APIRouters:

{!./src/bigger_applications/app/main.py!}

As the file app/routers/items.py is part of the same Python package, we can import it using "dot notation".

How the importing works

The section:

from .routers import items, users

Means:

• Starting in the same package that this module (the file app/main.py) lives in (the directory app/)...
• look for the subpackage routers (the directory at app/routers/)...
• and from it, import the submodule items (the file at app/routers/items.py) and users (the file at app/routers/users.py)...

The module items will have a variable router (items.router). This is the same one we created in the file app/routers/items.py. It's an APIRouter. The same for the module users.

We could also import them like:

from app.routers import items, users

!!! info The first version is a "relative import".

The second version is an "absolute import".

To learn more about Python Packages and Modules, read <a href="https://docs.python.org/3/tutorial/modules.html" target="_blank">the official Python documentation about Modules</a>.

Avoid name collisions

We are importing the submodule items directly, instead of importing just its variable router.

This is because we also have another variable named router in the submodule users.

If we had imported one after the other, like:

from .routers.items import router
from .routers.users import router

The router from users would overwrite the one from items and we wouldn't be able to use them at the same time.

So, to be able to use both of them in the same file, we import the submodules directly:

{!./src/bigger_applications/app/main.py!}

Include an APIRouter

Now, let's include the router from the submodule users:

{!./src/bigger_applications/app/main.py!}

!!! info users.router contains the APIRouter inside of the file app/routers/users.py.

With app.include_router() we can add an APIRouter to the main FastAPI application.

It will include all the routes from that router as part of it.

!!! note "Technical Details" It will actually internally create a path operation for each path operation that was declared in the APIRouter.

So, behind the scenes, it will actually work as if everything was the same single app.

!!! check You don't have to worry about performance when including routers.

This will take microseconds and will only happen at startup.

So it won't affect performance.

Include an APIRouter with a prefix, tags, and responses

Now, let's include the router from the items submodule.

But, remember that we were lazy and didn't add /items/ nor tags to all the path operations?

We can add a prefix to all the path operations using the parameter prefix of app.include_router().

As the path of each path operation has to start with /, like in:

@router.get("/{item_id}")
async def read_item(item_id: str):
    ...

...the prefix must not include a final /.

So, the prefix in this case would be /items.

We can also add a list of tags that will be applied to all the path operations included in this router.

And we can add predefined responses that will be included in all the path operations too.

{!./src/bigger_applications/app/main.py!}

The end result is that the item paths are now:

• /items/
• /items/{item_id}

...as we intended.

They will be marked with a list of tags that contain a single string "items".

The path operation that declared a "custom" tag will have both tags, items and custom.

These "tags" are especially useful for the automatic interactive documentation systems (using OpenAPI).

And all of them will include the the predefined responses.

The path operation that declared a custom 403 response will have both the predefined responses (404) and the 403 declared in it directly.

!!! check The prefix, tags, and responses parameters are (as in many other cases) just a feature from FastAPI to help you avoid code duplication.

!!! tip You could also add path operations directly, for example with: @app.get(...).

Apart from `app.include_router()`, in the same **FastAPI** app.

It would still work the same.

!!! info "Very Technical Details" Note: this is a very technical detail that you probably can just skip.

---

The `APIRouter`s are not "mounted", they are not isolated from the rest of the application.

This is because we want to include their path operations in the OpenAPI schema and the user interfaces.

As we cannot just isolate them and "mount" them independently of the rest, the path operations are "cloned" (re-created), not included directly.

Check the automatic API docs

Now, run uvicorn, using the module app.main and the variable app:

uvicorn app.main:app --reload

And open the docs at http://127.0.0.1:8000/docs.

You will see the automatic API docs, including the paths from all the submodules, using the correct paths (and prefixes) and the correct tags: