You can customize the JSON response from FastAPI using the @app.get decorator. For example:

```python
from typing import Optional
from fastapi import FastAPI

app = FastAPI()

@app.get("/items/{item_id}")
def read_item(item_id: int):
    if item_id == 1:
        return {item_id: "One"}
    else:
        return {item_id: None}
```

Acessing 127.0.0.1:8000/items/1 returns:
```
{"1":"One"}
```
And 127.0.0.1:8000/items/2 returns:
```
{"2":"null}
```

FastAPI URL Pathing

Create a file named main.py. A simple FastAPI instance:

```python
from fastapi import FastAPI

app = FastAPI()

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

To run the instance:

```
uvicorn main:app --reload
```


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FastAPI can be installed with pip via:

```
pip install fastapi
```

Additionally, you will need to install Uvicorn because FastAPI needs an AGSI server to operate. You can install it with pip as well via:

```
pip install uvicorn[standard]
```

Learn Before

Related

Learn After