Question
Answer and Explanation
The "TypeError: 'depends' object is not subscriptable" error in FastAPI typically arises when you're trying to access a dependency in a way that treats it like a list or dictionary when it's not.
Here's a breakdown of the common causes and solutions:
1. Incorrect Dependency Usage:
- You might be attempting to access a specific element or key within a dependency that's designed to return a single value. FastAPI's Depends function injects the result of a dependency function directly.
- Example (Incorrect):
from fastapi import FastAPI, Depends
app = FastAPI()
def get_user():
return {"username": "JohnDoe", "email": "john.doe@example.com"}
@app.get("/items/")
async def read_items(user: dict = Depends(get_user)):
username = user["username"] # Incorrect: user is the dict itself, not a list
return {"username": username}
- Solution: Access the dependency result directly without trying to subscript it if it's not a list or dictionary.
from fastapi import FastAPI, Depends
app = FastAPI()
def get_user():
return {"username": "JohnDoe", "email": "john.doe@example.com"}
@app.get("/items/")
async def read_items(user: dict = Depends(get_user)):
username = user["username"] # Correct: Access the dictionary directly
return {"username": username}
2. Nested Dependencies with Incorrect Return Types:
- In more complex scenarios with nested dependencies, ensure that each dependency returns the type of data you expect. A dependency returning None or a simple object when you're expecting a list can trigger this error.
- Example:
from fastapi import FastAPI, Depends
app = FastAPI()
def get_settings():
return None # Incorrect: Returning None
def get_database(settings = Depends(get_settings)):
db_url = settings["database_url"] # Error Here
return db_url
@app.get("/items/")
async def read_items(db_url: str = Depends(get_database)):
return {"database_url": db_url}
- Solution: Make sure get_settings always returns a dictionary with a "database_url" key, or handle the case where it might return None.
3. Confusing Depends with a Data Structure:
- Sometimes, developers mistakenly think that Depends itself is a data structure (like a list or dict), but it's actually a function that FastAPI uses to manage dependency injection.
- Example (Incorrect):
from fastapi import FastAPI, Depends
app = FastAPI()
def get_user():
return {"username": "Alice"}
@app.get("/items/")
async def read_items(user_dependency = Depends(get_user)):
username = Depends[user_dependency]["username"] # INCORRECT
return {"username": username}
- Solution: Access the result of the dependency directly, not through Depends. The error arises from treating Depends like a dictionary or list.
from fastapi import FastAPI, Depends
app = FastAPI()
def get_user():
return {"username": "Alice"}
@app.get("/items/")
async def read_items(user = Depends(get_user)):
username = user["username"] # CORRECT
return {"username": username}
Debugging this type of error involves carefully inspecting the return types of your dependency functions and how you're accessing their results. Use print statements or a debugger to examine the value of your dependencies before you try to access them.