Dependencies with parameters in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this FastAPI dependency with parameter?

Consider this FastAPI dependency that takes a parameter and returns a greeting message. What will be the response when accessing /hello?name=Alice?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def greet(name: str):
    return f"Hello, {name}!"

@app.get("/hello")
async def hello(message: str = Depends(greet)):
    return {"message": message}

A {"message": "Hello, Alice!"}

B {"message": "Hello, name!"}

C {"message": "Hello!"}

D {"message": "Hello, None!"}

💡 Hint

Think about how the Depends function injects the parameter from the query string.

The dependency greet takes the query parameter name. FastAPI automatically passes the query parameter name=Alice to greet. So the returned message is "Hello, Alice!".

📝 Syntax

intermediate

Which option correctly defines a FastAPI dependency with a default parameter value?

Choose the correct way to define a dependency function that takes an optional parameter lang with default value "en".

A

def get_language(lang: str):
    if not lang:
        lang = "en"
    return lang

B

def get_language(lang: str = "en"):
    return lang

C

def get_language(lang: str = Query("en")):
    return lang

D

def get_language(lang: str = Depends("en")):
    return lang

💡 Hint

Remember to use FastAPI's Query for default query parameters in dependencies.

Option C uses Query("en") to set a default query parameter value for lang. Option C sets a Python default but does not integrate with FastAPI's query system. Option C lacks FastAPI parameter declaration. Option C misuses Depends.

🔧 Debug

advanced

Why does this FastAPI dependency with parameter cause a runtime error?

Examine the code below. Why does calling /items cause a runtime error?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_multiplier(factor: int):
    return lambda x: x * factor

@app.get("/items")
async def read_items(multiplier = Depends(get_multiplier)):
    return {"result": multiplier(5)}

A RuntimeError because get_multiplier requires a parameter but FastAPI cannot provide it automatically

B TypeError because multiplier is not callable

C KeyError because 'factor' is missing in request

D No error, returns {"result": 25}

💡 Hint

Think about how FastAPI injects parameters into dependencies.

The dependency function get_multiplier requires a parameter factor, but FastAPI does not know how to provide it automatically. This causes a runtime error because the dependency cannot be resolved.

🧠 Conceptual

advanced

How does FastAPI handle dependencies with parameters that have default values?

Which statement best describes how FastAPI resolves dependencies with parameters that have default values?

A FastAPI raises an error if the parameter has a default value

B FastAPI always requires the parameter to be provided in the request, ignoring defaults

C FastAPI ignores default values and sets parameters to None

D FastAPI uses the default value if the parameter is not provided in the request

💡 Hint

Think about how query parameters with defaults behave in FastAPI.

FastAPI uses the default value for dependency parameters if the client does not provide them in the request. This allows optional parameters with defaults.

❓ state_output

expert

What is the output of this FastAPI dependency chain with parameters?

Given the following code, what is the JSON response when accessing /compute?value=3?

FastAPI

from fastapi import FastAPI, Depends, Query

app = FastAPI()

def double(value: int = Query(...)):
    return value * 2

def add_five(number: int = Depends(double)):
    return number + 5

@app.get("/compute")
async def compute(result: int = Depends(add_five)):
    return {"result": result}

A {"result": 8}

B {"result": 11}

C {"result": 16}

D {"result": 10}

💡 Hint

Trace the dependency calls and their returned values step-by-step.

The query parameter value=3 is passed to double, which returns 6. Then add_five receives 6 and returns 11. The endpoint returns {"result": 11}.

Practice

1. What is the main purpose of using dependencies with parameters in FastAPI?

easy

A. To automatically generate HTML templates

B. To create global variables accessible everywhere

C. To replace route functions with classes

D. To customize shared code by passing arguments to dependencies

Solution

1. Step 1: Understand dependency role

   Dependencies in FastAPI are reusable pieces of code that can be shared across routes.

2. Step 2: Recognize parameter use

   Adding parameters to dependencies allows customizing their behavior for different routes or situations.

3. Final Answer:

   To customize shared code by passing arguments to dependencies -> Option D

4. Quick Check:

   Dependencies with parameters = customize shared code [OK]

Hint: Dependencies with parameters customize shared logic easily [OK]

Common Mistakes:

• Thinking dependencies create global variables
• Confusing dependencies with route handlers
• Assuming dependencies generate HTML

2. Which of the following is the correct way to pass a parameter to a dependency in FastAPI?

easy

A. Depends(get_user(user_id=5))

B. Depends(get_user, user_id=5)

C. Depends(get_user)(user_id=5)

D. Depends(get_user)[user_id=5]

Solution

1. Step 1: Recall Depends usage

   Depends expects a callable or a call to a callable that returns a dependency.

2. Step 2: Passing parameters

   To pass parameters, you call the dependency function inside Depends, like Depends(get_user(user_id=5)).

3. Final Answer:

   Depends(get_user(user_id=5)) -> Option A

4. Quick Check:

   Call dependency inside Depends to pass parameters [OK]

Hint: Call dependency inside Depends() to pass parameters [OK]

Common Mistakes:

• Passing parameters directly to Depends without calling
• Using brackets [] instead of parentheses ()
• Trying to call Depends as a function with parameters

3. Given this code snippet, what will be the output when accessing the endpoint?

from fastapi import FastAPI, Depends

app = FastAPI()

def get_multiplier(factor: int):
    def multiplier(value: int):
        return value * factor
    return multiplier

@app.get("/multiply")
async def multiply(value: int, multiply_func = Depends(get_multiplier(3))):
    return {"result": multiply_func(value)}

medium

A. Error because Depends cannot take parameters

B. {"result": 6} when value=3

C. {"result": 9} when value=3

D. {"result": 3} when value=3

Solution

1. Step 1: Understand get_multiplier

   get_multiplier(3) returns a function that multiplies input by 3.

2. Step 2: Analyze endpoint call

   When calling /multiply with value=3, multiply_func(3) returns 3 * 3 = 9.

3. Final Answer:

   {"result": 9} when value=3 -> Option C

4. Quick Check:

   Dependency returns multiplier with factor 3, output 9 [OK]

Hint: Multiply value by factor passed in dependency [OK]

Common Mistakes:

• Assuming Depends cannot take parameters
• Confusing returned function with direct value
• Mixing up multiplication factor

4. Identify the error in this FastAPI dependency usage:

def get_limit(limit: int = 10):
    return limit

@app.get("/items")
async def read_items(limit = Depends(get_limit(limit=20))):
    return {"limit": limit}

medium

A. Default value in get_limit conflicts with parameter passed

B. Cannot pass parameters directly inside Depends like get_limit(limit=20)

C. Missing type annotation for limit in read_items

D. Depends should be imported from fastapi.dependencies

Solution

1. Step 1: Check Depends usage

   Depends expects a callable or a call to a callable without parameters directly inside Depends.

2. Step 2: Correct way to pass parameters

   To pass parameters, wrap get_limit in another function or use a lambda to supply parameters.

3. Final Answer:

   Cannot pass parameters directly inside Depends like get_limit(limit=20) -> Option B

4. Quick Check:

   Depends() must wrap callable, not call with parameters directly [OK]

Hint: Wrap parameterized dependency call outside Depends() [OK]

Common Mistakes:

• Calling dependency with parameters inside Depends directly
• Ignoring need for wrapper function
• Wrong import path for Depends

5. How can you create a reusable dependency with a parameter that changes per route in FastAPI? Choose the best approach.

def common_dep(param: str):
    def dependency():
        return f"Value is {param}"
    return dependency

@app.get("/route1")
async def route1(dep = Depends(common_dep("A"))):
    return {"msg": dep}

@app.get("/route2")
async def route2(dep = Depends(common_dep("B"))):
    return {"msg": dep}

hard

A. Define a function returning a dependency function with parameter, then call it inside Depends

B. Use global variables to store param values for each route

C. Pass parameters directly to Depends without wrapping

D. Create separate dependency functions for each parameter value

Solution

1. Step 1: Understand reusable dependency pattern

   Define a function that returns a dependency function customized by parameters.

2. Step 2: Apply pattern per route

   Call this function with different parameters inside Depends for each route to customize behavior.

3. Final Answer:

   Define a function returning a dependency function with parameter, then call it inside Depends -> Option A

4. Quick Check:

   Wrap dependency with parameter function, call inside Depends [OK]

Hint: Wrap parameterized dependency in function, call inside Depends [OK]

Common Mistakes:

• Using global variables instead of parameters
• Passing parameters directly to Depends without wrapping
• Duplicating dependency functions unnecessarily