API versioning strategies in FastAPI

Introduction

API versioning helps keep your app working well when you add new features or change things. It lets old and new users use the API without problems.

You want to add new features without breaking old apps using your API.

You need to fix bugs or improve your API but keep old versions running.

You want to support different clients that expect different API behaviors.

You plan to change the API structure but want to avoid sudden breaks.

You want to clearly organize your API updates over time.

Syntax

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/v1/items")
async def get_items_v1():
    return {"version": "v1", "items": ["apple", "banana"]}

@app.get("/v2/items")
async def get_items_v2():
    return {"version": "v2", "items": [{"name": "apple"}, {"name": "banana"}]}

Versioning can be done in the URL path, query parameters, or headers.

URL path versioning is the simplest and most common method.

Examples

Versioning using the URL path with 'v1' prefix.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/v1/users")
async def users_v1():
    return {"version": "v1", "users": ["Alice", "Bob"]}

Versioning using a custom header 'X-API-Version'.

FastAPI

from fastapi import FastAPI, Header, HTTPException

app = FastAPI()

@app.get("/users")
async def users(x_api_version: str | None = Header(default="1")):
    if x_api_version == "1":
        return {"version": "v1", "users": ["Alice", "Bob"]}
    elif x_api_version == "2":
        return {"version": "v2", "users": [{"name": "Alice"}, {"name": "Bob"}]}
    else:
        raise HTTPException(status_code=400, detail="Unsupported API version")

Versioning using a query parameter 'api_version'.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/users")
async def users(api_version: str = Query("1")):
    if api_version == "1":
        return {"version": "v1", "users": ["Alice", "Bob"]}
    elif api_version == "2":
        return {"version": "v2", "users": [{"name": "Alice"}, {"name": "Bob"}]}
    else:
        return {"error": "Unsupported API version"}

Sample Program

This FastAPI app uses a custom header 'X-API-Version' to decide which version of the items list to return. If the header is '1', it returns a simple list. If '2', it returns a list of objects. If the version is unsupported, it returns an error.

FastAPI

from fastapi import FastAPI, Header, HTTPException

app = FastAPI()

@app.get("/items")
async def read_items(x_api_version: str | None = Header(default="1")):
    if x_api_version == "1":
        return {"version": "v1", "items": ["apple", "banana"]}
    elif x_api_version == "2":
        return {"version": "v2", "items": [{"name": "apple"}, {"name": "banana"}]}
    else:
        raise HTTPException(status_code=400, detail="Unsupported API version")

Important Notes

Using URL path versioning is easy to understand and test.

Header or query parameter versioning keeps URLs clean but needs clients to send extra info.

Always document your API versions clearly for users.

Summary

API versioning helps keep old and new API users happy.

Common ways: URL path, headers, or query parameters.

Choose the method that fits your app and users best.

Practice

1. What is the main purpose of API versioning in FastAPI?

easy

A. To automatically generate API documentation

B. To speed up the API response time

C. To allow multiple versions of an API to coexist and support different clients

D. To encrypt API data for security

Solution

1. Step 1: Understand API versioning concept

   API versioning lets you keep old and new API versions available so different clients can use the version they need.

2. Step 2: Identify the main goal

   The main goal is to support multiple API versions simultaneously without breaking existing clients.

3. Final Answer:

   To allow multiple versions of an API to coexist and support different clients -> Option C

4. Quick Check:

   API versioning = support multiple versions [OK]

Hint: API versioning means supporting old and new API versions [OK]

Common Mistakes:

• Thinking versioning speeds up API
• Confusing versioning with security encryption
• Assuming versioning auto-generates docs

2. Which of the following is a correct way to define a versioned route in FastAPI using the URL path method?

easy

A. app.get('/items/v1')

B. app.get('/v1/items/')

C. app.get('v1items/')

D. app.get('items/v1')

Solution

1. Step 1: Check route path syntax

   FastAPI routes must start with a slash '/' to be valid paths.

2. Step 2: Identify correct version prefix

   Versioning by URL path usually prefixes the version like '/v1/' before the resource path.

3. Final Answer:

   app.get('/v1/items/') -> Option B

4. Quick Check:

   Versioned route starts with '/' and version prefix [OK]

Hint: Routes must start with '/' and include version prefix [OK]

Common Mistakes:

• Omitting leading slash in route path
• Placing version after resource path incorrectly
• Using quotes without leading slash

3. Given this FastAPI code snippet, what will be the response when accessing /api/items with header X-API-Version: 2?

from fastapi import FastAPI, Header, HTTPException
app = FastAPI()

@app.get('/api/items')
async def get_items(x_api_version: str | None = Header(None)):
    if x_api_version == '1':
        return {'version': 1, 'items': ['apple', 'banana']}
    elif x_api_version == '2':
        return {'version': 2, 'items': ['orange', 'grape']}
    else:
        raise HTTPException(status_code=400, detail='Unsupported API version')

medium

A. {'version': 1, 'items': ['apple', 'banana']}

B. HTTP 400 Unsupported API version

C. HTTP 404 Not Found

D. {'version': 2, 'items': ['orange', 'grape']}

Solution

1. Step 1: Check header value handling

   The function reads the header 'X-API-Version' and compares it to '1' or '2'.

2. Step 2: Match header '2' to response

   Since header is '2', it returns the dictionary with version 2 and items ['orange', 'grape'].

3. Final Answer:

   {'version': 2, 'items': ['orange', 'grape']} -> Option D

4. Quick Check:

   Header 'X-API-Version: 2' returns version 2 data [OK]

Hint: Match header value to if-elif branches for output [OK]

Common Mistakes:

• Ignoring header and returning default
• Confusing version 1 and 2 outputs
• Assuming 404 error instead of 400

4. Identify the error in this FastAPI versioning code using query parameters:

from fastapi import FastAPI
app = FastAPI()

@app.get('/items')
async def read_items(version: int = 1):
    if version == 1:
        return {'items': ['pen', 'notebook']}
    elif version == 2:
        return {'items': ['pencil', 'eraser']}
    else:
        return {'error': 'Unsupported version'}

medium

A. The function should raise HTTPException for unsupported versions instead of returning dict

B. The query parameter 'version' should be declared with Query() for validation

C. The route path should include the version like '/v1/items'

D. The default version value should be a string, not int

Solution

1. Step 1: Check error handling for unsupported versions

   Returning a dict with 'error' key is not standard; better to raise HTTPException with status code.

2. Step 2: Understand FastAPI error response best practice

   Raising HTTPException sends proper HTTP error status and message to client.

3. Final Answer:

   The function should raise HTTPException for unsupported versions instead of returning dict -> Option A

4. Quick Check:

   Use HTTPException for errors, not plain dict [OK]

Hint: Use HTTPException for errors, not just return dict [OK]

Common Mistakes:

• Returning error dict instead of raising exception
• Thinking query parameter must use Query() always
• Assuming version must be in URL path

5. You want to support two API versions in FastAPI: version 1 uses URL path versioning like /v1/users, and version 2 uses header versioning with X-API-Version: 2 on the same /users path. How can you implement this correctly?

hard

A. Create two separate routes: one with path '/v1/users' for version 1, and one with path '/users' that checks header 'X-API-Version' for version 2

B. Use a single route '/users' and check both path and header to decide version

C. Use only header versioning for both versions on '/users' path

D. Use query parameter 'version' for both versions on '/users' path

Solution

1. Step 1: Separate version 1 and version 2 routes

   Version 1 uses URL path '/v1/users', so define a route with that path.

2. Step 2: Define version 2 route with header check

   Version 2 uses header on '/users', so define a route '/users' that reads 'X-API-Version' header to confirm version 2.

3. Final Answer:

   Create two separate routes: one with path '/v1/users' for version 1, and one with path '/users' that checks header 'X-API-Version' for version 2 -> Option A

4. Quick Check:

   Separate routes for different versioning methods [OK]

Hint: Separate routes for path and header versioning [OK]

Common Mistakes:

• Mixing path and header versioning in one route
• Using only header or only path for both versions
• Trying to detect path version inside header route