Python aiohttp: How to Set Custom Headers

Introduction

Working with HTTP requests in asynchronous Python often requires the use of custom headers to communicate with APIs. This guide delves into setting up custom headers using the aiohttp library.

Setting the Stage

aiohttp is a powerful library in Python for making asynchronous HTTP requests. It presents an opportunity to leverage Python’s asynchronous features to handle HTTP calls efficiently. To get started with aiohttp, installation is straightforward:

pip install aiohttp

Creating a Simple GET Request with Custom Headers

Let’s begin with a basic example where we send a GET request:

import aiohttp
import asyncio

async def fetch_with_headers(url):
    headers = {'Custom-Header': 'MyValue', 'Another-Header': 'AnotherValue'}
    async with aiohttp.ClientSession() as session:
        async with session.get(url, headers=headers) as response:
            return await response.text()

url = 'https://example.com'
asyncio.run(fetch_with_headers(url))

In the code above, custom headers are included as a dictionary and passed to the get method.

Working with POST Requests

Sending POST requests with custom headers is quite similar:

async def post_with_headers(url, data):
    headers = {'Content-Type': 'application/json'}
    async with aiohttp.ClientSession() as session:
        async with session.post(url, data=data, headers=headers) as response:
            return await response.json()

url = 'https://api.example.com/post'
data = {'key': 'value'}
asyncio.run(post_with_headers(url, data))

Take notice that content-type is set as application/json, which is common for REST APIs.

Advanced Header Manipulation

Handling more sophisticated scenarios may involve dynamic headers, authentication, and even condition-based headers.

Dynamic Custom Headers

In cases where headers need to be generated dynamically based on certain logic:

async def dynamic_headers(url):
    def generate_headers():
        # Custom logic to create headers
        return {'Dynamic-Header': 'DynamicValue'}

    headers = generate_headers()
    async with aiohttp.ClientSession() as session:
        async with session.get(url, headers=headers) as response:
            return await response.text()

url = 'https://example.com/custom'
asyncio.run(dynamic_headers(url))

Authentication Headers

When dealing with secure endpoints, we might need to add authentication tokens:

async def fetch_with_auth(url):
    token = 'YourAuthToken'
    headers = {'Authorization': f'Bearer {token}'}
    async with aiohttp.ClientSession() as session:
        async with session.get(url, headers=headers) as response:
            return await response.text()

url = 'https://secure.example.com'
asyncio.run(fetch_with_auth(url))

Handling Exceptions and Connection Errors

While working with aiohttp, you might want to handle networking errors gracefully:

async def fetch_with_error_handling(url):
    headers = {'Custom-Header': 'MyValue'}
    async with aiohttp.ClientSession() as session:
        try:
            async with session.get(url, headers=headers) as response:
                response.raise_for_status()
                return await response.text()
        except aiohttp.ClientError as e:
            return f'HTTP Error: {str(e)}'

url = 'https://example.com/error'
asyncio.run(fetch_with_error_handling(url))

Chaining Headers

For more complex headers logic, you may chain or merge headers dynamically:

async def chained_headers(url):
    base_headers = {'Header1': 'Value1'}
    additional_headers = {'Header2': 'Value2'}
    headers = {**base_headers, **additional_headers}

    async with aiohttp.ClientSession() as session:
        async with session.get(url, headers=headers) as response:
            return await response.text()

url = 'https://example.com/chain'
asyncio.run(chained_headers(url))

Testing Custom Headers

When developing a solution, it’s important to test requests. Here is a small code example that tests aiohttp with pytest and pytest-aiohttp, which are popular testing frameworks for Python and aiohttp respectively.

To run this test, you need to install pytest and pytest-aiohttp first:

pip install pytest pytest-aiohttp

Code example:

# test_aiohttp.py
import pytest
from aiohttp import web

# Define a simple handler function
async def hello(request):
    return web.Response(text="Hello, world")

# Create a fixture for the aiohttp test server
@pytest.fixture
def cli(loop, aiohttp_client):
    app = web.Application()
    app.router.add_get('/', hello)
    return loop.run_until_complete(aiohttp_client(app))

# Write a test function that makes a GET request to the server and checks the response
async def test_hello(cli):
    resp = await cli.get('/')
    assert resp.status == 200
    text = await resp.text()
    assert text == "Hello, world"

Then you can run the test with:

Then you can run the test with:

You should see something similar to this:

============================= test session starts ==============================
platform linux -- Python 3.9.1, pytest-6.2.5, py-1.10.0, pluggy-0.13.1
rootdir: /home/user
plugins: aiohttp-0.3.0
collected 1 item                                                               

test_aiohttp.py .                                                        [100%]

============================== 1 passed in 0.12s ===============================

Conclusion

In conclusion, aiohttp is a flexible and powerful tool for handling asynchronous HTTP requests in Python. This tutorial has shown how to send get and post requests with custom headers, dynamic headers, handle authentication, and error handling. As always, test your code thoroughly to ensure it behaves as expected in a production environment.