Using httpx's AsyncClient for Asynchronous HTTP POST Requests

The httpx library in Python provides an AsyncClient class that makes it easy to send asynchronous HTTP requests without having to deal with some of the complexity of asyncio directly. In this article, we'll look at how to use AsyncClient to send POST requests asynchronously.

Why Asynchronous Requests?

Performing requests asynchronously allows your Python application to issue multiple requests in parallel without blocking. This can result in faster overall request times by avoiding delays from network roundtrips and server response times.

Some key benefits of asynchronous requests:

Creating an AsyncClient

To start, we need to create an AsyncClient instance. This handles sending HTTP requests asynchronously behind the scenes:

import httpx

async with httpx.AsyncClient() as client:
    # Use client for requests here    
    pass

Using async with ensures the client is properly closed when we're done with it.

Making a POST Request

Let's look at a simple POST request:

resp = await client.post("https://example.com/api", json={"key": "value"})
print(resp.status_code)

We use the client.post() method, passing the URL and optional data like a JSON body. Since it's an async method, we await the response.

The client handles encoding the JSON and adding the required Content-Type: application/json header for us automatically.

Posting a Multipart File

Uploading a file requires encoding the data as multipart form data. The AsyncClient handles this for us:

files = {"file": open("report.pdf", "rb")}
resp = await client.post("https://example.com/upload", files=files)

So file uploads are almost as straightforward as a normal POST.

Handling Timeouts

We can tell requests to time out if a response takes too long:

timeout = httpx.Timeout(10.0)

try:
    resp = await client.post("https://example.com", timeout=timeout)
except httpx.TimeoutException:
    print("Request timed out!")

The timeout is set per-request, so other requests won't be affected.

Limits & Connection Pooling

Creating a new HTTP connection for every request can be inefficient. The AsyncClient uses a connection pool internally, reusing connections when possible.

We can configure limits on the number of max connections and max keepalive time to tune performance:

client = httpx.AsyncClient(
    limits=httpx.Limits(max_connections=100, max_keepalive_connections=20)
)

Tuning these limits for your specific API usage can help reduce latency.

Wrapping Up

That covers some of the basics of sending asynchronous POST requests with the handy AsyncClient in httpx!

Key takeaways:

Following these patterns can help you speed up API requests and build more robust applications using asynchronous I/O.