README.md
# lowhaio [](https://circleci.com/gh/michalc/lowhaio) [](https://codeclimate.com/github/michalc/lowhaio/test_coverage)
A lightweight Python asyncio HTTP/1.1 client. No additional tasks are created; all code is in a single module; and other than the standard library only a single dependency is required, [aiodnsresolver](https://github.com/michalc/aiodnsresolver)
Lowhaio has a deliberately limited scope: it includes just enough code to be a useful HTTP client and allow more complex behaviour to be added on top if required.
Connections are DNS-aware, in that they are only re-used if they match a current A record for the domain.
## Installation
```bash
pip install lowhaio
```
## Usage
The API is streaming-first: for both request and response bodies, asynchronous iterators are used.
```python
import asyncio
from lowhaio import Pool
async def main():
request, close = Pool()
async def request_body():
yield b'a'
yield b'bc'
code, headers, response_body = await request(
b'POST', 'https://postman-echo.com/post',
headers=((b'content-length', b'3'), (b'content-type', b'text/plain'),),
body=request_body,
)
async for chunk in response_body:
print(chunk)
await close()
loop = asyncio.get_event_loop()
loop.run_until_complete(main())
```
However, there are helper functions `streamed` and `buffered` when this isn't required or possible.
```python
import asyncio
from lowhaio import Pool, streamed, buffered
async def main():
request, close = Pool()
request_body = streamed(b'abc')
code, headers, response_body = await request(
b'POST', 'https://postman-echo.com/post',
headers=((b'content-length', b'3'), (b'content-type', b'text/plain'),),
body=request_body,
)
print(await buffered(response_body))
await close()
loop = asyncio.get_event_loop()
loop.run_until_complete(main())
```
## Headers
The only header automatically added to requests is the `host` header, which is the idna/punycode-encoded domain name from the requested URL.
## Exceptions
Exceptions are usually subclasses of `HttpError`. If a lower-level exception caused this, it is set in the `__cause__` attribute of the `HttpError`. Specifically, before any data is sent `HttpConnectionError` is raised, and after data is sent, `HttpDataError` is raised. This difference is to make it possible to know if non-idempotent requests can be retried.
However, `asyncio.CancelledError` and exceptions that do not directive from `Exception`, such as `SystemExit`, are allowed to bubble up.
## Custom SSL context
Lowhaio can be used with an custom SSL context through through the `get_ssl_context` parameter to `Pool`. For example, to use the certifi CA bundle, you can install it by
```bash
pip install certifi
```
and use it as below.
```python
import asyncio
import ssl
import certifi
from lowhaio import Pool, buffered, streamed
async def main():
request, close = Pool(
get_ssl_context=lambda: ssl.create_default_context(cafile=certifi.where()),
)
request_body = streamed(b'abc')
code, headers, response_body = await request(
b'POST', 'https://postman-echo.com/post',
headers=((b'content-length', b'3'), (b'content-type', b'text/plain'),),
body=request_body,
)
print(await buffered(response_body))
await close()
loop = asyncio.get_event_loop()
loop.run_until_complete(main())
```
## Scope
The scope of the core functions is restricted to:
- (TLS) connection opening, closing and pooling;
- passing and receiving HTTP headers and streaming bodies;
- decoding chunked responses;
- raising exceptions on timeouts.
This is to make the core behaviour useful to a reasonable range of uses, but to _not_ include what can be added by layer(s) on top. Specifically not included:
- following redirects, implemented by [lowhaio-redirect](https://github.com/michalc/lowhaio-redirect);
- retrying failed requests, implemented by [lowhaio-retry](https://github.com/michalc/lowhaio-retry);
- encoding chunked requests, implemented by [lowhaio-chunked](https://github.com/michalc/lowhaio-chunked);
- authentication, such as AWS Signature Version 4 implemented by [lowhaio-aws-sigv4](https://github.com/michalc/lowhaio-aws-sigv4), or AWS Signature Version 4 with unsigned payload implemented by [lowhaio-aws-sigv4-unsigned-payload](https://github.com/michalc/lowhaio-aws-sigv4-unsigned-payload);
- compressing/decompressing requests/responses;
- cookies.