How do you send base64 source in Python code?

What is the equivalent of below code for sending source as bytes instead of a url in Python? Basically I have an image that is converted to bytes that I want to process.

Documentation for this service is horrendous by the way, so hard to find anything.

```
poller = document_intelligence_client.begin_analyze_document(
"prebuilt-read", AnalyzeDocumentRequest(url_source=formUrl)
)
```