Getting documents from a collection by their respective IDs can be done via the get() function on a client.collection(name) object: 
docs = ctx.client.collection("books").get(["lotr", "moby"])
The result is a key-value mapping: each key is a document ID, and each value is the document in the same shape it was upserted: 
# Example response
{
    "lotr": {
		"_id": "lotr",
        "title": "The Lord of the Rings",
        "author": "J.R.R. Tolkien",
        "published_year": 1954
    },
    "moby": {
		"_id": "moby",
        "title": "Moby Dick",
        "author": "Herman Melville",
        "published_year": 1851
    }
}
If a document with a given ID does not exist in the collection, it is simply omitted from the results; no error is thrown.