web3.py - How does web3.auto choose a provider node?

Question

I'm having this other issue which has forced me to rely on either Infura or web3.auto in order to call getTransactionReceipt. I've noticed that when I create a w3 instance from web3.auto it almost always successfully returns a receipt when I call getTransactionReceipt. This leads me to wonder how web3.auto chooses a provider node:

1. Does it only select full nodes?
2. Does it ever select parity pruning nodes or light nodes?
3. Are there any other restrictions?

I'm curious for 1, and for 2 I'd like to use this answer to apply it to my above-linked issue to see if I can determine why my parity node always returns None when I call getTransactionReceipt.

Answer 1

From the docs:

How Automated Detection Works

Web3 attempts to connect to nodes in the following order, using the first succesful connection it can make:

• The connection specified by an environment variable, see Provider via Environment Variable
• IPCProvider, which looks for several IPC file locations
• HTTPProvider, which attempts to connect to “http://localhost:8545”

The docs are getting a bit stale, but should be a good place to start. I just noticed it is missing the WebsocketProvider, which is attempted after HTTPProvider. In the end, the best documentation is the code. For example, IPCProvider's default location is set using:

def get_default_ipc_path(testnet=False):
    if testnet:
        testnet = "testnet"
    else:
        testnet = ""

    if sys.platform == 'darwin':
        ipc_path = os.path.expanduser(os.path.join(
            "~",
            "Library",
            "Ethereum",
            testnet,
            "geth.ipc"
        ))
        if os.path.exists(ipc_path):
            return ipc_path

        ipc_path = os.path.expanduser(os.path.join(
            "~",
            "Library",
            "Application Support",
            "io.parity.ethereum",
            "jsonrpc.ipc"
        ))
        if os.path.exists(ipc_path):
            return ipc_path

    elif sys.platform.startswith('linux') or sys.platform.startswith('freebsd'):
        ipc_path = os.path.expanduser(os.path.join(
            "~",
            ".ethereum",
            testnet,
            "geth.ipc"
        ))
        if os.path.exists(ipc_path):
            return ipc_path

        ipc_path = os.path.expanduser(os.path.join(
            "~",
            ".local",
            "share",
            "io.parity.ethereum",
            "jsonrpc.ipc"
        ))
        if os.path.exists(ipc_path):
            return ipc_path

    elif sys.platform == 'win32':
        ipc_path = os.path.join(
            "\\\\",
            ".",
            "pipe",
            "geth.ipc"
        )
        if os.path.exists(ipc_path):
            return ipc_path

        ipc_path = os.path.join(
            "\\\\",
            ".",
            "pipe",
            "jsonrpc.ipc"
        )
        if os.path.exists(ipc_path):
            return ipc_path