ERC-20 – Solving Connection Issues with Contract Deployed on Ropsten Testnet

[This might be] an error regarding HTTPProvider not connecting to the "https://ropsten.etherscan.io"

That's correct, you cannot use the etherscan.io API as a JSON-RPC server.

Instead, I'd recommend running an Ethereum node locally that is synced with Ropsten, similar to how (I presume) you ran the private testnet.

One way to do that would be:

parity --chain ropsten

You can then connect with:

from web3 import Web3, HTTPProvider
web3 = Web3(HTTPProvider("http://localhost:8545"))
assert web3.isConnected()

Naturally, you'll need to create an account before the balanceOf() call will work. For the transfer() function to work, you'll also need to send the account ropsten ether.

API Comparison

Let's look at how each API sends the latest block number.

Etherscan

To get the current block number with the etherscan.io API, you would use a GET call, specifying eth_blockNumber as the action parameter.

eth_blockNumber
Returns the number of most recent block

https://ropsten.etherscan.io/api?module=proxy&action=eth_blockNumber&apikey=YourApiKeyToken

like so:

curl "https://ropsten.etherscan.io/api?module=proxy&action=eth_blockNumber"

Ethereum JSON-RPC server

In contrast, JSON-RPC specifies the API as a POST call, with eth_blockNumber sent as the method parameter, like so:

// Request
curl -X POST --data '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":83}'

// Result
{
  "id":83,
  "jsonrpc": "2.0",
  "result": "0x4b7" // 1207
}

This error suggests that whatever account you are using to send the transaction from does not have enough ether in it. Though you don't have to post the answers to these, you need to know the answers to help debug this:

1. Exactly what account is this being sent from?
2. Exactly what is the balance of that account, on the network this transaction is being sent to?
3. What gas limit am I sending with?
4. What gas price am I sending with?
5. How much ether am I including as part of this transaction?

(your answer for #3)*(your answer for #4) + (your answer for #5) needs to be less than (your answer for #2). Is it? The error indicates it is not, so figure out which of these values is not as you expect.

Most likely, it's #2 and you don't have enough ether in the account you're trying to send from.

Keep in mind that if you bought/acquired Ether on the main Ethereum chain, or used the faucet to get some on Rinkeby, it doesn't matter on Ropsten. You could try the Ropsten faucet here to boost your balance a bit. You can check your balance on Ropsten by searching the address here.