Contract Deployment – How to Interact with Deployed Contract ABI Using Web3.py

Question : If I deploy my smart contract to the real Ethereum blockchain, then is there any user interface with which I could access the contract functions in the same way as I can do in MyEtherWalet?

You may use your node's console interface, like this:

<path-to-geth>/geth attach <path-to-datadir>/geth.ipc console

Console provides you with javascript interface, where you can manipulate with your node's state, call api methods, lookup blocks, account etc.

Question 2: What is the role of Web3 here?

Web3 is a ethereum client library. It can be written in different languages. Most popular (I think) is javascript version of Web3. Actually, using a geth's console you are using all the functionality of web3.

You will need to create an HTML page that uses the web3 library to interact with your Ethereum-enabled browser (MetaMask, Mist, etc.).

First include web3.js or web3.min.js in your HTML, then you will be able to call JavaScript functions that interact with accounts and contracts, check syncing status and block numbers, etc.

<script src="web3.min.js"></script>

Here is an example of how to instantiate a web3 instance:

if (typeof web3 !== 'undefined') {
  web3 = new Web3(web3.currentProvider);
} else {
  // browser is not web3-enabled
}

Then you can use it like this:

web3.eth.getBlock(48, function(error, result){
    if(!error)
        console.log(result)
    else
        console.error(error);
})

Here is the API reference: https://github.com/ethereum/wiki/wiki/JavaScript-API

Heads up: The API reference has a lot of examples of synchronous calls, but web3 has deprecated them in a lot of cases. Most of the function calls will need to specify callbacks.