In this challenge we're going to play the role of a fintech engineer who's working at one of the largest banks, with the task to build a blockchain-based ledger system, complete with a user-friendly web interface. On this web-based ledger, users would be allowed to transfer money between each other while guaranteeing the integrity of the data in the ledger.

Programming Language:

• Python 3.7.11 loaded from the distribution Anaconda 4.10.3

Libraries:

• streamlit 1.8.1, a library that allows to build and share data apps
  Install through pip install streamlit
• hashlib, module that implements a common interface to many different secure hash and message digest algorithms
• typing, module that provides runtime support for type hints
• datetime, module that supplies classes for manipulating dates and times

To code this app we make an extensive use of data classes, programming the app in an object oriented programming (OOP) logic. We define our transaction at the most elementary level through the "Record" class, which contains information of the single transaction, specifically the sender, receiver and the amount. This class will be itself an attribute of the class "Block", which have also the creator ID, the previous block's hash, a timestamp and a nonce as attributes and a method consisting in a function that encodes this information. Next we have the "PyChain" class that contains the list of blocks' that make up our blockchain. This class contains three methods: one for the proof of work, one to add a new block and another to check if the new added block is valid.

After defining the classes we'll be using, we customize our web interface with the streamlit commands. We can try the result by opening the CLI in the directory where the script is and type:
streamlit run pychain.py
The following pictures show how it looks like:

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