61 — Project — a mini bank

Guide 48 kept a shop's money in a dict; guide 43 put the storage logic in a module. A bank account is the same pair one step further: a dict with a balance and a history list of every transaction, saved to account.json by a bank.nme module.

#Steps

1. An account is one dict with two parts: balance starts at 0, and history is a list that grows with every deposit or withdrawal. It prints 100 and ['+100']:

   account = {"balance": 0, "history": []}
   account["balance"] = account["balance"] + 100
   account["history"].append("+100")
   show account["balance"]
   show account["history"]
   

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   Each transaction is one string in the history list, with a + for deposits and a - for withdrawals.

2. Storage lives in a module, bank.nme, exporting load() and save. load() returns a fresh account when no file exists yet — the same pattern as the store module in guide 48:

   # bank.nme — file storage for the mini bank.
   
   import os
   use file latest
   
   def load():
       if os.path.exists("account.json"):
           return json_load("account.json")
       return {"balance": 0, "history": []}
   
   def save(account):
       json_save("account.json", account)
   

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   json_load returns the saved dict, so balance and history come back exactly as they were written.

3. The whole bank. Save account.nme next to bank.nme:

   # account.nme — a mini bank kept in a JSON file.
   # Run: nme r account
   # Type deposit, withdraw, balance, history, or quit.
   
   from "bank.nme" import load, save
   account = load()
   
   show "Mini bank — balance is kept in account.json"
   while True:
       show "Commands: deposit, withdraw, balance, history, quit"
       ask command, "? "
       if command == "deposit":
           ask amount_text, "Amount? "
           amount = int(amount_text)
           account["balance"] = account["balance"] + amount
           account["history"].append(f"+{amount}")
           save(account)
           show f"Deposited {amount}"
       elif command == "withdraw":
           ask amount_text, "Amount? "
           amount = int(amount_text)
           if amount <= account["balance"]:
               account["balance"] = account["balance"] - amount
               account["history"].append(f"-{amount}")
               save(account)
               show f"Withdrew {amount}"
           else:
               show "Not enough money"
       elif command == "balance":
           show f"Balance: {account['balance']}"
       elif command == "history":
           show f"{len(account['history'])} transactions"
           for entry in account["history"]:
               show entry
       elif command == "quit":
           show "Bye!"
           break
       else:
           show "Unknown command"
   

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   deposit adds to the balance and records +amount; withdraw checks the balance first and records -amount. Both call save, so every change is written to account.json immediately. history walks the list the way list did in guide 31.

4. Run it and feed the commands through a pipe. deposit 100 then withdraw 30 leaves 70, and history shows both transactions:

   printf 'deposit\n100\nwithdraw\n30\nbalance\nhistory\nquit\n' | nme r account
   

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   Mini bank — balance is kept in account.json
   Commands: deposit, withdraw, balance, history, quit
   ? Amount? Deposited 100
   Commands: deposit, withdraw, balance, history, quit
   ? Amount? Withdrew 30
   Commands: deposit, withdraw, balance, history, quit
   ? Balance: 70
   Commands: deposit, withdraw, balance, history, quit
   ? 2 transactions
   +100
   -30
   Commands: deposit, withdraw, balance, history, quit
   ? Bye!
   

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   Look at account.json — it now holds the whole state:

   {"balance": 70, "history": ["+100", "-30"]}
   

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   Withdrawing more than the balance prints Not enough money and saves nothing, so the account can never go negative:

   printf 'withdraw\n500\nbalance\nquit\n' | nme r account
   

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   Mini bank — balance is kept in account.json
   Commands: deposit, withdraw, balance, history, quit
   ? Amount? Not enough money
   Commands: deposit, withdraw, balance, history, quit
   ? Balance: 70
   Commands: deposit, withdraw, balance, history, quit
   ? Bye!
   

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#Try it yourself

Add a transfer command that withdraws from this account and deposits into a second one — load both, change both, save both. Or refuse deposits of zero or negative amounts with an if amount <= 0: check.

#What you learned

• An account is a dict of {balance, history}; history is a list of strings.
• load() / save() in bank.nme keep the file format in one module.
• withdraw checks amount <= account["balance"] before spending.
• Every change calls save, so the account survives between runs.