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Clean a UK bank export with plain Python, no pandas

Practice: Setting limits and rules. About 5 minutes.

About this exercise

Write a Python script that reads bank_export.csv and writes cleaned.csv with only Date, Description and Amount, keeping a header row. The bank is in the UK, so dates are day/month/year and must come out as YYYY-MM-DD. Amounts in parentheses are money out and must become negative numbers without commas, e.g. (1,250.00) becomes -1250.00. Use only Python's standard library, because pandas isn't installed on the shared laptop. One code block, at most 20 lines, then under 25 words outside the code.

Common mistakes when you ask AI for this

  • Not saying what's installed. The AI reaches for pandas, which isn't on the shared laptop. Try: Add: "Standard library only, no pandas."
  • Not saying where the bank is. The AI reads 03/11 as March 11, so every date shifts. Try: Add: "Dates are day/month/year (UK)."
  • Not explaining the parentheses. The AI keeps (1,250.00) as text or as a positive number. Try: Add: "Parentheses mean negative: (1,250.00) becomes -1250.00."
  • Getting a menu of options. The AI likes to offer two or three versions of the script, so you're left to pick and test them yourself. Try: Add: "Give one script only, in one code block, no alternatives."
  • Getting a tutorial. By default the AI explains every part of the code at length, which you then have to scroll past. Try: Add: "After the code, one line under 15 words. Nothing else."

What a strong prompt includes

  • Rules out pandas
  • Gives the date order
  • Explains negative amounts
  • Lists the output columns
  • Caps the code length

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