Requirements for Outputs

All Excel Files

Zero Formula Errors

• Every Excel model MUST be delivered with ZERO formula errors (#REF!, #DIV/0!, #VALUE!, #N/A, #NAME?)

Preserve Existing Templates (when updating templates)

• Study and EXACTLY match existing format, style, and conventions when modifying files

• Never impose standardised formatting on files with established patterns

• Existing template conventions ALWAYS override these guidelines

Financial Models

For financial models, DCFs, and valuations - read references/financial-model-standards.md for colour coding, number formatting, formula construction rules, and documentation requirements.

XLSX Creation, Editing, and Analysis

Overview

A user may ask you to create, edit, or analyse the contents of an .xlsx file. You have different tools and workflows available for different tasks.

Important Requirements

LibreOffice Required for Formula Recalculation: You can assume LibreOffice is installed for recalculating formula values using the recalc.py script. The script automatically configures LibreOffice on first run.

Reading and Analysing Data

Data analysis with pandas

For data analysis, visualisation, and basic operations, use pandas which provides powerful data manipulation capabilities:

import pandas as pd

Read Excel

df = pd.read_excel('file.xlsx') # Default: first sheet all_sheets = pd.read_excel('file.xlsx', sheet_name=None) # All sheets as dict

Analyse

df.head() # Preview data df.info() # Column info df.describe() # Statistics

Write Excel

df.to_excel('output.xlsx', index=False)

CRITICAL: Use Formulas, Not Hardcoded Values

Always use Excel formulas instead of calculating values in Python and hardcoding them. This ensures the spreadsheet remains dynamic and updateable.

Bad - Hardcoding Calculated Values

Bad: Calculating in Python and hardcoding result

total = df['Sales'].sum() sheet['B10'] = total # Hardcodes 5000

Bad: Computing growth rate in Python

growth = (df.iloc[-1]['Revenue'] - df.iloc[0]['Revenue']) / df.iloc[0]['Revenue'] sheet['C5'] = growth # Hardcodes 0.15

Bad: Python calculation for average

avg = sum(values) / len(values) sheet['D20'] = avg # Hardcodes 42.5

Correct - Using Excel Formulas

Good: Let Excel calculate the sum

sheet['B10'] = '=SUM(B2:B9)'

Good: Growth rate as Excel formula

sheet['C5'] = '=(C4-C2)/C2'

Good: Average using Excel function

sheet['D20'] = '=AVERAGE(D2:D19)'

This applies to ALL calculations - totals, percentages, ratios, differences, etc. The spreadsheet should be able to recalculate when source data changes.

Common Workflow

• Choose tool: pandas for data, openpyxl for formulas/formatting

• Create/Load: Create new workbook or load existing file

• Modify: Add/edit data, formulas, and formatting

• Save: Write to file

• Recalculate formulas (MANDATORY IF USING FORMULAS): Use the recalc.py script python recalc.py output.xlsx

• Verify and fix any errors:

• The script returns JSON with error details

• If status is errors_found , check error_summary for specific error types and locations

• Fix the identified errors and recalculate again

• Common errors to fix:

• #REF! : Invalid cell references

• #DIV/0! : Division by zero

• #VALUE! : Wrong data type in formula

• #NAME? : Unrecognised formula name

For detailed code examples (creating/editing files), read references/openpyxl-patterns.md .

Recalculating Formulas

Excel files created or modified by openpyxl contain formulas as strings but not calculated values. Use the provided recalc.py script to recalculate:

python recalc.py <excel_file> [timeout_seconds]

Example:

python recalc.py output.xlsx 30

The script:

• Automatically sets up LibreOffice macro on first run

• Recalculates all formulas in all sheets

• Scans ALL cells for Excel errors (#REF!, #DIV/0!, etc.)

• Returns JSON with detailed error locations and counts

• Works on both Linux and macOS

For the formula verification checklist and recalc.py output interpretation, read references/formula-verification.md .

Code Style Guidelines

IMPORTANT: When generating Python code for Excel operations:

• Write minimal, concise Python code without unnecessary comments

• Avoid verbose variable names and redundant operations

• Avoid unnecessary print statements

For Excel files themselves:

• Add comments to cells with complex formulas or important assumptions

• Document data sources for hardcoded values

• Include notes for key calculations and model sections

References

• references/financial-model-standards.md

• Colour coding, number formatting, formula construction rules, documentation requirements for financial models

• references/openpyxl-patterns.md

• Code examples for creating/editing files, library selection guide, openpyxl and pandas tips

• references/formula-verification.md

• Verification checklist, common pitfalls, recalc.py output interpretation