Data Formats Skill

Excel (.xlsx) Files

Reading Excel

import pandas as pd
df = pd.read_excel('input.xlsx', engine='openpyxl')
# Check what you got
print(df.columns.tolist())
print(df.dtypes)
print(df.head())

Writing Excel

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

CRITICAL: Excel Formula Evaluation

openpyxl writes formula strings but does NOT compute them. Verifiers read VALUES, not formulas.

Problem: Cell with =SUM(A1:A3) shows as 0 or #N/A when read back.

Solutions (in order of preference):

1. Compute values in Python and write computed values directly
2. Use gnumeric to recalculate: ssconvert --recalc file.xlsx file.xlsx
3. Use LibreOffice: libreoffice --headless --calc --convert-to xlsx file.xlsx

Common Excel Pitfalls

• Type mismatch: Number 2025 vs string "2025" breaks MATCH/VLOOKUP
• Missing packages: pip3 install --break-system-packages openpyxl xlsxwriter
• Sheet names: Check with pd.ExcelFile('input.xlsx').sheet_names
• Multiple sheets: pd.read_excel('input.xlsx', sheet_name='Sheet2')

CSV Files

import pandas as pd
# Always specify encoding
df = pd.read_csv('input.csv', encoding='utf-8')
# Check for issues
print(f"Shape: {df.shape}")
print(f"Columns: {df.columns.tolist()}")
print(df.head())

# Write
df.to_csv('output.csv', index=False, encoding='utf-8')

CSV Pitfalls

• Delimiter: Some files use ; or \t — check with head -2 file.csv
• Encoding: Try encoding='latin-1' if utf-8 fails
• Header: Some files have no header — use header=None
• Mixed types: Use dtype=str to read everything as strings first

JSON Files

import json
with open('input.json', encoding='utf-8') as f:
    data = json.load(f)

# Write with proper formatting
with open('output.json', 'w', encoding='utf-8') as f:
    json.dump(data, f, indent=2, ensure_ascii=False)

YAML Files

# Install: pip3 install --break-system-packages pyyaml
import yaml
with open('input.yaml') as f:
    data = yaml.safe_load(f)

General Tips

• Always check output file exists and has content before finishing
• Verify column names match exactly what the task expects
• Watch for NaN values: df.fillna(0) or df.dropna()
• Numeric precision: use round(value, N) for expected decimal places