feat: Add Opt-in Formula Reading Support

[PSU3D0]

Overview

This PR introduces optional formula reading capabilities to python-calamine, allowing users to access the underlying formulas in spreadsheet cells rather than just their calculated values.

Motivation

Previously, python-calamine only provided access to cell values (the calculated results of formulas). Users had no way to access the actual formula expressions, which is essential for:

• Spreadsheet analysis and auditing
• Formula debugging and validation
• Converting between different spreadsheet formats while preserving formulas
• Building tools that need to understand the calculation logic

Implementation

Core Changes

1. Opt-in Design: Formula reading is disabled by default (read_formulas=False) to maintain backward compatibility and avoid performance overhead for users who don't need formulas.

2. New API Parameter: Added read_formulas parameter to all workbook creation methods:

   • CalamineWorkbook.from_object(path_or_filelike, read_formulas=False)
   • CalamineWorkbook.from_path(path, read_formulas=False)
   • CalamineWorkbook.from_filelike(filelike, read_formulas=False)
   • load_workbook(path_or_filelike, read_formulas=False)

3. Formula Iterator: Added CalamineSheet.iter_formulas() method that returns a CalamineFormulaIterator with consistent dimensions matching the data iterator.

4. Performance Optimization: Implemented on-demand coordinate mapping instead of pre-allocating expanded ranges, ensuring minimal memory overhead.

Technical Details

• Coordinate Consistency: Formula iterator returns the same dimensions as data iterator, with empty strings for cells without formulas
• Graceful Error Handling: Clear error message when attempting to access formulas without enabling the feature
• Memory Efficient: Uses get_value() lookups rather than expanding formula ranges upfront
• Iterator Properties: Both CalamineCellIterator and CalamineFormulaIterator expose position, start, width, and height properties for coordinate recovery

File Format Support

Supports formula reading across all major spreadsheet formats:

• Excel (.xlsx, .xls, .xlsb)
• OpenDocument (.ods)

Formula syntax varies by format (e.g., ODS uses of:=SUM([.A1:.B1]) vs Excel's SUM(A1:B1)).

API Examples

# Default behavior - formulas disabled
wb = load_workbook("spreadsheet.xlsx")
sheet = wb.get_sheet_by_name("Sheet1")
# sheet.iter_formulas()  # Raises ValueError

# Enable formula reading
wb = load_workbook("spreadsheet.xlsx", read_formulas=True)
sheet = wb.get_sheet_by_name("Sheet1")

# Access both data and formulas
data = list(sheet.iter_rows())      # [[10.0, 15.0, 25.0]]
formulas = list(sheet.iter_formulas())  # [["", "", "SUM(A1:B1)"]]

# Iterator properties for coordinate mapping
formula_iter = sheet.iter_formulas()
print(f"Start: {formula_iter.start}, Size: {formula_iter.width}x{formula_iter.height}")

Backward Compatibility

• Default behavior unchanged (read_formulas=False)
• No breaking changes to existing APIs
• Existing code continues to work without modification

Testing

Comprehensive test suite covering:

• All supported file formats (.xlsx, .xls, .ods)
• All workbook creation methods
• Error handling for disabled formulas
• Iterator property validation
• Coordinate consistency between data and formula iterators

Performance Impact

• Zero overhead when formulas are disabled (default)
• Minimal memory usage when formulas are enabled due to on-demand lookup strategy
• No upfront range expansion or pre-allocation

[PSU3D0]

A human addendum to the AI's diff summary ^:

This lib is much faster than openpyxl but remained unusable for my use-case due to:

• Lack of formula support
• Lack of coordinate recovery

This PR should be a big step forward for broader usability and more use-cases. The next thing I'd hit would be a unified CellData iterator that exposes value, coordinate, optional formula, and potentially style information (still need to check Calamine source on this).

Cheers!

[PSU3D0]

@dimastbk bumping this