opensheet-core 0.2.0

A fast, memory-efficient spreadsheet I/O library for Python, powered by Rust

Features  •  Benchmarks  •  Installation  •  Quick Start  •  API  •  Roadmap  •  Contributing

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Why OpenSheet Core?

Existing Python spreadsheet libraries force you to choose between performance, memory efficiency, broad format support, and easy installation. OpenSheet Core eliminates that tradeoff with a native Rust core exposed through a clean Python API — installable with a single pip install.

Features

• Streaming XLSX reader — row-by-row iteration without loading the entire file into memory
• Streaming XLSX writer — write millions of rows with constant memory usage
• Formula support — read and write formulas with optional cached values
• Date/time support — read and write datetime.date and datetime.datetime cells with automatic Excel serial number conversion
• Merged cells — read and write merged cell ranges
• Column widths & row heights — set and read custom column widths and row heights
• Freeze panes — freeze rows and/or columns so they stay visible when scrolling
• Auto-filter — add drop-down filter controls to column headers
• Number formats — write and read cells with custom number formats (currency, percentage, custom format strings)
• Cell styling — fonts (bold, italic, underline, name, size, color), fills, borders (thin, medium, thick, dashed, dotted, double), alignment (horizontal, vertical, wrap text, rotation), and number formats on styled cells
• Typed cell extraction — strings, numbers, booleans, dates, datetimes, formulas, and empty cells are returned as native Python types
• Context manager support — Pythonic with statement for safe resource management
• AI/RAG-ready — convert spreadsheets to markdown, chunked text, or plain text for LLM pipelines (planned)
• Cross-platform — tested on Linux, macOS, and Windows across Python 3.9–3.13
• Pandas integration — read XLSX files into DataFrames and write DataFrames to XLSX (pip install opensheet-core[pandas])
• Zero Python dependencies — single native extension, no dependency tree to manage

Benchmarks

Benchmarked against openpyxl 3.1.5 on a 100,000-row x 10-column dataset (1M cells), 5 interleaved runs, current RSS measurement (not high-water mark):

Operation	OpenSheet Core	openpyxl	Speedup	Memory (RSS delta)
Write	2.3s	3.7s	1.6x faster	1.7x less (1.2 MB vs 2.1 MB)
Read	253ms	3.5s	13.8x faster	2.5x less (13.5 MB vs 33.3 MB)

OpenSheet Core is faster and uses less memory for both reads and writes. The speed advantage comes from a Rust streaming parser with deferred shared-string resolution — strings are stored as indices during parsing and only converted to Python objects at the boundary. Write memory is low because the Rust writer streams data directly to disk.

Run it yourself: python benchmarks/benchmark.py

See the Benchmarking Methodology doc for details on how we measure and avoid common benchmarking pitfalls.

Installation

pip install opensheet-core

# With pandas support
pip install opensheet-core[pandas]

From source (requires Rust toolchain)

pip install maturin
git clone https://github.com/0xNadr/opensheet-core
cd opensheet-core
maturin develop --release

Quick Start

Reading an XLSX file

from opensheet_core import read_xlsx, read_sheet

# Read all sheets
sheets = read_xlsx("report.xlsx")
for sheet in sheets:
    print(f"Sheet: {sheet['name']}")
    for row in sheet["rows"]:
        print(row)  # List of typed Python values

# Read a specific sheet
rows = read_sheet("report.xlsx", sheet_name="Data")

Writing an XLSX file

from opensheet_core import XlsxWriter

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Data")
    writer.write_row(["Name", "Age", "Active"])
    writer.write_row(["Alice", 30, True])
    writer.write_row(["Bob", 25, False])

Writing dates

import datetime
from opensheet_core import XlsxWriter

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Events")
    writer.write_row(["Event", "Date", "Timestamp"])
    writer.write_row(["Launch", datetime.date(2025, 3, 15), datetime.datetime(2025, 3, 15, 14, 30)])

Merging cells

from opensheet_core import XlsxWriter

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Report")
    writer.write_row(["Title spanning three columns", "", ""])
    writer.write_row(["A", "B", "C"])
    writer.merge_cells("A1:C1")

Column widths and row heights

from opensheet_core import XlsxWriter

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Data")
    writer.set_column_width("A", 25.0)   # By letter
    writer.set_column_width(1, 15.0)     # By 0-based index
    writer.set_row_height(1, 30.0)       # Row 1 (1-based)
    writer.write_row(["Name", "Age"])
    writer.write_row(["Alice", 30])

Freeze panes

from opensheet_core import XlsxWriter

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Data")
    writer.freeze_panes(row=1, col=0)    # Freeze top row
    writer.write_row(["Header1", "Header2", "Header3"])
    writer.write_row(["data", "data", "data"])

Auto-filter

from opensheet_core import XlsxWriter

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Data")
    writer.write_row(["Name", "Age", "City"])
    writer.write_row(["Alice", 30, "NYC"])
    writer.write_row(["Bob", 25, "LA"])
    writer.auto_filter("A1:C1")

Number formats

from opensheet_core import XlsxWriter, FormattedCell

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Finance")
    writer.write_row(["Item", "Price", "Tax Rate"])
    writer.write_row([
        "Widget",
        FormattedCell(19.99, "$#,##0.00"),   # Currency
        FormattedCell(0.08, "0.00%"),         # Percentage
    ])

Cell styling

from opensheet_core import XlsxWriter, CellStyle, StyledCell

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Report")
    # Bold header with fill color
    writer.write_row([
        StyledCell("Name", CellStyle(bold=True, fill_color="4472C4", font_color="FFFFFF")),
        StyledCell("Score", CellStyle(bold=True, fill_color="4472C4", font_color="FFFFFF")),
    ])
    # Data with borders and alignment
    writer.write_row([
        StyledCell("Alice", CellStyle(border="thin", horizontal_alignment="left")),
        StyledCell(95, CellStyle(border="thin", number_format="0.0")),
    ])

Writing formulas

from opensheet_core import XlsxWriter, Formula

with XlsxWriter("output.xlsx") as writer:
    writer.add_sheet("Budget")
    writer.write_row(["Item", "Cost"])
    writer.write_row(["Rent", 1200])
    writer.write_row(["Food", 400])
    writer.write_row(["Total", Formula("SUM(B2:B3)", cached_value=1600)])

Pandas integration

import pandas as pd
from opensheet_core import read_xlsx_df, to_xlsx

# Read XLSX into a DataFrame
df = read_xlsx_df("data.xlsx", sheet_name="Sheet1")

# Write a DataFrame to XLSX
df = pd.DataFrame({"Name": ["Alice", "Bob"], "Age": [30, 25]})
to_xlsx(df, "output.xlsx", sheet_name="Results")

API Reference

read_xlsx(path: str) -> list[dict]

Reads an XLSX file and returns a list of dicts with "name" (str), "rows" (list of lists), "merges" (list of range strings like "A1:C1"), "column_widths" (dict of 0-based col index to width), "row_heights" (dict of 0-based row index to height), "freeze_pane" (tuple of (rows_frozen, cols_frozen) or None), and "auto_filter" (range string like "A1:C1" or None). Each cell is a typed Python value (str, int, float, bool, datetime.date, datetime.datetime, Formula, FormattedCell, or None).

read_sheet(path, sheet_name=None, sheet_index=None) -> list[list]

Reads a single sheet by name or index. Returns the first sheet by default.

sheet_names(path: str) -> list[str]

Returns the list of sheet names in a workbook.

XlsxWriter(path: str)

Streaming XLSX writer. Use as a context manager.

Method	Description
add_sheet(name: str)	Create a new worksheet
write_row(values: list)	Write a row of values to the current sheet
merge_cells(range: str)	Merge a range of cells (e.g. "A1:C1")
set_column_width(column, width)	Set column width (column is a letter or 0-based int)
set_row_height(row, height)	Set row height in points (row is 1-based)
freeze_panes(row=0, col=0)	Freeze top row rows and left col columns
auto_filter(range)	Set auto-filter on a range (e.g. "A1:C1")
close()	Finalize and close the file

read_xlsx_df(path, sheet_name=None, sheet_index=None, header=True)

Reads a single XLSX sheet into a pandas DataFrame. Requires pip install opensheet-core[pandas]. When header=True (default), the first row is used as column names. Formulas are unwrapped to cached values, FormattedCell values are unwrapped to plain numbers.

to_xlsx(df, path, sheet_name="Sheet1", header=True, index=False)

Writes a pandas DataFrame to an XLSX file. Handles numpy int/float/bool types, NaN/NaT (written as empty cells), and datetime64/Timestamp columns. Set index=True to include the DataFrame index as column(s).

CellStyle(**kwargs)

Style properties for a cell. All parameters are keyword-only. Properties: bold (bool), italic (bool), underline (bool), font_name (str), font_size (float), font_color (str, hex RGB), fill_color (str, hex RGB), border (str, shorthand for all 4 sides), border_left/border_right/border_top/border_bottom (str: "thin", "medium", "thick", "dashed", "dotted", "double"), border_color (str, hex RGB), horizontal_alignment (str: "left", "center", "right"), vertical_alignment (str: "top", "center", "bottom"), wrap_text (bool), text_rotation (int, 0-180), number_format (str, Excel format code).

StyledCell(value, style: CellStyle)

A cell value with styling. Pass as a cell value in write_row(). Returned by read_xlsx() and read_sheet() for cells that have visual styling. The inner value can be a string, number, bool, date, datetime, or formula.

FormattedCell(value, number_format: str)

A numeric value with a custom Excel number format code. Pass as a cell value in write_row(). Returned by read_xlsx() for cells with non-default number formats. Common format codes: "$#,##0.00" (currency), "0.00%" (percentage), "#,##0" (thousands separator).

Formula(formula: str, cached_value=None)

Represents a spreadsheet formula. Pass as a cell value when writing, and received when reading cells that contain formulas.

Architecture

┌──────────────────────────┐
│      Python API          │  ← opensheet_core (PyO3 bindings)
├──────────────────────────┤
│      Rust Core           │  ← Streaming parser & writer
│  ┌────────┐ ┌──────────┐ │
│  │ Reader │ │  Writer  │ │
│  │ (SAX)  │ │ (Stream) │ │
│  └────────┘ └──────────┘ │
├──────────────────────────┤
│  quick-xml  │    zip     │  ← Dependencies
└──────────────────────────┘

Feature Comparison vs openpyxl

OpenSheet Core is designed to be a faster, memory-efficient alternative to openpyxl for the most common spreadsheet workflows. Here's where we stand:

What we already do better

	OpenSheet Core	openpyxl
Write 1M cells	~2.3s	~3.7s
Read 1M cells	~0.25s	~3.5s
Write memory	1.2 MB RSS delta	2.1 MB RSS delta
Read memory	13.5 MB RSS delta	33.3 MB RSS delta
Python dependencies	Zero	Several
Architecture	Rust streaming core	Pure Python DOM

Memory optimization: shared strings are stored as indices during parsing and resolved to Python objects at the boundary via pre-interned lookup, avoiding duplicate string allocations. A future streaming iterator API will bring constant-memory reads.

Feature coverage

Category	Feature	openpyxl	OpenSheet Core
Formats	.xlsx read/write	Yes	Yes
	.xlsm (macro-enabled)	Yes	Planned
	.xltx/.xltm (templates)	Yes	—
Cell Types	Strings, numbers, booleans	Yes	Yes
	Dates and datetimes	Yes	Yes
	Formulas with cached values	Yes	Yes
	Rich text	Yes	Planned
	Error values	Yes	Planned
Styling	Fonts (name, size, bold, italic, color)	Yes	Yes
	Fill (solid, pattern, gradient)	Yes	Solid
	Borders (14 styles)	Yes	6 styles
	Alignment (horizontal, vertical, wrap, rotation)	Yes	Yes
	Number formats (30+ builtins + custom)	Yes	Yes
	Named styles	Yes	Planned
	Conditional formatting (6 rule types)	Yes	Planned
Worksheet	Merged cells	Yes	Yes
	Freeze panes	Yes	Yes
	Auto-filter	Yes	Yes
	Column widths / row heights	Yes	Yes
	Data validation (7 types)	Yes	Planned
	Sheet protection	Yes	Planned
	Row/column insert/delete	Yes	—
	Print settings	Yes	Planned
	Row/column grouping	Yes	—
Workbook	Named ranges / defined names	Yes	Planned
	Document properties	Yes	Planned
	Workbook protection	Yes	—
	Multiple sheet states (hidden, veryHidden)	Yes	Planned
Charts	12+ chart types (bar, line, pie, scatter, etc.)	Yes	Planned
	3D variants and combined charts	Yes	—
Images	Embed PNG/JPEG	Yes	Planned
Tables	Structured tables with styles	Yes	Planned
Pivot Tables	Read/preserve existing	Yes	—
VBA/Macros	Preserve on load (.xlsm)	Yes	Planned
Integration	Pandas DataFrame I/O	Yes	Yes
	NumPy type support	Yes	Yes
AI/RAG	Markdown/text extraction for LLMs	—	Planned
	Embedding-sized chunking	—	Planned
	LangChain / LlamaIndex loaders	—	Planned
Performance	Streaming read (constant memory)	Yes (read_only mode)	Yes (default)
	Streaming write (constant memory)	Yes (write_only mode)	Yes (default)

Legend: Yes = implemented, Planned = on the roadmap, — = not planned for now

Our approach

We are not trying to clone openpyxl. We are building a fast, safe, memory-efficient core for the most common Excel workflows. The goal is to cover the ~80% of features that people use day-to-day, while being up to 14x faster and using 2–3x less memory. Streaming is the default, not an opt-in mode.

Roadmap

Done

• XLSX reading with typed cell extraction
• Streaming XLSX writing with low memory usage
• Formula read/write support with cached values
• Date/time cell support with automatic serial number conversion
• Merged cell metadata (read and write)
• Python bindings via PyO3
• Type stubs (.pyi) and py.typed marker for IDE autocomplete
• CI across Linux, macOS, Windows (Python 3.9–3.13)
• Prebuilt wheels on PyPI
• Benchmarks vs openpyxl
• Runnable benchmark script (python benchmarks/benchmark.py)
• Zero Python dependencies
• Column widths and row heights
• Freeze panes
• Auto-filter
• Number formats (currency, percentage, custom format strings)
• Pandas DataFrame integration (read_xlsx_df / to_xlsx)
• Basic cell styling (fonts, fills, borders, alignment)

Phase 1.5 — AI/RAG integration

• xlsx_to_markdown() — convert sheets to structured markdown tables for LLM consumption
• xlsx_to_chunks() — yield embedding-sized chunks with header attachment and merge-aware boundaries
• xlsx_to_text() — plain text extraction for simple pipelines and search indexes
• LangChain OpenSheetLoader document loader
• LlamaIndex OpenSheetReader data connector

Phase 2 — Broader compatibility

• Named ranges / defined names
• Data validation
• Comments and hyperlinks
• .xlsm read support (preserve macros)
• Sheet protection
• Structured tables with styles
• Multiple sheet states (hidden, veryHidden)

Phase 3 — Rich content and ecosystem

• Charts (bar, line, pie, scatter — most common types)
• Image embedding (PNG, JPEG)
• Conditional formatting
• Document and custom properties
• NumPy type support
• Broader test corpus and fuzzing
• Security hardening (XML attack prevention)

Docs & community

• Migration guide: openpyxl → opensheet-core (side-by-side code comparisons)
• FastAPI/Flask streaming XLSX download examples
• Benchmark methodology documentation
• Dedicated benchmark page with chart visualizations

Project Status

v0.2.0 — streaming reader and writer with formula, date/time, merged cell, column width/row height, freeze pane, auto-filter, number format, cell styling, and pandas DataFrame support. 135 passing tests and prebuilt wheels on PyPI. The API may change before 1.0.

Contributing

Contributions are welcome! Here are some great ways to get involved:

• Report bugs or real-world spreadsheet edge cases
• Submit representative sample files for testing
• Suggest benchmark scenarios
• Improve documentation
• Open PRs for roadmap items

License

MIT

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