excelreader (Python)

Read and write XLSX, XLSB, XLS and CSV through ExcelReader's NativeAOT library. No .NET runtime required — the shared library is self-contained.

Install (from source)

python python/scripts/build_native.py   # requires the .NET 10 SDK, once per machine
pip install -e "python[dev]"

build_native.py publishes src/ExcelReader.Native for your platform and copies the resulting ExcelReader.Native.{dll,so,dylib} into excelreader/_lib/. To point at a binary you built elsewhere, set EXCELREADER_NATIVE_LIB to its full path.

Usage

from excelreader import open_workbook

with open_workbook("book.xlsx") as workbook:
    print(len(workbook.sheets), workbook.sheets[0].name)
    for row in workbook.sheets[0].rows():
        for cell in row:
            print(cell.column, cell.type.name, cell.value)

Sheets

A workbook hands out sheets. workbook.sheets is a sequence: len(), iteration, and lookup by index (negative allowed), slice or name (names match without regard to case; a miss raises KeyError). A Sheet is the workbook and an index; it holds no native resource, and every read on it opens its own cursor.

import excelreader

with excelreader.open_workbook("report.xlsx") as workbook:
    for sheet in workbook.sheets:
        print(sheet.index, sheet.name, sheet.visibility)

    totals = workbook.sheets["Totals"]
    for row in totals.rows():
        ...

One open workbook can be read from several threads, which loads the shared-string table once. The native calls release the GIL:

from concurrent.futures import ThreadPoolExecutor

with excelreader.open_workbook("report.xlsx") as workbook, ThreadPoolExecutor() as pool:
    tables = list(pool.map(lambda sheet: sheet.to_arrow(sheet.infer_schema()), workbook.sheets))

rows() and iter_parse_typed() are generators: the native cursor opens at the first next(). A generator that has already started keeps working after workbook.close(); one that has not started raises at its first next(). to_arrow_stream() opens its stream at the call, so it works after close() even if never started.

Formats

open_workbook sniffs XLS/XLSX/XLSB by file signature. CSV has no signature, so it is chosen by the .csv extension — or explicitly:

open_workbook("data.txt", format="csv")

Dates

cell.value is always the raw text as stored, so CellType.DATE cells hold Excel serial numbers. Use Cell.as_date() to convert, passing the workbook's epoch flag:

as_date = cell.as_date(workbook.is_date1904)

as_date() returns None for any cell that isn't CellType.DATE.

Reading a sheet

A sheet reads in one of these shapes:

infer_schema() guesses the schema every typed method takes.

Typed columns — the fastest path

rows() hands back cell text, which means the library formats every value to a string on the way out and builds one Python object per cell. parse_typed() skips that entirely: you give it a schema, and the conversion happens natively, straight into typed column buffers. On a 65,536 × 14 sheet it is ~30× faster than rows(), and faster than polars.read_excel() — see Reading below for the measured numbers.

from excelreader import ColumnSpec, ColumnType

with open_workbook("sales.xlsb") as workbook:
    table = workbook.sheets[0].parse_typed([
        ColumnSpec(ColumnType.STRING, name="Region"),
        ColumnSpec(ColumnType.DATE, name="Order Date"),
        ColumnSpec(ColumnType.F64, name="Total Revenue", nullable=True),
    ])

table.row_count            # rows read
table.names                # ["Region", "Order Date", "Total Revenue"]
region, day, revenue = table.columns
region[0]                  # "Asia" — strings decode on demand, not one str per row up front
day[0]                     # 15477 — days since 1970-01-01
revenue[0]                 # 14862.69
table.validity[2]          # bit-packed nulls, or None when the column has none

Each non-string column is a stdlib array.array, or a NumPy array if NumPy is installed (pip install -e "python[numpy]") — NumPy is optional and never required.

Leave name out to resolve a column by position instead: ColumnSpec(ColumnType.I64, index=3). header_row defaults to 1 (the first row names the columns); pass header_row=0 for a sheet with no header, where every spec must resolve by index.

A column that fails to convert is an error unless its spec sets nullable=True, which records the failure in table.validity and keeps reading.

parse_typed() always reads the whole sheet from its first row, independent of any rows() generator in progress.

Reading a sheet a batch at a time

For a sheet too large to hold in memory at once, iter_parse_typed() is parse_typed() a batch at a time — a generator yielding one TypedTable per batch:

with open_workbook("sales.xlsb") as workbook:
    schema = [
        ColumnSpec(ColumnType.STRING, name="Region"),
        ColumnSpec(ColumnType.DATE, name="Order Date"),
        ColumnSpec(ColumnType.F64, name="Total Revenue", nullable=True),
    ]
    for batch in workbook.sheets[0].iter_parse_typed(schema, batch_size=10_000):
        region, day, revenue = batch.columns
        ...

batch_size is rows per batch; 0 means one unbounded batch, identical to parse_typed(), and a negative value is an error. Chunked reads are independent: several can be open on one workbook at once, typed or Arrow (see Arrow below), and none disturbs another.

Being a generator, iter_parse_typed() opens nothing until the first iteration, so a bad batch_size is only raised there, not at the call.

Reading a large sheet on several threads

parse_typed(), to_arrow(), to_pandas() and to_polars() take parallelism: 1 (the default) reads on one thread, 0 uses every core, n up to n threads. XLSX, XLSB and CSV are split. XLS, or a sheet under 256 KiB, is read on one thread with the same result. The table is identical either way. Every partition's columns are held until they are merged, so the peak memory is higher than a sequential read of the same file. to_pandas() and to_polars() with parallelism other than 1 parse the whole file instead of streaming it in batches.

xl_parse_typed on a Ryzen 7 5700X (8 cores, 16 threads), 14 typed columns, file read from memory:

The same call on the real-data workbooks (XLSX 6.7 MB compressed, XLSB 3.7 MB; measured with the published NativeAOT library): XLSX 54.1 ms at parallelism=1 against 20.3 ms at 0 (2.6x), XLSB 28.2 ms against 15.2 ms (1.85x). Inflate runs on one thread, so under the JIT the sheet stage stops scaling at about three threads.

Peak working set on the 160 MB file, above the interpreter's own, went from 470 MiB to 503 MiB (both include the file's bytes and the handle's copy of them).

Guessing a schema

Writing the ColumnSpec list by hand means already knowing every column's name and type. When you don't, infer_schema() samples the sheet and guesses one for you:

with open_workbook("sales.xlsb") as workbook:
    sheet = workbook.sheets[0]
    schema = sheet.infer_schema()   # header_row=1, sample_size=100 by default
    table = sheet.parse_typed(schema)

By default each column's type comes from the CellType Excel already stored for its sampled cells, so it costs nothing beyond the sample and is exact for XLSX/XLSB/XLS. A column with a real mix of kinds, only formula/error results, or nothing sampled falls back to ColumnType.STRING; nullable is set when any sampled row left the column empty. CSV cells carry no such type tag, so by default every CSV column is guessed ColumnType.STRING. Pass parse_text=True to type text cells from their exact shape instead: integers, decimals, true/false and ISO dates or date-times. Codes with a leading zero (00123), scientific notation (12E4), padded or comma-decimal numbers and non-ISO dates stay strings. It is still a guess over the sample, so a value further down can fail to convert:

with open_workbook("sales.csv") as workbook:
    sheet = workbook.sheets[0]
    schema = sheet.infer_schema(parse_text=True)
    table = sheet.parse_typed(schema, parallelism=0)

Writing

write_workbook() writes a TypedTable (what parse_typed() returns) back out as a single sheet, through the same xl_write_typed native export — one-shot, no writer handle before or after the call:

from excelreader import ColumnType, write_workbook

with open_workbook("sales.xlsb") as workbook:
    sheet = workbook.sheets[0]
    table = sheet.parse_typed(sheet.infer_schema())

types = [ColumnType.STRING, ColumnType.DATE, ColumnType.F64]  # one per table.columns, in order
write_workbook("sales_copy.xlsx", table, types)

types is required because a TypedTable column is a raw buffer (array/StringColumn/NumPy array) and nothing about the buffer alone tells I64 from TIME apart — both are 8-byte-per-row arrays. format is inferred from the path's extension (one of xlsx/xlsb/xls/csv) or set explicitly:

write_workbook("report.dat", table, types, format="csv")

The destination can also be a binary object with write(), such as io.BytesIO or an open file. The workbook is built in memory and written in one write() call, and with no extension to infer it from, format is required:

import io

buffer = io.BytesIO()
write_workbook(buffer, table, types, format="xlsx")
payload = buffer.getvalue()

write_workbook() also takes Arrow-shaped data in place of a TypedTable: a pyarrow.RecordBatch or Table, a pandas DataFrame (its index is not written), or any table exposing __arrow_c_stream__, such as a polars DataFrame. Each column's type comes from its Arrow type, so leave types out. This path requires pyarrow:

write_workbook("report.xlsx", df)          # pandas
write_workbook("report.xlsx", polars_df)   # polars
write_workbook("report.xlsx", record_batch)

WriteOptions sets the sheet name and CSV dialect, mirroring xl_write_options — every field defaults to None, meaning "use the library default":

from excelreader import WriteOptions

write_workbook(
    "report.xlsx", table, types,
    options=WriteOptions(sheet_name="Q3 Results", use_shared_strings=True),
)

Phase-1 limits, stated plainly: a single sheet only (no multi-sheet workbooks); the whole table must already be in memory (no streaming/chunked writes); no styling beyond the temporal number formats xl_write_typed applies to DATE/TIME/TIMESTAMP columns. format="auto" is not accepted — sniffing reads a file's existing signature bytes, and a file being created has none.

Writing a sheet row by row

open_workbook's counterpart for writing one row at a time, instead of building a whole table first:

import datetime
from excelreader import open_writer

with open_writer("out.xlsx") as writer:
    writer.start_sheet("Data")
    writer.write_row(["name", "qty", "when"])
    writer.write_row(["widget", 7, datetime.date(2026, 1, 31)])
    writer.end_sheet()

write_row picks each cell's type from the Python value: str, bool, int (64-bit), float, datetime.datetime (an aware one is converted to UTC), datetime.date and datetime.time. A None becomes a blank string cell; the tuple (None, ColumnType.I64) writes a blank cell of that type:

writer.write_row(["widget", (None, ColumnType.I64), 2.5])

To build a workbook without touching the filesystem, open the writer over a binary object with write(), such as io.BytesIO. It needs format, and receives the finished workbook in one write() call when the writer closes:

import io

from excelreader import open_writer

buffer = io.BytesIO()
with open_writer(buffer, "xlsx") as writer:
    writer.start_sheet("Data")
    writer.write_row(["name", "qty"])
    writer.end_sheet()
payload = buffer.getvalue()

Arrow

With pyarrow installed, to_arrow() runs the same read and hands the buffers to pyarrow zero-copy over the Arrow C Data Interface, as one column-named pyarrow.RecordBatch:

with open_workbook("sales.xlsb") as workbook:
    batch = workbook.sheets[0].to_arrow(schema)

pyarrow owns the buffers from that point on, so the result stays valid after the workbook is closed.

Streaming: Arrow streams, pandas, and polars

to_arrow_stream() is to_arrow() a batch at a time — peak memory is one batch rather than one sheet. It returns a single-use stream implementing the Arrow PyCapsule interface (__arrow_c_stream__) and needs no third-party package itself; hand it to any consumer, such as pyarrow for a RecordBatchReader:

import pyarrow as pa

with open_workbook("sales.xlsb") as workbook:
    reader = pa.RecordBatchReader.from_stream(workbook.sheets[0].to_arrow_stream(schema, batch_size=10_000))
    for batch in reader:
        frame = batch.to_pandas()   # or polars.from_arrow(batch), for one DataFrame per batch

Same batch_size rules as iter_parse_typed() above, except to_arrow_stream() raises immediately rather than on the first iteration. The stream keeps working after the workbook is closed.

to_pandas()/to_polars() materialize the whole sheet as a single DataFrame and both take batch_size and parallelism, but they spend batch_size differently — do not blur them. to_polars() genuinely streams: polars consumes the stream batch by batch, so the whole sheet is never resident as Arrow buffers at once. to_pandas() does not bound peak memory the same way — it concatenates every batch before pandas ever sees them. What batch_size buys to_pandas() instead is the conversion: self_destruct=True frees each Arrow chunk as pandas consumes it, so the sheet is never held twice, once as Arrow and once as the DataFrame.

A faulted stream surfaces differently from every other native error in this library: pyarrow's RecordBatchReader reports a failed batch as a plain OSError carrying the native error message, not ExcelReaderError — the failure crosses the Arrow C Data Interface before this library's own exception wrapping ever runs.

From memory, a stream, or your own bytes

open_workbook() takes more than a path. bytes, bytearray, memoryview and io.BytesIO are read from memory; the format comes from the content, so pass format only for a CSV or to skip the check.

with open_workbook(payload) as workbook:
    ...

An object with a size and a read_at(offset, buffer) that fills buffer and returns the count is read in blocks on demand: the library asks for 4 MiB blocks and caches them, and every sheet can still be read on its own thread.

import urllib.request

from excelreader import open_workbook

class HttpSource:
    def __init__(self, url):
        self.url = url
        with urllib.request.urlopen(urllib.request.Request(url, method="HEAD")) as response:
            self.size = int(response.headers["Content-Length"])

    def read_at(self, offset, buffer):
        request = urllib.request.Request(self.url, headers={"Range": f"bytes={offset}-{offset + len(buffer) - 1}"})
        with urllib.request.urlopen(request) as response:
            data = response.read()
        buffer[: len(data)] = data
        return len(data)

with open_workbook(HttpSource("https://example.com/book.xlsx")) as workbook:
    ...

Any other object with readinto() or read(), such as an open file or sys.stdin.buffer, is read as a stream. A CSV is read as it arrives, and its first 16 MiB are kept so that infer_schema() followed by a read works; an XLSX, XLSB or XLS is read whole first.

import sys

from excelreader import open_workbook

with open_workbook(sys.stdin.buffer, "csv") as workbook:
    ...

Both callbacks run with the GIL held. For a file on disk, pass its path: that is faster.

Reader options

open_workbook() takes an optional OpenOptions for CSV dialect settings and reader resource limits. Every field defaults to None, meaning "use the library default", so you set only what you want to change.

from excelreader import OpenOptions, open_workbook

# A semicolon-delimited CSV, which the default comma dialect would read as one column per row.
with open_workbook("export.csv", format="csv", options=OpenOptions(csv_delimiter=ord(";"))) as workbook:
    for row in workbook.sheets[0].rows():
        ...

csv_delimiter and csv_quote are byte values, so pass ord(";") rather than ";". max_cell_bytes and intern_strings apply to every format, a CSV included.

The max_* fields are resource limits rather than tuning knobs: they bound what a malformed or hostile file can make the reader allocate, and exceeding one raises ExcelReaderError. Lower them when parsing untrusted uploads.

options = OpenOptions(
    max_total_decompressed_bytes=64 * 1024 * 1024,  # zip-bomb budget for XLSX/XLSB
    max_cell_bytes=1024 * 1024,
    max_zip_entries=1024,
)

prefetch_decompression=True overlaps inflating an XLSX/XLSB sheet with parsing it — worth it for single-file batch work, not for a server already reading many files in parallel. See the root README for the measured trade.

Each ZIP entry of an XLSX or XLSB is checked against its CRC-32 as it is read, so a damaged file raises ExcelReaderError instead of returning wrong cells. verify_zip_checksums=False turns the check off, for a file whose producer wrote wrong checksums.

Encrypted workbooks

open_workbook() takes a password keyword to open a password-protected OOXML workbook (.xlsx/.xlsb/.xlsm):

from excelreader import PasswordIncorrectError, open_workbook

try:
    with open_workbook("protected.xlsx", password="hunter2") as workbook:
        ...
except PasswordIncorrectError:
    ...  # ask again

Omitting password for an encrypted file raises PasswordRequiredError; a wrong one raises PasswordIncorrectError — both subclass ExcelReaderError, so a caller that doesn't care about the distinction can just catch that. Any other native failure (an unsupported encryption scheme, a corrupt file) also raises ExcelReaderError but is not worth retrying.

An explicit format="xlsx"/format="xlsb" works for an encrypted file too, the same as leaving format unset — both routes decrypt correctly given the right password.

Writing an encrypted workbook is a second step: write the plaintext package with write_workbook or open_writer, then wrap it with encrypt_package, which produces an agile-encrypted (ECMA-376 4.4) CFB container — AES-256-CBC, SHA-512, 100,000 spin iterations, with a dataIntegrity HMAC:

from excelreader import encrypt_package, write_workbook

write_workbook("plain.xlsx", table, types)
encrypt_package("plain.xlsx", "secret.xlsx", "hunter2")

The package must already be a finished file; it is read, not removed. Encryption parameters are fixed at Excel's own defaults — there are no options — and only XLSX/XLSB packages can be encrypted, matching what open_workbook can decrypt.

Either side can stay in memory, so the plaintext package never has to be written to disk: the package can be bytes or an io.BytesIO, and the destination any binary object with write():

import io

from excelreader import encrypt_package, write_workbook

plain, secret = io.BytesIO(), io.BytesIO()
write_workbook(plain, table, types, format="xlsx")
encrypt_package(plain, secret, "hunter2")

Benchmarks

benchmarks/bench_read.py and benchmarks/bench_write.py over tests/ExcelReader.Benchmarks/Data/65K_Records_Data.xlsb (65,535 data rows, 14 columns), the same fixture the .NET, C++ and Rust suites use. Measured on Windows 10 (22H2), AMD Ryzen 7 5700X (16 logical CPUs @ 3.39 GHz), CPython 3.14.7, 10 runs each (medians shown; min is in the scripts' own output), against a library published from commit 65945b4. Competitors: polars 1.44.1 (with fastexcel 0.21.0), pandas 3.0.5 (with openpyxl 3.1.5), xlsxwriter 3.2.9.

Reading

The batched readers cost the same as their whole-sheet counterparts and lower the peak memory: to_arrow_stream(batch_size=10000) peaks at 11.4 MiB against 18.8 MiB for one whole-sheet record batch on this file.

Only the to_polars() / polars.read_excel() pair is a like-for-like comparison (~3.6x), and even that one is loose: both produce a typed columnar DataFrame with inferred types, but the inference rules are not identical. The rows above it produce different things and are listed to show what each API costs, not to rank them — rows() is ~30x slower than parse_typed() because it materializes 917,504 Python objects, which is the price of that shape, not a slow parser.

CSV, from 65K_Records_Data.csv (8.2 MB), same 14 typed columns:

polars' CSV reader is ahead (multithreaded, and it infers types from the text without our schema step); ExcelReader is ~10x faster than pandas on either engine. These include opening the file, so they are above the in-memory xl_parse_typed figures in Reading a large sheet on several threads.

Writing

The two DataFrame comparisons are matched work — same DataFrame in, xlsx out both times: write_workbook(polars_df) is ~9.6x faster than polars' own write_excel(), and write_workbook(pandas_df) ~14.6x faster than to_excel(). Both of ours pay a conversion the raw path does not: the DataFrame goes through Arrow and then a Python list before reaching the native columns, which is most of the gap between the ~490 ms rows and the 90 ms one. Handing write_workbook() buffers that are already columnar — what parse_typed() returns — skips all of it.

write_workbook(xlsx) at 90.6 ms lands within ~5 ms of the C++ binding's xl::write_columns on the same 14 columns (85.4 ms), which is the expected result: both are thin wrappers over the same xl_write_typed call, and neither adds work per cell.

xls being both the largest file and faster than xlsb and xlsx is not a paradox — BIFF8 writes fixed-width records with no compression, so it trades 3.5x the bytes for less work per cell.

Notes

• A Workbook can be read from several threads at once; see Sheets.
• Empty cells are skipped, so cell.column may skip indices. Do not assume row[i].column == i.
• The ABI is documented in src/ExcelReader.Native/include/excelreader.h.