Loading Excel file chunk by chunk with Python instead of loading full file into memory

Question

I want to read just 10 lines from Excel files (xlsx) without loading the whole file at once, as it can't be done on one of my machines (low memory).

I tried using

import xlrd
import pandas as pd
def open_file(path):
    xl = pd.ExcelFile(path)
    reader = xl.parse(chunksize=1000)
    for chunk in reader:
        print(chunk)

It seems like the file is loaded first then divided into parts.

How to read only first lines?

Answer 1

Due to the nature of xlsx files (which are essentially a bunch of xml files zipped together) you can't poke the file at an arbitrary byte and hope for it to be the beginning of Nth row of the table in the sheet you are interested in.

The best you can do is use pandas.read_excel with the skiprows (skips rows from the top of the file) and skip_footer (skips rows from the bottom) arguments. This however will load the whole file to memory first and then parse the required rows only.

# if the file contains 300 rows, this will read the middle 100
df = pd.read_excel('/path/excel.xlsx', skiprows=100, skip_footer=100,
                   names=['col_a', 'col_b'])

Note that you have to set the headers manually with the names argument otherwise the column names will be the last skipped row.

If you wish to use csv instead then it is a straightforward task since csv files are plain-text files.

But, and it's a big but, if you are really desperate you can extract the relevant sheet's xml file from the xlsx archive and parse that. It's not going to be an easy task though.

An example xml file that represents a sheet with a single 2 X 3 table. The <v> tags represent the cells' value.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="x14ac" xmlns:x14ac="http://schemas.microsoft.com/office/spreadsheetml/2009/9/ac">
    <dimension ref="A1:B3"/>
    <sheetViews>
        <sheetView tabSelected="1" workbookViewId="0">
            <selection activeCell="C10" sqref="C10"/>
        </sheetView>
    </sheetViews>
    <sheetFormatPr defaultColWidth="11" defaultRowHeight="14.25" x14ac:dyDescent="0.2"/>
    <sheetData>
        <row r="1" spans="1:2" ht="15.75" x14ac:dyDescent="0.2">
            <c r="A1" t="s">
                <v>1</v>
            </c><c r="B1" s="1" t="s">
                <v>0</v>
            </c>
        </row>
        <row r="2" spans="1:2" ht="15" x14ac:dyDescent="0.2">
            <c r="A2" s="2">
                <v>1</v>
            </c><c r="B2" s="2">
                <v>4</v>
            </c>
        </row>
        <row r="3" spans="1:2" ht="15" x14ac:dyDescent="0.2">
            <c r="A3" s="2">
                <v>2</v>
            </c><c r="B3" s="2">
                <v>5</v>
            </c>
        </row>
    </sheetData>
    <pageMargins left="0.75" right="0.75" top="1" bottom="1" header="0.5" footer="0.5"/>
</worksheet>