Streams API (High Performance I/O)

For extremely large datasets where allocating millions of Cell objects (or even Python lists) simultaneously would consume too much memory, pyopenxlsx provides XLStreamWriter and XLStreamReader.

These classes bypass the standard document model and write/read directly to/from the underlying XML streams on disk, offering the lowest possible memory footprint.

Stream Writer

stream_writer allows you to append rows sequentially. Important: Once a stream writer is active, you should not use standard cell assignment methods on that worksheet until writer.close() is called.

from pyopenxlsx import Workbook

with Workbook("large_output.xlsx") as wb:
    ws = wb.active
    
    # Open a stream writer for this worksheet
    writer = ws.stream_writer()
    
    # 1. Append rows of plain values
    writer.append_row(["ID", "Name", "Score"])
    
    # 2. Append rows with styles (using tuples: (value, style_index))
    bold_style = wb.add_style(font=wb.styles.fonts().create(bold=True))
    writer.append_row([
        (1, bold_style), 
        ("Alice", bold_style), 
        99.9  # Plain value, inherits default style
    ])

    for i in range(1000000):
        # Appends a row immediately to the XML stream
        writer.append_row([i, f"User_{i}", 99.9])
        
    # Close the stream to finalize the XML structure
    writer.close()

Stream Reader

stream_reader allows you to iterate through rows sequentially without loading the entire worksheet into memory.

from pyopenxlsx import Workbook

with Workbook("large_input.xlsx") as wb:
    ws = wb.active
    
    # Open a stream reader for this worksheet
    reader = ws.stream_reader()
    
    # Iterate through rows sequentially
    while reader.has_next():
        current_row_idx = reader.current_row()
        row_data = reader.next_row() # Returns a list of values
        
        # Process row_data...
        # print(f"Row {current_row_idx}: {row_data}")

Use Cases

Exporting database query results directly to Excel.
Parsing multi-gigabyte .xlsx files where loading the DOM would trigger Out-Of-Memory errors.