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GroupDocs.Watermark can render document pages as images so you can show page thumbnails without fully opening a document in a viewer. You drive the render with a create_page_stream callback that supplies a writable file stream for each page — the library writes the page image to it and closes it for you.

Set PreviewOptions with a create_page_stream callback, choose the preview_format, and list the page_numbers to render.

from groupdocs.watermark import Watermarker
from groupdocs.watermark.options import PreviewOptions, PreviewFormats

def generate_preview():
    # Supply a writable file stream for each page to render
    def create_page_stream(page_number):
        return open(f"./output-page-{page_number}.png", "wb")

    with Watermarker("./sample.pdf") as watermarker:
        options = PreviewOptions(create_page_stream)
        options.preview_format = PreviewFormats.PNG
        options.page_numbers = [1, 2]
        watermarker.generate_preview(options)

if __name__ == "__main__":
    generate_preview()

output-page-1.png (600 KB)
output-page-2.png (94 KB)

The page images are written to the current directory — one PNG per requested page. Use PreviewFormats.JPEG or PreviewFormats.BMP for other formats, and set options.width / options.height to control the rendered size.

The library closes each file create_page_stream returns once the page is written. To act on a page as soon as it is ready, pass a second callback: PreviewOptions(create_page_stream, release_page_stream) calls release_page_stream(page_number, stream) with the object create_page_stream returned for that page, after the whole image is in it. A one-argument release_page_stream(page_number) works too. create_page_stream may also return an io.BytesIO: it stays open, and holds the page image by the time release_page_stream gets it.

Use case: Generate page previews to display in a web app, dashboard, or document-management system without requiring users to download the full file.