Why does saving a presentation to a file-like object create an empty presentation?

Question

As an answer to this answer to my previous question, I wrote the following short program to try to reproduce the problem.

from pptx import Presentation
from io import BytesIO

p = Presentation()
slide = p.slides.add_slide(p.slide_layouts[0])
slide.shapes[0].text = 'asdf'

p.save('test.pptx')

out = BytesIO()
p.save(out)

out_file = open('bytes_test.pptx', 'wb', buffering=0)
out_file.write(out.read())
out_file.close()

This created two pptx files.

The first, test.pptx, contained one slide with the layout "Title Slide" and contained the string "asdf". File size was 28 KB.

Secondly, bytes_test.pptx, when it was opened in PowerPoint, showed only a large gray square that said "Click to add the first slide." File size: 0.

Works on Windows 10 with Anaconda Python 3.6.1 and python-pptx 0.6.6

Why is this happening?

Answer 1

, , , .

out.getvalue() out.read(). , , - .

, out.flush() out.seek(0) out.read(). BytesIO - , read(). , , read() , seek(0) reset .

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