How to send zip files in the python Flask framework?

BytesIO() needs to be passed bytes data, but a ZipFile() object is not bytes-data; you actually created a file on your harddisk.

You can create a ZipFile() in memory by using BytesIO() as the base:

memory_file = BytesIO()
with zipfile.ZipFile(memory_file, 'w') as zf:
    files = result['files']
    for individualFile in files:
        data = zipfile.ZipInfo(individualFile['fileName'])
        data.date_time = time.localtime(time.time())[:6]
        data.compress_type = zipfile.ZIP_DEFLATED
        zf.writestr(data, individualFile['fileData'])
memory_file.seek(0)
return send_file(memory_file, attachment_filename="capsule.zip", as_attachment=True)

The with statement ensures that the ZipFile() object is properly closed when you are done adding entries, causing it to write the required trailer to the in-memory file object. The memory_file.seek(0) call is needed to ‘rewind’ the read-write position of the file object back to the start.

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