Modern Python Standard Library Cookbook

How it works...

tempfile.SpooledTemporaryFile has an internal _file property that keeps the real data stored in a BytesIO store until it can fit in memory, and then moves it to a real file once it gets bigger than max_size.

You can easily see this behavior by printing the value of _file while you are writing data:

>>> with tempfile.SpooledTemporaryFile(max_size=30) as temp:
...     for i in range(3):
...         temp.write(b'Line of text\n')
...         print(temp._file)
... 
<_io.BytesIO object at 0x10d539ca8>
<_io.BytesIO object at 0x10d539ca8>
<_io.BufferedRandom name=4>