>>> f"{number:#0x}"  # using integer format specifier

It’s not clear what your purpose was adding the above line, but the zero flag (0) does nothing because there is no “width” field. I think it could be misleading, because it is actually the “#x” codes that generate the “0x” prefix.

If you want to illustrate a minimum width, I suggest something like

>>> f"{number:#06x}"
'0x0400'

or (if the number is never negative)

>>> f"0x{number:04X}"
'0x0400'

Or if you don’t care about the width:

>>> f"{number:#x}"
'0x400'