Python TypeError: non-empty format string passed to object.__format__

Python TypeError: non-empty format string passed to object.format

bytes objects do not have a __format__ method of their own, so the default from object is used:

>>> bytes.__format__ is object.__format__
True
>>> {:20}.format(object())
Traceback (most recent call last):
  File <stdin>, line 1, in <module>
TypeError: non-empty format string passed to object.__format__

It just means that you cannot use anything other than straight up, unformatted unaligned formatting on these. Explicitly convert to a string object (as you did by decoding bytes to str) to get format spec support.

You can make the conversion explicit by using the !s string conversion:

>>> {!s:20s}.format(bHi)
bHi               
>>> {!s:20s}.format(object())
<object object at 0x1100b9080>

object.__format__ explicitly rejects format strings to avoid implicit string conversions, specifically because formatting instructions are type specific.

This also happens when trying to format None:

>>> {:.0f}.format(None)
Traceback (most recent call last):
  File <stdin>, line 1, in <module>
TypeError: non-empty format string passed to object.__format__

That took a moment to work out (in my case, when None was being returned by an instance variable)!