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

Question

I hit this TypeError exception recently, which I found very difficult to debug. I eventually reduced it to this small test case:

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

This is very non-obvious, to me anyway. The workaround for my code was to decode the byte string into unicode:

 >>> "{:20}".format(b"hi".decode("ascii"))
 'hi                  '

What is the meaning of this exception? Is there a way it can be made more clear?

Probably worth noting: this only became a TypeError in Python 3.4+, and it affects anything that inherits from object without defining __format__ along the way (e.g. None; class T(object): pass, etc.). — Henry Keiter
– Henry Keiter, Commented Jul 8, 2014 at 13:54
I had this problem too, finally found out that a variable that should have contained a float, actually contained None. Thus '{0:.4f}'.format(bla) caused this exception — SomethingSomething
– SomethingSomething, Commented May 27, 2018 at 7:39

Martijn Pieters · Accepted Answer · 2014-06-11 19:06:13Z

79

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(b"Hi")
"b'Hi'               "
>>> '{!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.

edited Jun 11, 2014 at 19:06

answered Jun 11, 2014 at 18:59

Martijn Pieters

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1 Comment

Chris AtLee Over a year ago

makes sense, thanks! I wish the error were more intuitive though!

Jeremy Field · Accepted Answer · 2016-03-12 11:50:38Z

27

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)!

answered Mar 12, 2016 at 11:50

Jeremy Field

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