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up vote 22 down vote favorite
2

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?

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5  
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 Jul 8 '14 at 13:54
1  
For background information see bugs.python.org/issue7994 – gerrit Mar 19 '15 at 23:25
up vote 22 down vote accepted

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.

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4  
makes sense, thanks! I wish the error were more intuitive though! – Chris AtLee Jun 12 '14 at 15:38
up vote 2 down vote

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)!

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