From the doc (https://docs.python.org/3/library/string.html#format-specification-mini-language):

> The '_' option signals the use of an underscore for a thousands separator for floating point presentation types and for integer presentation type 'd'. For integer presentation types 'b', 'o', 'x', and 'X', underscores will be inserted every 4 digits. For other presentation types, specifying this option is an error.

>>> '{0:_}'.format(123456789)
'123_456_789'
>>> '{0:x}'.format(123456789)
'75bcd15'
>>> '{0:x_}'.format(123456789)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier

What am I doing wrong?  I read the doc as saying that using `type` of `x` would result in the `_` separator to be inserted every 4 characters, so I was expecting the output to be '75b_cd15'.

Thanks!

>>> format(123456789, '_x')
'75b_cd15'

The format specifier (here, 'x') always goes last.

>>> '{0:_x}'.format(123456789)
'75b_cd15'

See https://docs.python.org/3/library/string.html#formatstrings for the details.

Serhiy is correct that it's often easier to use format() instead of ''.format() for testing things like this.

Thanks guys.

I really thought I tried '{0:_x}'.format(123456789), but I guess I hadn't.  Good to know that using format(123456879, '_x) is better.