This is just the old style of string formatting. I think you can just use a ".L", where L is the length to truncate the string to whatever length you like. eg:
'format': '%(name).5s'
would truncate the length to 5 characters.
It's a little hard to find, but they actually do mention it in the docs:
The precision is a decimal number indicating how many digits should be displayed after the decimal point for a floating point value formatted with 'f' and 'F', or before and after the decimal point for a floating point value formatted with 'g' or 'G'. For non-number types the field indicates the maximum field size - in other words, how many characters will be used from the field content