Understanding the TypeError: String Formatting Mismatch

When trying to incorporate placeholders like {0} in string formatting, it's essential to understand the nuances of the formatting method being used. The error message "TypeError: not all arguments converted during string formatting" typically arises when the %-based formatting method is misused.

Old-style % Formatting

The % formatting method employs % codes to specify formatting operations. When formatting a single value, it can be included directly in the string:

'It will cost $%d dollars.' % 95

However, for multiple values, a tuple must be provided:

"'%s' is longer than '%s'" % (name1, name2)

New-style {} Formatting

In contrast, the {} formatting method utilizes {} placeholders and the .format() method. It's crucial to avoid mixing these methods. If the "template" string contains {} placeholders, use .format(), not %.

# Values as method arguments
'It will cost ${0} dollars.'.format(95)
"'{0}' is longer than '{1}'".format(name1, name2)