Why do I get “TypeError: not all arguments converted during string formatting” trying to substitute a placeholder like {0} using %?

Old-style % formatting uses % codes for formatting:

# A single value can be written as is:
'It will cost $%d dollars.' % 95

# Multiple values must be provided as a tuple:
"'%s' is longer than '%s'" % (name1, name2)

New-style {} formatting uses {} codes and the .format method. Make sure not to mix and match – if the “template” string contains {} placeholders, then call .format, don’t use %.

# The values to format are now arguments for a method call,
# so the syntax is the same either way:
'It will cost ${0} dollars.'.format(95)

"'{0}' is longer than '{1}'".format(name1, name2)