Why does Python raise a TypeError: not all arguments converted during string formatting when using % with {0} placeholders?

General Discussions
, ,
1

I’m trying to print a message comparing two input strings using string formatting.

However, when I mix % formatting with {} placeholders, I get the error TypeError: not all arguments converted during string formatting.

What causes this mismatch, and how should I correctly format the string in this situation?

2

Ah yes, I’ve hit this exact error before! The problem is you’re mixing two different string formatting styles. The {0} and {1} syntax is for the .format() method, not for % formatting. So when you do something like:

print("'{0}' is longer than '{1}'" % (name1, name2))

Python gets confused - it expects %s, not {}. You should either use one or the other.

For %, do:

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

Or better yet, switch fully to .format():

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