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

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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))

3

You’re not alone - I see this a lot with students new to Python. Basically, % formatting is the old-style method and doesn’t understand {} placeholders.

The error happens because Python tries to insert values where it finds %, but it can’t parse the {0} - it’s expecting %s, %d, etc.

To keep things clean, I’d recommend using .format() or f-strings if you’re on Python 3.6+:

print(f"'{name1}' is longer than '{name2}'")

Much more readable and modern!