How to Avoid \'TypeError: Not All Arguments Converted\' When Using Placeholder Formatting in Python

TypeError When Substituting Placeholder with % Formatting

When attempting to substitute a placeholder like {0} using % formatting, developers may encounter the following error: "TypeError: not all arguments converted during string formatting." This error stems from improper formatting, specifically due to a mix-up between old-style % formatting and new-style {} formatting.

Old-style % formatting employs placeholders like %d for formatting, as exemplified below:

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

However, when utilizing multiple values, they must be provided as a tuple:

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

On the other hand, new-style {} formatting employs placeholders like {} and the .format method. It's crucial to avoid mixing these two styles. If the template string contains {} placeholders, .format should be used, not %.

# Correct:
'It will cost ${0} dollars.'.format(95)
"'{0}' is longer than '{1}'".format(name1, name2)

# Incorrect (Do not mix % and {}):
'It will cost ${0} dollars.' % 95
"'%0' is longer than '%1'" % (name1, name2)

By adhering to these formatting guidelines, developers can resolve the "TypeError: not all arguments converted during string formatting" error and format their strings correctly.

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