Explicit str() conversion needed for old-style string formatting?

Question

I'm maintaining an old Python codebase that has some very strange idioms scattered throughout it. One thing I've come across is string formatting using the percent-encoded style:

'Error %s: %s' % (str(err), str(message))

Ignoring the existence of .format() for modern string interpolation, the question I have is:

Is it necessary to explicitly convert %s parameters with str() or is that exactly what %s does?

Answer 1

No, there is no need for the explicit str() calls, the %s formatter includes a str() call already:

>>> class Foo(object):
...     def __str__(self):
...         return "I am a string for Foo"
... 
>>> '%s' % Foo()
'I am a string for Foo'

This is also explicitly documented in the String Formatting Operations section:

's'
String (converts any Python object using str()).

Answer 2

No need to do that. The %s is a string formatting syntax used for printing. This might help add some more context:

What does %s mean in Python?