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?

I did and it worked. I even checked with a dummy class to see if the `__str__` method was called but I wanted to check whether that was default behaviour or part of a fall-back chain. — Milliams, Sep 25 '15 at 12:34
Ah, I see; note that it's helpful to mention that kind of thing in the question. — jonrsharpe, Sep 25 '15 at 12:35

score 6 · Accepted Answer · answered Sep 25 '15 at 12:18

6

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()).

answered Sep 25 '15 at 12:18

Martijn Pieters

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Thank you. I had struggled to find that part in the documentation when I looked. – Milliams Sep 25 '15 at 12:31

score 2 · Answer 2 · edited May 23 '17 at 11:58

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?

edited May 23 '17 at 11:58

Community

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answered Sep 25 '15 at 12:18

idjaw

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spectras · Answer 3 · 2015-09-25T12:25:51.500

0

It depends on the exact specification you use. The old style formatting using a syntax close to that of C lanuage's printf. For instance,

%s shows a string (invokes str() on the passed argument).
%r shows a representation (invokes repr() on the passed argument).
%d takes a number.
etc.

You have the whole list on the documentation.

edited Sep 25 '15 at 12:25

answered Sep 25 '15 at 12:19

spectras

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The wording for `%s` was not ideal, I fixed it. But I explained python would call `str` on it. For the number however, python will only take an integer, you can check it easily: `"%d" % "42"` will raise an exception. – spectras Sep 25 '15 at 12:27

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

3 Answers3