What does %s mean in a Python format string?

Question

What does %s mean in Python? And what does the following bit of code do?

For instance...

 if len(sys.argv) < 2:
     sys.exit('Usage: %s database-name' % sys.argv[0])

 if not os.path.exists(sys.argv[1]):
     sys.exit('ERROR: Database %s was not found!' % sys.argv[1])

The `%` operator is deprecated in favor of the more powerful `str.format` method, see [PEP-3101](http://docs.python.org/3.1/whatsnew/2.6.html#pep-3101). — Paulo Scardine, Jul 17 '13 at 17:37
Actually that PEP says "In Python 3.0, the % operator is supplemented by a more powerful string formatting method" and that it is backported to Python 2.6. Where I come from "supplemented" means added to, not replaced. The PEP does not say "supplanted" and in no part of the PEP does it say the % operator is deprecated (yet it does say other things are deprecated down the bottom). You might prefer str.format and that's fine, but until there's a PEP saying it is deprecated there's no sense in claiming it is when it isn't. — Ben, Oct 29 '13 at 08:07

score 248 · Accepted Answer · edited Mar 01 '22 at 16:10

It is a string formatting syntax (which it borrows from C).

Please see "PyFormat":

Python supports formatting values into strings. Although this can include very complicated expressions, the most basic usage is to insert values into a string with the %s placeholder.

Here is a really simple example:

#Python 2
name = raw_input("who are you? ")
print "hello %s" % (name,)

#Python 3+
name = input("who are you? ")
print("hello %s" % (name,))

The %s token allows me to insert (and potentially format) a string. Notice that the %s token is replaced by whatever I pass to the string after the % symbol. Notice also that I am using a tuple here as well (when you only have one string using a tuple is optional) to illustrate that multiple strings can be inserted and formatted in one statement.

Note that this kind of string interpolation is deprecated in favor of the more powerful `str.format` method. — Paulo Scardine, Jul 17 '13 at 17:35
In python3 raw_input() is now just input() for those of you who are trying for yourself. — Peter Girnus, Jun 03 '18 at 18:44

score 153 · Answer 2 · edited Mar 01 '22 at 16:28

153

Andrew's answer is good.

And just to help you out a bit more, here's how you use multiple formatting in one string:

"Hello %s, my name is %s" % ('john', 'mike') # Hello john, my name is mike".

If you are using ints instead of string, use %d instead of %s.

"My name is %s and I'm %d" % ('john', 12) #My name is john and I'm 12

edited Mar 01 '22 at 16:28

Peter Mortensen

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answered Jun 15 '09 at 20:17

sqram

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nice. %d saves you from casting str(int). any idea what the %s and %d stand for? i guess i'll remember them as string and digit. – user391339 Apr 03 '16 at 19:29
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@user391339 stands for decimal :) they're all here https://docs.python.org/2/library/stdtypes.html#string-formatting-operations – sqram Apr 04 '16 at 04:47
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I don't know for earlier versions, but at least for 3.6 it works the same even if you use `%s` on integers, it will just be converted into a string. – lapin Mar 21 '20 at 01:15
1

@lapin you are correct :) . but that may not always be what you want. Say you want to pad a digit, for example. `print('This number will be padded with 4 zeros: %05d ' % 1)` - this will work. `print('This number will be padded with 4 zeros: %05s ' % 1)` - this won't ` – sqram Mar 24 '20 at 16:49
@sqram Hi, Is there a way to change the position of John and mike without changing the position of john and mike in input? – Imtango30 Sep 15 '20 at 18:21

score 36 · Answer 3 · edited Jul 25 '18 at 17:53

36

The format method was introduced in Python 2.6. It is more capable and not much more difficult to use:

>>> "Hello {}, my name is {}".format('john', 'mike')
'Hello john, my name is mike'.

>>> "{1}, {0}".format('world', 'Hello')
'Hello, world'

>>> "{greeting}, {}".format('world', greeting='Hello')
'Hello, world'

>>> '%s' % name
"{'s1': 'hello', 's2': 'sibal'}"
>>> '%s' %name['s1']
'hello'

edited Jul 25 '18 at 17:53

Community

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answered Jul 17 '13 at 17:50

dansalmo

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This answer would be improved if it explained that the syntax in the question was formatting text and _then_ demonstrated the newer method. That way it could stand on its own. Providing an example that was equivalent to the example in the question would also be a plus. – Steve S Jul 17 '13 at 18:03

Om Sao · Answer 4 · 2022-08-10T04:43:36.223

%sand %d are format specifiers or placeholders for formatting strings, decimals, floats, etc.

The most common used format specifiers:

%s: string

%d: decimals

%f: float

Self explanatory code:

name = "Gandalf"
extendedName = "the Grey"
age = 84
IQ = 149.9
print('type(name): ', type(name)) # type(name): <class 'str'>
print('type(age): ', type(age))   # type(age): <class 'int'>
print('type(IQ): ', type(IQ))     # type(IQ): <class 'float'>

print('%s %s\'s age is %d with incredible IQ of %f ' %(name, extendedName, age, IQ)) # Gandalf the Grey's age is 84 with incredible IQ of 149.900000

# The same output can be printed in following ways:


print ('{0} {1}\'s age is {2} with incredible IQ of {3} '.format(name, extendedName, age, IQ))          # With the help of an older method
print ('{} {}\'s age is {} with incredible IQ of {} '.format(name, extendedName, age, IQ))          # With the help of an older method

print("Multiplication of %d and %f is %f" %(age, IQ, age*IQ)) # Multiplication of 84 and 149.900000 is 12591.600000

# Storing formattings in a string

sub1 = "python string!"
sub2 = "an arg"

a = "I am a %s" % sub1
b = "I am a {0}".format(sub1)

c = "with %(kwarg)s!" % {'kwarg':sub2}
d = "with {kwarg}!".format(kwarg=sub2)

print(a)  # "I am a python string!"
print(b)  # "I am a python string!"
print(c)  # "with an arg!"
print(d)  # "with an arg!"

score 14 · Answer 5 · edited Mar 01 '22 at 16:12

14

%s indicates a conversion type of string when using Python's string formatting capabilities. More specifically, %s converts a specified value to a string using the str() function. Compare this with the %r conversion type that uses the repr() function for value conversion.

Take a look at the documentation for string formatting.

edited Mar 01 '22 at 16:12

Peter Mortensen

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answered Jun 15 '09 at 19:09

Brandon E Taylor

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score 7 · Answer 6 · edited Mar 01 '22 at 16:13

To answer your second question: What does this code do?...

This is fairly standard error-checking code for a Python script that accepts command-line arguments.

So the first if statement translates to: if you haven't passed me an argument, I'm going to tell you how you should pass me an argument in the future, e.g. you'll see this on-screen:

Usage: myscript.py database-name

The next if statement checks to see if the 'database-name' you passed to the script actually exists on the filesystem. If not, you'll get a message like this:

ERROR: Database database-name was not found!

From the documentation:

argv[0] is the script name (it is operating system dependent whether this is a full pathname or not). If the command was executed using the -c command line option to the interpreter, argv[0] is set to the string '-c'. If no script name was passed to the Python interpreter, argv[0] is the empty string.

score 3 · Answer 7 · edited Apr 07 '22 at 21:03

3

Here is a good example in Python 3.

>>> a = input("What is your name? ")
What is your name? Peter

>>> b = input("Where are you from? ")
Where are you from? DE

>>> print("So you are %s of %s." % (a, b))
So you are Peter of DE.

edited Apr 07 '22 at 21:03

Peter Mortensen

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answered Jun 03 '18 at 18:43

Peter Girnus

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What does %s mean in a Python format string?

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