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])

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.

Edit: Here is a really simple example:

#Python2
name = raw_input("who are you? ")
print "hello %s" % (name,)

#Python3+
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.

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

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'

%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 docs for string formatting.

%sand %d are Format Specifiers or placeholders for formatting strings/decimals/floats etc.

MOST common used Format specifier:

%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 

#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 help of older method
print ('{} {}\'s age is {} with incredible IQ of {} '.format(name, extendedName, age, IQ))          # with help of 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 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!"

In answer to 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.

Here is a good example in Python3.

  >>> 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