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]) 

Answer 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.

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.

Answer 2

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