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])
This question is related to
python
string-formatting
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
%splaceholder.
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.
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
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'
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
%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.
Source: Stackoverflow.com