print "%r" vs
print "%r %r"%("hello") I want to compare above two lines of code in python.The latter statement gives an error with not enough arguements to print but in first case we have no arguements but still it works.Can anybody explain this to someone who is new to programming. Any help would be greatly appreciated.
The % inside the string only comes into play once Python has established you're using the % printf-like operator outside the string.
In the first case, you're not, so it doesn't complain. All you're doing in that case is printing a string.
The second case, you are using the % operator so it complains that you haven't provided enough arguments. In that case, you're formatting a string before printing it.
print itself knows nothing of the string formatting functions, it simply prints what you give it. In the string formatting case, something like "Hello, %s"%("Pax") is processed by the % operator to give you "Hello, Pax" and then that is given to print for output:
print "Hello, %s"%("Pax") # ^ \_________________/ <- This bit done first # | # Then the print is done So basically, it's the % operator that decides you're doing printf-style processing, not the fact you have a string containing the % character. That's confirmed by the fact that:
print "%r %r %r %r %r" also has no issue with argument count mismatch, printing out the literal %r %r %r %r %r.
% character, you need to write %%.print "%r" just prints the string %r.
print "%r %r" % ("hello"). % after the string %r %r is called a formatting operator and, because of it, %r here have a special meaning - it's a placeholder for the something you want to put on it's place.
You have two %r in the string - you should have two strings to put there:
print "%r %r" % ("hello", "world") Also see docs on string formatting.
You're not reading the error message:
>>> print "%r %r"%("hello") Traceback (most recent call last): File "<pyshell>", line 1, in <module> print "%r %r"%("hello") TypeError: not enough arguments for format string It's the format string which doesn't have enough arguments, not the print. This shows the same problem:
>>> "%r %r"%("hello") Traceback (most recent call last): File "<pyshell>", line 1, in <module> "%r %r"%("hello") TypeError: not enough arguments for format string Whereas this works:
>>> "%r %r"%("hello", "world") "hello world" It's worth noting that the only argument you are giving to print in all cases is the string. The Python interpreter evaluates any string formatting before print is called.
As @paxdiablo says, when you don't give any arguments to the %r format specifier, Python assumes you want the string as is, and does not complain.
The second line demands two arguments - there are two "%r", so you need two arguments in the parenthesis, like this:
print "%r %r" % ("hello", "world") Refer to the documentation.
print "%r %r"also works no problem