formatter in python

Question

I'm going through exercises in a python book and I'm a bit puzzled by what is going on in this code.

formatter = "%r %r %r %r"

print formatter % (1, 2, 3, 4)
print formatter % ("one", "two", "three", "four")
print formatter % (True, False, False, True)
print formatter % (formatter, formatter, formatter, formatter)
print formatter % (
    "I had this thing.",
    "That you could type up right.",
    "But it didn't sing.",
    "So I said goodnight."
)

The author offer no explanation as to what "formatter" is doing after each "print". If I remove them, everything prints out exactly the same. Am I missing something here?

Answer 1

No, it does not print out the exact same thing. There are no commas, and no parenthesis if you use the formatter % part.

It'll be clearer if you expand the formatter. I suggest you use:

formatter = "One: %r, Two: %r, Three: %r, Four: %r"

instead.

The formatter acts as a template, each %r acting as a place holder for the values in the tuple on the right-hand side.

Answer 2

formatter is a string. so, the first line is the same as:

"%r %r %r %r" % (1, 2, 3, 4)

which calls repr on each of the items in the tuple on the right and replaces the corresponding %r by the result. Of course, it does the exact same thing for

formatter % ("one", "two", "three", "four")

and so on as well.

Note that you'll often also see:

"%s %s %s %s" % (1, 2, 3, 4)

which calls str instead of repr . (In your example, I think that str and repr return the same things for all of those objects, so the output will be exactly the same if you change formatter to use %s instead of %r )

Answer 3

That's the classic format for string formatting, print "%r" % var will print the raw value of var, four %r expects 4 variables to be passed after the %.

A better example would be:

formatter = "first var is %r, second is %r, third is %r and last is %r"
print formatter % (var1, var2, var3, var4)

The use of a formatter variable is just to avoid using a long line in the print but usually there's no need of that.

print "my name is %s" % name
print "the item %i is $%.2f" % (itemid, price)

%.2f is float with 2 values after comma.

There's a newer variant of string formatting you may wish to try: (if you're using at least 2.6)

print "my name is {name} I'm a {profession}".format(name="sherlock holmes", profession="detective")

More at:

http://www.python.org/dev/peps/pep-3101/

http://pythonadventures.wordpress.com/2011/04/04/new-string-formatting-syntax/