Difference between Python print/format methods

Question

When combining a variable and a string to be printed in Python, there seem to be a number of ways to do the same thing;

test = "Hello"
print "{} World".format(test) #Prints 'Hello World'
print test+" World" #Prints 'Hello World'
print "%s World" % test #Prints 'Hello World'

What (if any) is the difference between these methods in terms of performance, compatibility and general preference. Even between open source projects all three methods seem to be used interchangeably.

Answer 1

As you mentioned, various open source projects will use all of these methods for string formatting. However, I would stick to one method for one project so as not to confuse other developers with differing styles.

print test+" World" is the most efficient, performance-wise, but gives you the least amount flexibility

print "%s World" % test #Prints 'Hello World' is basically like C's sprintf which does string interpolation. I like to use this method a lot, because you can pass in not just a regular string, but a dictionary.

print "Good morning %(name), there are %(count)d new articles in %(topic)s today. Would you like to <a href='%(url)s'>read them</a>?" % values

I haven't used "{} World".format(test) personally.

In real applications, the performance difference between these methods are insignificant, and it's really about adhering to style and not over-coding.

Answer 2

A little benchmark:

>>> a = lambda: "{} World".format("Hello")
>>> b = lambda: "Hello" + " World"
>>> c = lambda: "%s World" % "Hello"
>>> d = lambda: "".join(("Hello", " World"))
>>> a(), b(), c(), d()
('Hello World', 'Hello World', 'Hello World', 'Hello World')
>>> timeit.timeit(a)
0.7830071449279785
>>> timeit.timeit(b)
0.18782711029052734
>>> timeit.timeit(c)
0.18806695938110352
>>> timeit.timeit(d)
0.3966488838195801

Seems like b and c are fastest, after d, an surprisingly a is slowest.

But, if you don't do a lot of processing, it doesn't really matter which one to use, just it is better not to mix them.

I personally prefer "".join for just simple concentenations, and str.format for placing values, like "Hello, my name is {}.".format(name).

There was a rumor that % formatting will be deprecated and removed in Python 3, but it didn't.