print format? which one is better python

Question

I have always been curious

Why does python have different way to print stuff?

for example

print "this is ",a," and " ,b

vs

print "this is %d and %d" %(a,b)

Any performance issues? and stuff?

Answer 1

There are many more ways to "print stuff" in Python, too, including the more modern str.format() approach, sys.stdout.write('foo'), and probably others.

In terms of the difference between print a, b, c and % formatting, I generally use the former mostly for debugging output, when you just want spaces between a bunch of variables. When I want exact formatting or more control over the formatting, I use % (actually, these days I always use str.format).

For example:

print 'DEBUG:', var1, var2

Versus:

print 'Benchmarks took {0:.3f}s to run'.format(seconds)

Also, % and str.format formatting are much more general -- they don't actually print anything, they return a string, which you can print, write to a file, store in a database, send as a web response, etc.

Regarding performance -- just don't worry about it. All three are almost certainly fast, and premature optimization is the root of all evil.

I don't really want to post numbers (as that may encourage the wrong mindset), but timeit is so simple to use I couldn't help myself.

C:\>python -m timeit -s "import cStringIO; s = cStringIO.StringIO()"
                     "print >>s, 'this is', 'a', 'test'"
100000 loops, best of 3: 3.39 usec per loop

c:\>python -m timeit -s "import cStringIO; s = cStringIO.StringIO()"
                     "print >>s, 'this is %s test' % 'a'"
1000000 loops, best of 3: 1.32 usec per loop

C:\>python -m timeit -s "import cStringIO; s = cStringIO.StringIO()"
                     "print >>s, 'this is {0} test'.format('a')"
1000000 loops, best of 3: 1.64 usec per loop

I'm not quite sure why the print a, b, c approach is significantly slower, probably an implementation detail. But still, don't worry about it -- the time it takes to print to your file or screen probably far outweighs the string formatting part.

Answer 2

The second method allows better formatting of your printed arguments. For example, you can specify to format a number as hexadecimal, or specify a number of decimals to be displayed of a floating point number.