7.1 Fancier Output Formatting

So far we've encountered two ways of writing values: expression statements and the print statement. (A third way is using the write() method of file objects; the standard output file can be referenced as sys.stdout. See the Library Reference for more information on this.)

Often you'll want more control over the formatting of your output than simply printing space-separated values. There are two ways to format your output; the first way is to do all the string handling yourself; using string slicing and concatenation operations you can create any lay-out you can imagine. The standard module string  contains some useful operations for padding strings to a given column width; these will be discussed shortly. The second way is to use the % operator with a string as the left argument. % interprets the left argument as a C sprintf()-style format string to be applied to the right argument, and returns the string resulting from this formatting operation.

One question remains, of course: how do you convert values to strings? Luckily, Python has a way to convert any value to a string: pass it to the repr() function, or just write the value between reverse quotes (``). Some examples:

>>> x = 10 * 3.14
>>> y = 200*200
>>> s = 'The value of x is ' + `x` + ', and y is ' + `y` + '...'
>>> print s
The value of x is 31.4, and y is 40000...
>>> # Reverse quotes work on other types besides numbers:
... p = [x, y]
>>> ps = repr(p)
>>> ps
'[31.4, 40000]'
>>> # Converting a string adds string quotes and backslashes:
... hello = 'hello, world\n'
>>> hellos = `hello`
>>> print hellos
'hello, world\012'
>>> # The argument of reverse quotes may be a tuple:
... `x, y, ('spam', 'eggs')`
"(31.4, 40000, ('spam', 'eggs'))"

Here are two ways to write a table of squares and cubes:

>>> import string
>>> for x in range(1, 11):
...     print string.rjust(`x`, 2), string.rjust(`x*x`, 3),
...     # Note trailing comma on previous line
...     print string.rjust(`x*x*x`, 4)
...
 1   1    1
 2   4    8
 3   9   27
 4  16   64
 5  25  125
 6  36  216
 7  49  343
 8  64  512
 9  81  729
10 100 1000
>>> for x in range(1,11):
...     print '%2d %3d %4d' % (x, x*x, x*x*x)
... 
 1   1    1
 2   4    8
 3   9   27
 4  16   64
 5  25  125
 6  36  216
 7  49  343
 8  64  512
 9  81  729
10 100 1000

(Note that one space between each column was added by the way print works: it always adds spaces between its arguments.)

This example demonstrates the function string.rjust(), which right-justifies a string in a field of a given width by padding it with spaces on the left. There are similar functions string.ljust() and string.center(). These functions do not write anything, they just return a new string. If the input string is too long, they don't truncate it, but return it unchanged; this will mess up your column lay-out but that's usually better than the alternative, which would be lying about a value. (If you really want truncation you can always add a slice operation, as in "string.ljust(x, n)[0:n]".)

There is another function, string.zfill(), which pads a numeric string on the left with zeros. It understands about plus and minus signs:

>>> string.zfill('12', 5)
'00012'
>>> string.zfill('-3.14', 7)
'-003.14'
>>> string.zfill('3.14159265359', 5)
'3.14159265359'

Using the % operator looks like this:

>>> import math
>>> print 'The value of PI is approximately %5.3f.' % math.pi
The value of PI is approximately 3.142.

If there is more than one format in the string you pass a tuple as right operand, e.g.

>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> for name, phone in table.items():
...     print '%-10s ==> %10d' % (name, phone)
... 
Jack       ==>       4098
Dcab       ==>    8637678
Sjoerd     ==>       4127

Most formats work exactly as in C and require that you pass the proper type; however, if you don't you get an exception, not a core dump. The %s format is more relaxed: if the corresponding argument is not a string object, it is converted to string using the str() built-in function. Using * to pass the width or precision in as a separate (integer) argument is supported. The C formats %n and %p are not supported.

If you have a really long format string that you don't want to split up, it would be nice if you could reference the variables to be formatted by name instead of by position. This can be done by using an extension of C formats using the form %(name)format, e.g.

>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> print 'Jack: %(Jack)d; Sjoerd: %(Sjoerd)d; Dcab: %(Dcab)d' % table
Jack: 4098; Sjoerd: 4127; Dcab: 8637678

This is particularly useful in combination with the new built-in vars() function, which returns a dictionary containing all local variables.