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. The key to nice formatting in
Python 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.
Finally, the % operator (modulo) with a string left argument
interprets this string as a C sprintf 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: 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 = `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, ('foo', 'bar')`
"(31.4, 40000, ('foo', 'bar'))"
>>>
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'
>>>