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(python2.1-tut.info)Fancier Output Formatting

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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.  The `%' operator interprets the
left argument much like a `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 31.400000000000002, and y is 40000...
     >>> # Reverse quotes work on other types besides numbers:
     ... p = [x, y]
     >>> ps = repr(p)
     >>> ps
     '[31.400000000000002, 40000]'
     >>> # Converting a string adds string quotes and backslashes:
     ... hello = 'hello, world\n'
     >>> hellos = `hello`
     >>> print hellos
     'hello, world\n'
     >>> # The argument of reverse quotes may be a tuple:
     ... `x, y, ('spam', 'eggs')`
     "(31.400000000000002, 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:

     >>> import string
     >>> 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': 7678}
     >>> for name, phone in table.items():
     ...     print '%-10s ==> %10d' % (name, phone)
     ...
     Jack       ==>       4098
     Dcab       ==>       7678
     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.