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Python Cookbook by David Ascher, Alex Martelli

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Decorating an Object with Print-Like Methods

Credit: Jürgen Hermann


You want functionality similar to that of the print statement on a file object that is not necessarily standard output, and you want to access this functionality in an object-oriented manner.


Statement print is quite handy, but we can emulate (and optionally tweak) its semantics with nicer, object-oriented syntax by writing a suitable class:

class PrintDecorator:
    """ Add print-like methods to any writable file-like object. """

    def _ _init_ _(self, stream, do_softspace=1):
        """ Store away the stream for later use. """
        self.stream = stream
        self.do_softspace = do_softspace
        self.softspace = 0

    def Print(self, *args, **kw):
        """ Print all arguments as strings, separated by spaces.

            Take an optional "delim" keyword parameter to change the
            delimiting character and an optional "linend" keyword
            parameter to insert a line-termination string. Ignores
            unknown keyword parameters for simplicity.
        delim = kw.get('delim', ' ')
        linend = kw.get('linend', '')
        if self.do_softspace and self.softspace and args: start = delim
        else: start = ''
        self.stream.write(start + delim.join(map(str, args)) + linend)
        self.softspace = not linend

    def PrintLn(self, *args, **kw):
        """ Just like self.Print(  ), but linend defaults to line-feed.
        self.Print(*args, **kw)

if _ _name_ _ == '_ _main_ _':
    # Here's how you use this:
    import sys
    out = PrintDecorator(sys.stdout)
    out.PrintLn(1, "+", 1, "is", 1+1)
    out.Print("Words", "Smashed", "Together", delim='')
    out.PrintLn(  )


This recipe shows how to decorate objects with new functions, specifically by decorating an arbitrary writable stream (file-like object opened for writing) with two methods that work like the built-in print statement.

The Print method takes any number of positional arguments, converts them to strings (via the map and str built-ins), joins these strings with the given delim, then finally writes the resulting string to the stream. An optional linend, the empty string by default, allows line termination.

The PrintLn method delegates to Print, changing the default for the linend argument to '\n'. Other ways of sharing common code between Print and PrintLn run into difficulties—for example, when delim is nonwhitespace or on multitasking environments where printing operations need to be atomic (a single call to the stream’s method write per call to the decorator’s Print or PrintLn methods).

Softspace functionality is also provided to emulate the print statement’s ability to avoid inserting a useless trailing space if a newline should immediately follow. This seems simple, and it’s definitely useful, but it can be tricky to implement. Furthermore, this wrapper supports softspace functionality independently of the decorated stream’s support for setting and getting the softspace attribute. Softspace behavior can, however, appear somewhat strange if successive Print calls use different delim strings. The softspace functionality can be turned off at instantiation time.

The code uses Python 2.x syntax (string methods, new-style argument passing), but it can be easily ported to Python 1.5.2 (if necessary) by using apply for function calling and the string module instead of string methods.

See Also

The documentation for the string built-in module and built-in file objects in the Library Reference.

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