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5.10 Decorating an Object with Print-Like Methods
Credit: Jürgen Hermann
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. """ kw.setdefault('linend','\n') 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梖or 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.
5.10.4 See Also
The documentation for the string built-in module and built-in file objects in the Library Reference.
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