A Generic Display Tool

We can put these interfaces to work in a superclass that displays accurate class names and formats all attributes of an instance of any class. Open a new file in your text editor to code the following—it’s a new, independent module named classtools.py that implements just such a class. Because its __str__ print overload uses generic introspection tools, it will work on any instance, regardless of its attributes set. And because this is a class, it automatically becomes a general formatting tool: thanks to inheritance, it can be mixed into any class that wishes to use its display format. As an added bonus, if we ever want to change how instances are displayed we need only change this class, as every class that inherits its __str__ will automatically pick up the new format when it’s next run:

# File classtools.py (new)
"Assorted class utilities and tools"

class AttrDisplay:
    """
    Provides an inheritable print overload method that displays
    instances with their class names and a name=value pair for
    each attribute stored on the instance itself (but not attrs
    inherited from its classes). Can be mixed into any class,
    and will work on any instance.
    """
    def gatherAttrs(self):
        attrs = []
        for key in sorted(self.__dict__):
            attrs.append('%s=%s' % (key, getattr(self, key)))
        return ', '.join(attrs)
    def __str__(self):
        return '[%s: %s]' % (self.__class__.__name__, self.gatherAttrs())


if __name__ == '__main__':
    class TopTest(AttrDisplay):
        count = 0
        def __init__(self):
            self.attr1 = TopTest.count
            self.attr2 = TopTest.count+1
            TopTest.count += 2

    class SubTest(TopTest):
        pass

    X, Y = TopTest(), SubTest()
    print(X)                         # Show all instance attrs
    print(Y)                         # Show lowest class name

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Notice the docstrings here—as a general-purpose tool, we want to add some functional documentation for potential users to read. As we saw in Chapter 15, docstrings can be placed at the top of simple functions and modules, and also at the start of classes and their methods; the help function and the PyDoc tool extracts and displays these automatically (we’ll look at docstrings again in Chapter 28).

When run directly, this module’s self-test makes two instances and prints them; the __str__ defined here shows the instance’s class, and all its attributes names and values, in sorted attribute name order:

C:\misc> classtools.py
[TopTest: attr1=0, attr2=1]
[SubTest: attr1=2, attr2=3]