Ignore changes in the amount of whitespace
Differences between version dated 2004-11-16 10:59:39 and 2005-01-08 03:13:09
(spanning 3 versions)
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Additions are marked like this.
The "Classic" Visitor Pattern
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The "Classic" Visitor pattern, from the `Gang of Four Book`__, tries to solve this problem by putting the algorithm into a "Visitor" class, and using a ``visit()`` method in the target classes, that selects the correct behavior from the Visitor class. For example, if we were writing a "pretty printer" Visitor, we might make a ``PrettyPrinter`` class with methods like ``visit_list()``, ``visit_string()``, and so on, that have the code for printing that kind of object. Then, we would add a simple ``accept()`` method like this::
The "Classic" Visitor pattern, from the `Gang of Four Book`__, tries to solve this problem by putting the algorithm into a "Visitor" class, and using an ``accept()`` method in the target classes, that selects the correct behavior from the Visitor class. For example, if we were writing a "pretty printer" Visitor, we might make a ``PrettyPrinter`` class with methods like ``visit_list()``, ``visit_string()``, and so on, that have the code for printing that kind of object. Then, we would add a simple ``accept()`` method like this::
def accept(self, visitor):
return visitor.visit_list(self)
def print_list(target,stream):
# pretty-print a list (or subclass thereof)
stream.write("[")
for item in list:
for item in target:
pretty_print(item,stream)
stream.write(",")
stream.write("]")
