Visitor¶

The Visitor pattern solves the problem of many complex algorithms that an application uses. Its purpose is to separate the algorithm definitions from the objects on which they can be used.

Visitor it is a rarely used pattern due to its complexity level.

Desing¶

In order to use the Visitor pattern we need to create:

a common interface representing the elements on which a certain algorithm can be executed. In addition to methods specific to possible actions, there should be a method to "accept" the Visitor object
implementations of the above interfaces
interface representing the Visitor, which has the ability to "visit" individual implementations of elements (usually a method per implementation), and its implementation.

visitor

Example¶

The example is based on the HTMLFile object and the file validation algorithm.

The HTMLFile interface has a method to "accept" the Visitor and has three implementations:

HTML4File
HTML5File
XHTMLFile

In addition, the example implements the Visitor interface, whose implementation of HTMLFileValidator has the ability to perform HTML file syntax validation.

class HTMLFile:
    def get_doctype_declaration(self):
        pass

    def get_head(self):
        pass

    def get_body(self):
        pass

    def accept(self, visitor):
        pass

class HTML4File(HTMLFile):
    def __init__(self, head, body, visitor):
        self._head = head
        self._body = body
        self._visitor = visitor

    def get_doctype_declaration(self):
        return ('<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"\n'
                '        "http://www.w3.org/TR/html4/loose.dtd">')

    def get_head(self):
        return self._head

    def get_body(self):
        return self._body

    def accept(self, visitor):
        visitor.validate_html4_file(self)

class HTML5File(HTMLFile):
    def __init__(self, head, body, visitor):
        self._head = head
        self._body = body
        self._visitor = visitor

    def get_doctype_declaration(self):
        return '<!DOCTYPE html>'

    def get_head(self):
        return self._head

    def get_body(self):
        return self._body

    def accept(self, visitor):
        visitor.validate_html5_file(self)

class XHTMLFile(HTMLFile):
    def __init__(self, head, body, visitor):
        self._head = head
        self._body = body
        self._visitor = visitor

    def get_doctype_declaration(self):
        return ('<?xml version="1.0" encoding="UTF-8"?>\n'
                '<!DOCTYPE html\n'
                '        PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"\n'
                '        "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">\n'
                '<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">')

    def get_head(self):
        return self._head

    def get_body(self):
        return self._body

    def accept(self, visitor):
        visitor.validate_xhtml_file(self)

class Visitor:
    def validate_html4_file(self, html4_file):
        pass

    def validate_html5_file(self, html5_file):
        pass

    def validate_xhtml_file(self, xhtml_file):
        pass

The implementation of the Visitor interface only simulates the execution of validation:

class HTMLFileValidator(Visitor):
    def validate_html4_file(self, html4_file):
        print("Validating HTML 4 schema with https://validator.w3.org/#validate_by_uri+with_options")

    def validate_html5_file(self, html5_file):
        print("Validating HTML 5 schema with https://validator.w3.org/#validate_by_uri+with_options")

    def validate_xhtml_file(self, xhtml_file):
        print("Validating XHTML schema with https://validator.w3.org/#validate_by_uri+with_options")

def main():
    visitor = HTMLFileValidator()

    html4_file = HTML4File('<head>\n    <title>Title</title>\n</head>','<body>\n<p>HTML4 FILE</p>\n</body>', visitor)
    html5_file = HTML5File('<head>\n    <meta charset="UTF-8">\n    <title>Title</title>\n</head>'
                           , '<body>\n<p>HTML5 FILE</p>\n</body>',  visitor)
    xhtml_file = XHTMLFile('<head>\n    <title>Title</title>\n</head>', '<body>\n<p>XHTML file</p>\n</body>', visitor)

    html4_file.accept(visitor)
    html5_file.accept(visitor)
    xhtml_file.accept(visitor)


if __name__ == '__main__':
    main()