# File: sgmllib-example-3.py

import sgmllib

class WellFormednessChecker(sgmllib.SGMLParser):
    # check that an SGML document is 'well formed'
    # (in the XML sense).

    def __init__(self, file=None):
        sgmllib.SGMLParser.__init__(self)
        self.tags = []
        if file:
            self.load(file)

    def load(self, file):
        while 1:
            s = file.read(8192)
            if not s:
                break
            self.feed(s)
        self.close()

    def close(self):
        sgmllib.SGMLParser.close(self)
        if self.tags:
            raise SyntaxError, "start tag %s not closed" % self.tags[-1]

    def unknown_starttag(self, start, attrs):
        self.tags.append(start)

    def unknown_endtag(self, end):
        start = self.tags.pop()
        if end != start:
            raise SyntaxError, "end tag %s does't match start tag %s" %\
                  (end, start)

try:
    c = WellFormednessChecker()
    c.load(open("samples/sample.htm"))
except SyntaxError:
    raise # report error
else:
    print "document is wellformed"

## Traceback (innermost last):
## ...
## SyntaxError: end tag head does't match start tag meta