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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def generateRevision(self):
 """ Generates the revision file for this builder. """

        revpath = self.sourcePath()
        if not os.path.exists(revpath):
            return

        # determine the revision location
        revfile = os.path.join(revpath, self.revisionFilename())
        mode = ''
        # test for svn revision
        try:
            args = ['svn', 'info', revpath]
            proc = subprocess.Popen(args, stdout=subprocess.PIPE)
            mode = 'svn'
        except WindowsError:
            try:
                args = ['git', 'rev-parse', 'HEAD', revpath]
                proc = subprocess.Popen(args, stdout=subprocess.PIPE)
                mode = 'git'
            except WindowsError:
                return

        # process SVN revision
        rev = None

        if mode == 'svn':
            for line in proc.stdout:
                data = re.match('^Revision: (\d+)', line)
                if data:
                    rev = int(data.group(1))
                    break

        if rev is not None:
            try:
                f = open(revfile, 'w')
                f.write('__revision__ = {0}\n'.format(rev))
                f.close()
            except IOError:
                pass