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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 updateActiveMarkupClass(self):
		'''
		Update the active markup class based on the default class and
		the current filename. If the active markup class changes, the
		highlighter is rerun on the input text, the markup object of
		this tab is replaced with one of the new class and the
		activeMarkupChanged signal is emitted.
		'''
		previousMarkupClass = self.activeMarkupClass

		self.activeMarkupClass = find_markup_class_by_name(globalSettings.defaultMarkup)

		if self._fileName:
			markupClass = get_markup_for_file_name(
				self._fileName, return_class=True)
			if markupClass:
				self.activeMarkupClass = markupClass

		if self.activeMarkupClass != previousMarkupClass:
			self.highlighter.docType = self.activeMarkupClass.name if self.activeMarkupClass else None
			self.highlighter.rehighlight()

			self.activeMarkupChanged.emit()
			self.triggerPreviewUpdate()