<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_index(self, filename):

 """Return index associated with filename"""

        index = self.fsmodel.index(filename)
        if index.isValid() and index.model() is self.fsmodel:
            return self.proxymodel.mapFromSource(index)