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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 columns(self):
 """ Returns the list of selected column names. """

        fields = [f.label for f in self.form_fields
                  if self.cleaned_data["field_%s_export" % f.id]]
        if self.cleaned_data["field_0_export"]:
            fields.append(self.entry_time_name)
        return fields