Write MongoDB aggregation pipeline statements that transform documents, in database named 'market' and collection named 'stocks', into aggregated results using multiple pipeline stages as appropriate. Specifically create a function or method in Python or Java that will take as input a string. The function or method will find documents for which the input string matches the document key "Sector" and returns the total outstanding shares grouped by document key "Industry." Examples of sector string inputs are "Healthcare," "Basic Materials," and so on. You will also need to create a simple main application to call your function. Provide screenshots of the results of their execution.