When should i use str() and when should i use repr() ?
Almost always use str() when creating output for end users.
repr() is mainly useful for debugging and exploring. For example, if you suspect a string has non printing characters in it, or a float has a small rounding error, repr() will show you; str() may not.
repr() can also be useful for generating literals to paste into your source code. It can also be used for persistence (with ast.literal_eval or eval), but this is rarely a good idea--if you want editable persisted values, something like JSON or YAML is much better, and if you don't plan to edit them, use pickle.
2.In which cases i can use either of them ?
Well, you can use them almost anywhere. You shouldn't generally use them except as described above.
3.What can str() do which repr() can't ?
Give you output fit for end-user consumption--not always (e.g., str(['spam', 'eggs']) isn't likely to be anything you want to put in a GUI), but more often than repr().
4.What can repr() do which str() can't
Give you output that's useful for debugging--again, not always (the default for instances of user-created classes is rarely helpful), but whenever possible.
And sometimes give you output that's a valid Python literal or other expression--but you rarely want to rely on that except for interactive exploration.
