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Tokens

        Tokens are the smallest meaningful unit of a program. Python supports the various tokens. Let us learn about
        them.


        Keywords
        Words that have special meaning are reserved by Python for special purposes and are not allowed to be used
        as identifiers. We can see the list of the reserved keywords by giving the following command at the python
        command prompt:

             >>> import keyword
             >>>keyword.kwlist
        Following is the list of keywords displayed by running the above commands:

                     False           class           finally         is              return
                     None            continue        for             lambda          try
                     True            def             from            nonlocal        while
                     and             del             global          not             with
                     as              elif            if              or              yield
                     assert          else            import          pass
                     break           except          in              raise

        Identifiers (Names)

        Identifiers are the user-defined names of variables, list, dictionary, tuples, classes etc. They identify an element
        in Python.

        Naming the identifiers follows rules as given below:
           • An identifier can have letters in uppercase or lowercase, numbers or underscore(_).
           • It cannot begin with a number or an underscore.
           • No special characters like $, period(.), space, etc. are allowed.
           • It should not be a Python keyword.
           • Uppercase and lowercase letters are different.
        Example of invalid identifiers are:

        First name    :   Spaces not allowed.
        Last&Name  :     Cannot have special character.

        _class9th     :   Cannot begin with underscore.
        9thclass      :   Cannot begin with number

        else          :   Keyword not allowed.
        Example of valid identifiers are:

           Myclass, class9, Address, address, first_name, city, student_id, data123, _class9th

        Variables

        Variable is a name given to a memory location to hold a specific value. It is just like a container that can hold any
        type of data that can be changed later in the program.



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