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Engineering LibreTexts

15.3: Variable Names and Keywords

  • Page ID
    15357
  • Programmers generally choose names for their variables that are meaningful—they document what the variable is used for.

    Variable names can be arbitrarily long. They can contain both letters and numbers, but they have to begin with a letter. It is legal to use uppercase letters, but it is a good idea to begin variable names with a lowercase letter (you’ll see why later).

    The underscore character, _, can appear in a name. It is often used in names with multiple words, such as my_name or airspeed_of_unladen_swallow.

    If you give a variable an illegal name, you get a syntax error:

    >>> 76trombones = 'big parade'
    SyntaxError: invalid syntax
    >>> more@ = 1000000
    SyntaxError: invalid syntax
    >>> class = 'Advanced Theoretical Zymurgy'
    SyntaxError: invalid syntax
    

    76trombones is illegal because it does not begin with a letter. more@ is illegal because it contains an illegal character, @. But what’s wrong with class?

    It turns out that class is one of Python’s keywords. The interpreter uses keywords to recognize the structure of the program, and they cannot be used as variable names.

    Python 2 has 31 keywords:

    and       del       from      not       while    
    as        elif      global    or        with     
    assert    else      if        pass      yield    
    break     except    import    print              
    class     exec      in        raise              
    continue  finally   is        return             
    def       for       lambda    try
    

    In Python 3, exec is no longer a keyword, but nonlocal is.

    You might want to keep this list handy. If the interpreter complains about one of your variable names and you don’t know why, see if it is on this list.

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