2.3. Variable names and keywords

Programmers generally choose names for their variables that are meaningful and document what the variable is used for.

Variable names can be arbitrarily long. They can contain both letters and numbers, but they cannot start with a number. 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. Variable names can start with an underscore character, but we generally avoid doing this unless we are writing library code for others to use.

If you give a variable an illegal name, you get a syntax error when you try to execute the code.

csp-10-2-1: Click the portion of the variable names that cause syntax errors.Remember that variables cannot start with a number. If you're stuck, look at the list of keywords below.

76trombones = "big parade"
more@ = 1000000
class = "Advanced Theoretical Zymurgy"

The variable name 76trombones is illegal because it begins with a number. The name 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 reserves 33 keywords:

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

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.

csp-10-2-2: Which of the following is not a legal variable name?

• _a1
• You can use an underscore as the first character in a variable name
• my_name
• You can use an underscore between words in a variable name.
• amountOfStuff
• You can use both uppercase and lowercase letters in a variable name.
• BMP
• You can use only uppercase letters in a variable name.
• 1A
• You can't use a digit as the first letter in a variable name.

csp-10-2-3: Which of the following is not a legal variable name?

• _my_name
• This is legal, but you don't usually start a variable name with an underscore.
• my name
• You can't have a space in a variable name.
• myname
• This may be hard to read, but it is legal.
• myName
• Since you can't have spaces in names, one way to make variable names easier to read is to use camel case (uppercase the first letter of each new word).
• my_name
• Since you can't have spaces in names, one way to make variable names easier to read is to use an underscore between two words.

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