Why are 008 and 009 invalid keys for Python dicts?

Question

Why is it that I can't have 008 or 009 be keys for a Python dict, but 001-007 are fine? Example:

some_dict = {
    001: "spam",
    002: "eggs",
    003: "foo",
    004: "bar",
    008: "anything", # Throws a SyntaxError
    009: "nothing"   # Throws a SyntaxError
    }

Update: Problem solved. I wasn't aware that starting a literal with a zero made it octal. That seems really odd. Why zero?

Answer 1

Python takes 008 and 009 as octal numbers, therefore...invalid.

You can only go up to 007, then the next number would be 010 (8) then 011 (9). Try it in a Python interpreter, and you'll see what I mean.

Answer 2

In Python (and many other languages), starting a number with a leading "0" indicates an octal number (base 8). Using this leading-zero notation is called an octal literal. Octal numbers go 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, etc. So 08 (in octal) is invalid.

If you remove the leading zeros, your code will be fine:

some_dict = 
{ 
    1: "spam",
    2: "eggs",
    3: "foo",
    4: "bar",
    8: "anything",
    9: "nothing" 
}

Answer 3

In python and some other languages, if you start a number with a 0, the number is interpreted as being in octal (base 8), where only 0-7 are valid digits. You'll have to change your code to this:

some_dict = { 
    1: "spam",
    2: "eggs",
    3: "foo",
    4: "bar",
    8: "anything",
    9: "nothing" }

Or if the leading zeros are really important, use strings for the keys.

Answer 4

@DoxaLogos is right. It's not that they're invalid keys - they're invalid literals. If you tried to use them in any other context, you'd get the same error.

Answer 5

That is because, when you start a number with a 0, it is interpreted as an octal number. Since 008 and 009 are not octal numbers, it fails.

A 0 precedes an octal number so that you do not have to write (127)₈. From the Wikipedia page: "Sometimes octal numbers are represented by preceding a value with a 0 (e.g. in Python 2.x or JavaScript 1.x - although it is now deprecated in both)."