Newbie: Simple question about string slices

[Ray Smith]

Hi there,

I'm completely new to Python and was reading the standard tutorial for
Python and came across string slices., The tutorial says:

"Strings can be subscripted (indexed); like in C, the first character of a
string has subscript (index) 0. There is no separate character type; a
character is simply a string of size one. Like in Icon, substrings can be
specified with the slice notation: two indices separated by a colon."

Then later:

"Here's a useful invariant of slice operations: s[:i] + s[i:]equals s."

Now,
if I have a string defined as

s = "abcdefg"

I assumed s[2:2] to be equal to 'c',
but when I tried it it equals '' (empty).
The way I read this is give me the slice from index 2 to index 2.

I also "feel like" s[:2] + s[2:] should be equal to 'abccdef'

Simple testing shows me
s[2:3] = 'c', and
s[:2] + s[2:] = "abcdef"

My question is:
If given a slice s[a:b]
can I assume index "b" means upto but not including the index "b"?

eg.  s="abcdef"

s[:3] = "abc", meaning from index 0, upto but not including index 3
s[3:] = "def", meaning from index 3 to the end of the string

I guess another way to ask my question is:
Why is s[a:a] always '' (empty) ?

Hope someone out there in Python land can make sense of my question!

Thanks

Ray Smith