The colon is required after the numerical value. In your example you have (a+b):

The colon and the curly brackets, it's all the same thing really, these are just symbols that we are using as part of the format. It's just the syntax. You should always have some symbol to signify the format.

Other languages use semi-colons at the end of the line, some languages use parenthesis when invoking a method, etc.

You are not correct to assume that you are doing a+b just as they are. just as "a" and "b"?

The truth is that everything is following a syntax, there has to be a colon after the value, then the width in curly brackets, then the precision. The curly brackets are just a way to signify the start and end of the width and the beginning of the precision. It's just how python chose to make their syntax.