Issue with writing a Custom language specification for Python

I'm trying to find a way to perform auto indentation. The issue is described below:

"Most of the programming languages like C, C++, Java use braces { } to define a block of code. Python uses indentation.
A code block (body of a function, loop etc.) starts (in python) with indentation and ends with the first unindented line. The amount of indentation is up to you, but it must be consistent throughout that block."

I can start the block fine.  All blocks start with a ":" eg:

def test():   
     print("hello world")  

The problem is how do I indicate an end of block, so that the auto indentation stops, as the end of block is signalled by an unindented line rather than a symbol such as '}'

Thanks