Let's look at rst's example:

   File.open("...") do |f|
     ... stuff that might throw an exception ...
   end

The syntax you see here is for passing a block to a method. In this case, you're passing a block to File.open, which opens the file, executes your block with it and then makes sure to close the file no matter what.

In Python, you would do:

   with open("x.txt") as f:
     ... stuff that might throw an exception ...

What this does is evaluate open("x.txt"), call the __enter__ method on the resulting value (called the context guard), assign the result of the __enter__ method to f, executes the body of the with statement and makes sure to call __exit__ method of the guard.

The difference is that the syntax used in the Ruby example uses is not syntactic sugar for Dispose pattern, it's part of Ruby's syntax for working with blocks in general, whereas the syntax used in Python example is syntactic sugar meant for Dispose pattern (but can be used for other stuff too).