Maybe spelunking in the Python 2 branch will help? It seems it was introduced in 2005 by Jeremy Hylton with this comment:

/* The distinction between NEW_BLOCK and NEXT_BLOCK is subtle. (I'd
   like to find better names.) NEW_BLOCK() creates a new block and sets
   it as the current block. NEXT_BLOCK() also creates an implicit jump
   from the current block to the new block.
*/

That comment (and NEW_BLOCK()) are no longer found in the Python 3 source.

On Wed, Mar 21, 2018 at 3:52 AM, Serhiy Storchaka <storchaka@gmail.com> wrote:

There is the NEXT_BLOCK() macro in compile.c. It creates a new block, creates an implicit jump from the current block to the new block, and sets it as the current block.

But why it is used? All seems working if remove NEXT_BLOCK(). If there was a need of NEXT_BLOCK() (if it reduces the computational complexity of compilation or allows some optimizations), it should be documented, and we should analyze the code and add missed NEXT_BLOCK() where they are needed, and perhaps add new tests. Otherwise it can be removed.

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