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Python 3.8 AST updates

| by Thomas Binetruy

On this Friday night, I decided to give my mini Python to pseudo-Michelson compiler 1 a little polish. I remembered leaving it working flawlessly, so that it’d be easy to get back grinding at it anytime I so desired to. Using the Python AST module as a compiler frontend, I was sure it’d be pretty stable, unlike Marshall code, purposely left undocumented 2.

So I gave my code a go, and somehow, my integration test wasn’t working anymore! What could possibly have gone wrong. I noticed I was using the Python 3.8 interpreter and that I had only tested my code on Python 3.7. Could this update contain breaking changes with respect to my code? I figured if this had been the case, then surely it’d be in the AST module. I compared the bytecode output I was getting with the one my test expected and surprisingly, it was not the same! It wasn’t pushing my constants onto the stack when assigning them to variables. A little more digging, with the help of Python’s new breakpoint(), until I noticed that my condition on ast.Num, to push a number on the stack, had been replaced by ast.Constant… Hence the weird bug I was noticing. A little Googling to find that indeed:

Deprecated since version 3.8: Class ast.Constant is now used for all constants. Old classes ast.Num, ast.Str, ast.Bytes, ast.NameConstant and ast.Ellipsis are still available, but they will be removed in future Python releases. 3

Good thing it’s now fixed :)

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