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FAT Python: a new static optimizer for Python 3.6


Victor Stinner - FAT Python: a new static optimizer for Python 3.6 [EuroPython 2016] [20 July 2016] [Bilbao, Euskadi, Spain] (

The Python language is hard to optimize. Let's see how guards checked at runtime allows to implement new optimizations without breaking the Python semantic.

(Almost) everything in Python is mutable which makes Python a language very difficult to optimize. Most optimizations rely on assumptions, for example that builtin functions are not replaced. Optimizing Python requires a trigger to disable optimization when an assumption is no more true. FAT Python exactly does that with guards checked at runtime. For example, an optimization relying on the builtin len() function is disabled when the function is replaced.

Guards allows to implement various optimizations. Examples: loop unrolling (duplicate the loop body), constant folding (propagates constants), copy builtins to constants, remove unused local variables, etc.

FAT Python implements guards and an optimizer rewriting the Abstract Syntax Tree (AST). The optimizer is implemented in Python so it's easy to enhance it and implement new optimizations.

FAT Python uses a static optimizer, it is less powerful than a JIT compiler like PyPy with tracing, but it was written to be integrated into CPython.

I wrote 3 PEP (509, 510, 511) targeting Python 3.6. Some changes to support FAT Python have already been merged into Python 3.6.

We will also see other pending patches to optimize CPython core, and the bytecode project which allows to modify bytecode, it also includes a peephole optimizer written in pure Python.


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