Python semantics and Limitations

fatoptimizer bets that the Python code is not modified when modules are loaded, but only later, when functions and classes are executed. If this assumption is wrong, fatoptimizer changes the semantics of Python.

Python semantics

It is very hard, to not say impossible, to implementation and keep the exact behaviour of regular CPython. CPython implementation is used as the Python “standard”. Since CPython is the most popular implementation, a Python implementation must do its best to mimic CPython behaviour. We will call it the Python semantics.

fatoptimizer should not change the Python semantics with the default configuration. Optimizations modifting the Python semantics must be disabled by default: opt-in options.

As written above, it’s really hard to mimic exactly CPython behaviour. For example, in CPython, it’s technically possible to modify local variables of a function from anywhere, a function can modify its caller, or a thread B can modify a thread A (just for fun). See Everything in Python is mutable for more information. It’s also hard to support all introspections features like locals() (vars(), dir()), globals() and sys._getframe().

Builtin functions replaced in the middle of a function

fatoptimizer uses guards to disable specialized function when assumptions made to optimize the function are no more true. The problem is that guard are only called at the entry of a function. For example, if a specialized function ensures that the builtin function chr() was not modified, but chr() is modified during the call of the function, the specialized function will continue to call the old chr() function.

The copy builtin functions to constants optimization changes the Python semantics. If a builtin function is replaced while the specialized function is optimized, the specialized function will continue to use the old builtin function. For this reason, the optimization is disabled by default.


def func(arg):
    x = chr(arg)

    with unittest.mock.patch('builtins.chr', result='mock'):
        y = chr(arg)

    return (x == y)

If the copy builtin functions to constants optimization is used on this function, the specialized function returns True, whereas the original function returns False.

It is possible to work around this limitation by adding the following configuration at the top of the file:

__fatoptimizer__ = {'copy_builtin_to_constant': False}

But the following use cases works as expected in FAT mode:

import unittest.mock

def func():
    return chr(65)

def test():
    with unittest.mock.patch('builtins.chr', return_value="mock"):



The test() function doesn’t use the builtin chr() function. The func() function checks its guard on the builtin chr() function only when it’s called, so it doesn’t use the specialized function when chr() is mocked.

Guards on builtin functions

When a function is specialized, the specialization is ignored if a builtin function was replaced after the end of the Python initialization. Typically, the end of the Python initialization occurs just after the execution of the site module. It means that if a builtin is replaced during Python initialization, a function will be specialized even if the builtin is not the expected builtin function.


import builtins

builtins.chr = lambda: mock

def func():
    return len("abc")

In this example, the func() is optimized, but the function is not specialize. The internal call to func.specialize() is ignored because the chr() function was replaced after the end of the Python initialization.

Guards on type dictionary and global namespace

For other guards on dictionaries (type dictionary, global namespace), the guard uses the current value of the mapping. It doesn’t check if the dictionary value was “modified”.

Tracing and profiling

Tracing and profiling works in FAT mode, but the exact control flow and traces are different in regular and FAT mode. For example, loop unrolling removes the call to range(n).

See sys.settrace() and sys.setprofiling() functions.

Expected limitations

Function inlining optimization makes debugging more complex:

  • sys.getframe()
  • locals()
  • pdb
  • etc.
  • don’t work as expected anymore

Bugs, shit happens:

  • Missing guard: specialized function is called even if the “environment” was modified

FAT python! Memory vs CPU, fight!

  • Memory footprint: loading two versions of a function is memory uses more memory
  • Disk usage: .pyc will be more larger

Possible worse performance:

  • guards adds an overhead higher than the optimization of the specialized code
  • specialized code may be slower than the original bytecode