[issue25843] lambdas on the same line may incorrectly share code objects

[STINNER Victor]

STINNER Victor added the comment:

Let me try to explain this issue again.

"f1, f2 = lambda: 1, lambda: 1.0" is compiled to two MAKE_FUNCTION instructions, MAKE_FUNCTION takes a code object as parameter (and a name). The Python compiler merges constants which are seen as "equal", with exceptions to not merge values of different types, or float of different sign.

Merging duplicate code objects is a cool micro optimization, I prefer to keep it. My patch keeps this micro optimization, but fix the bug: it ensures that equal constants having different types are not seen as equal. For example, 0 is equal to 0.0, but if when used for code constants, the code objects are seen a different.

Patch version 3:

* as suggested by Armin Rigo & Serhiy Storchaka: use id(obj) in the constant key for unknown constant types -- in practice, this patch is never taken, it's just to be extra safe (I checked manually by running the whole test suite when an assertion, assertion not in the posted patch)
* add a lot of unit tests
* add a documentation to _PyCode_ConstantKey()

@Serhiy: does it look good to you now?

> Would option (1) work if wrap 1 and 1.0 in a tuple? In a list? In a custom collection?

My patch now uses id(obj) in the "constant key" for unknown types. The compiler only emits simple type (int, str, ...), tuple, frozenset and code objects.

You *can* other types if you patch manually constants with custom objects, since code_richcomp() now uses the "constant key" function to compare constants. For example, my fat project has a replace_consts() function to inject builtin functions in constants:
http://fatoptimizer.readthedocs.org/en/latest/fat.html#replace_consts

There is also @asconstants decorator of codetransformer which allow to inject arbitrary types in function constants:
https://pypi.python.org/pypi/codetransformer

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Added file: http://bugs.python.org/file41671/code_richcompare-3.patch

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<http://bugs.python.org/issue25843>
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