Yup, another Python Quiz


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Using the following source code as a test happily in my Python compiler Nuitka for some years now.

# Testing dict optimization with all constants for compatibility.
print(
    "Dictionary entirely from constant args", dict(
        q='Guido',
        w='van',
        e='Rossum',
        r='invented',
        t='Python',
        y=''
     )
)

Quiz Question

Lately, when adding Python 3.4 support, this and other code changed. So lets do this manually:

PYTHONHASHSEED=0 python3.3
Python 3.3.5 (default, Mar 22 2014, 13:24:53)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> dict(
...                q='Guido',
...                w='van',
...                e='Rossum',
...                r='invented',
...                t='Python',
...                y=''
...             )
{'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van', 'y': ''}
>>> {'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van', 'y': ''}
{'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van', 'y': ''}
>>> {'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van', 'y': ''}
{'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van', 'y': ''}
>>> {'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van', 'y': ''}
{'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van', 'y': ''}

See, the dictionary is stable, once it gets reordered, due to hash values, but then it stays fixed. Which is pretty OK, and using a fixed hash value, it's determinstic. Random hashing is not good for comparison testing, so I disable it for tests.

Now things get interesting, repeat with 3.4:

PYTHONHASHSEED=0 python3.4
Python 3.4.1rc1 (default, May  5 2014, 14:28:34)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> dict(
...                q='Guido',
...                w='van',
...                e='Rossum',
...                r='invented',
...                t='Python',
...                y=''
...             )
{'y': '', 'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van'}
>>> {'y': '', 'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van'}
{'r': 'invented', 'q': 'Guido', 'y': '', 'e': 'Rossum', 't': 'Python', 'w': 'van'}
>>> {'y': '', 'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van'}
{'r': 'invented', 'q': 'Guido', 'y': '', 'e': 'Rossum', 't': 'Python', 'w': 'van'}
>>> {'y': '', 'q': 'Guido', 'r': 'invented', 'e': 'Rossum', 't': 'Python', 'w': 'van'}
{'r': 'invented', 'q': 'Guido', 'y': '', 'e': 'Rossum', 't': 'Python', 'w': 'van'}

Nuitka builds this as the argument dictionary, before it is
passed to dict. Since it's all compile time constants, we can do that, right, and
we can use the result instead. So see this:

Look at how the result of "dict" is not reproducing itself, when used as a constant. I am only feeding the dict result to the interpreter, and it changes.

So the quizz this time is, why does this happen. What change in CPython3.4 makes this occur. Obviously it has to do with dictionary sizes.

Solution

I had a theory, but I couldn't confirm it looking at all of CPython sources "ceval.c" and "dictobject.c" differences between the two versions.

I am suspecting a difference between presized and non-presized dictionaries, or that change to dictionary grow. When dict is being called, the amount of keys is know though, as well as when building the constant. So this ought to not play any role.

Hm, actually. I don't know the solution yet. :-)

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