CODEWITHSUNDEEP
pythonpython-3.xhash
stanley dodds
stanley dodds

Reputation: 161

Limit for hashing nested tuples?

A few lines of code that demonstrates what I'm asking are:

>>> x = ()
>>> for i in range(1000000):
...     x = (x,)


>>> x.__hash__()

=============================== RESTART: Shell ===============================

The 1000000 may be excessive, but it demonstrates that there is some form of limit when hashing nested tuples (and I assume other objects). Just to clarify, I didn't restart the shell, it did that automatically when I attempted the hash.

What I want to know is what is this limit, why does it happen (and why did it not raise an error), and is there a way around it (so that I can put tuples like this into sets or dictionaries).

Upvotes: 5

Views: 357

Answers (1)

MSeifert
MSeifert

Reputation: 152677

The __hash__ method of tuple calculates the hash of each item in the tuple - in your case like a recursive function. So if you have a deeply nested tuple then it ends up with a very deep recursion. At some point there is probably not enough memory on the stack to go "one level deeper". That's also why the "shell restarts" without Python exception - because the recusion is done in C (at least for CPython). You could use, i.e. gdb to get more information about the exception or debug it.

There will be no global hard limit, the limit depends on your system (e.g. how much stack) and how many function calls (internally) are involved and how much of the "stack" each function call requires.

However that could qualify as Bug in the implementation, so it would be a good idea to post that on the Python issue tracker: CPython issue tracker.

Upvotes: 4

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