Show HN: Inside Python Dict – an explorable explanation

https://just-taking-a-ride.com/inside_python_dict/

by eatitraw • 8 years ago

111 27 8 years ago

deepakg• 8 years ago

stinos• 8 years ago

Python lists are actually arrays — contiguous chunks of memory.

Just wondering here, is this guaranteed to always be the case? Practically it probably is, but does the Python spec (as in: the laguage, not one of it's implementations) say a list must be implemented using contiguous memory of slots with Python objects? That seems so low-level and C-ish. Or does the OP actually mean CPython here for instance?

lozenge• 8 years ago

Python doesn't really have a spec in the way that, say, C# does. The docs are missing things like whether the argument is positional-only, exact exceptions raised in all cases, and time complexity of common operations.

That said, I think every implementation uses contiguous memory. PyPy has list "strategies" int[], double[] and PyObject[] and probably more, switching transparently between them.

mlthoughts2018• 8 years ago

With numba or other jit tools in CPython, you have things equivalent to pypy list strategies as well. If you add jitted helper functions or Cython, you have even more control.

chrisseaton• 8 years ago

It seems like a reasonable shorthand to say that Python does something, but to really mean that CPython happens to be implemented to do it that way.

eatitraw5• 8 years ago

It is indexable, and it is used as an array, so I guess it is an array in all implementations.

But good point, I should check this.

Dicts are definktely implemented differently in different implementations, and I mention this.

johnisgood• 8 years ago

According to this post: https://news.ycombinator.com/item?id=18707403, "Lists aren't called arrays in Python because they're not arrays. Next!".

I really am confused now. Is it an array, or is it not?

joatmon-snoo• 8 years ago

Strictly speaking, lists are not arrays.

I'm not aware of what the specification for a list would be, this is the closest I'm aware of: https://docs.python.org/3/reference/datamodel.html#objects-v..., 3.2 > Sequences > Mutable Sequences > Lists, "The items of a list are arbitrary Python objects. Lists are formed by placing a comma-separated list of expressions in square brackets"

CPython implementation is here: https://github.com/python/cpython/blob/e42b705188271da108de4...

If I'm talking about arrays with someone else, I would expect us to be thinking of arrays in the sense of C. Both C++ and Java use the same definition. Other languages that I'm aware of (e.g. Javascript, Go) have their own notions of arrays that you can't conflate with the C notion of an array.

From the C11 spec, §6.2.5.20: "An array type describes a contiguously allocated nonempty set of objects with a particular member object type, called the element type."

eatitraw5• 8 years ago

I guess it depends on your definition of arrays, and what you expect from them. For example, Ruby calls a similar thing array: https://docs.ruby-lang.org/en/2.0.0/Array.html

To me it is kind of self-growing array that can hold objects of different types. The C++-ish equivalent would be something like std::vector<void*> / std::vector<MagicObjectBoxType> (there is no MagicObjectBoxType of course).

_ZeD_• 8 years ago

No, they are not. At "best" they are implemented "like" an array of pointers to the objects (scattered somewhere else in the memory) in the list. If you want an array (in the "c-like" sense of it) you can look at the array module https://docs.python.org/3/library/array.html

c3534l• 8 years ago

To me, the difference between a list and an array is that an array is a fixed length allocation of memory storing multiple items of the same size. This way, to jump to the third item in the list, you just need to know the data type (or rather, how many registers a single item of that type takes up in memory) and the index. That way you can jump right to the correct memory address by just doing ```index * cell size```.

You compare this to, say, another fundamental data structure, the stack. In a stack you don't know how much memory each item takes up, so you can't just jump right to that item. Thus, the limitation is that you can only pop or push the last item on that stack. Once upon a time, stack-based languages were quite common.

A list, in contrast, is an ordered, polymorphic collection of objects which can have variable size. You can create resizable arrays by using a linked list. A resizable list would typically be implemented as an array with the last item being a pointer to the next array. I believe (but don't quote me on this), that Python uses links to increasingly large arrays so that there aren't too many links. That is, it dynamically increases how much memory to allocate to that array depending on how much data you're actually storing in that list.

A list is also polymorphic, meaning it can house multiple types of data. I sometimes hear a non-polymorphic, but resizable array called a "vector," but that's super confusing terminology. You can implement this by creating a resizable array of pointers, where each pointer points to a location in memory where the value, along with the data type, are stored. This way, all the pointers are the same size and you can jump right to that location by calculating it from the index, but you still get to have multiple value types.

The distinction is not universal, though. But in my humble opinion everyone who disagrees with me is wrong.

noobiemcfoob• 8 years ago

An array is a list, but a list is not always an array

black-tea• 8 years ago

It's not implemented like that and couldn't be. They aren't arrays unless you consider them arrays of pointers but this isn't a useful definition in Python.

An array contains a number of elements of the same size. That means you can find the memory address of any element by simple arithmetic: array_loc + index * element_size.

Python lists can contain objects of any type and the objects can't be found using arithmetic like that.

mcguire2• 8 years ago

Does Python actually have a spec? AFAIK, it's always been "what CPython does."

IIRC, an old joke was that Python lists are arrays and Perl arrays are lists.

whoisnnamdi• 8 years ago

Thanks. Slowing realizing some of the benefits of dicts and trying to use them more in my code where I would have otherwise defaulted to another data structure.

They really are quite useful and intuitive to work with once you understand basic structure

I do sometimes worry about speed relative to other options though, especially in large chunks of data

Show HN: Inside Python Dict – an explorable explanation

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