Creating "C Array" in Python

Question

I am reading a book which is using ctypes to create C Array in python.

import ctypes

class Array:
    def __init__(self, size):        
        array_data_type = ctypes.py_object * size
        self.size = size
        self.memory = array_data_type()
        
        for i in range(size):
            self.memory[i] = None

I understand that self.memory = array_data_type() is creating a memory chunk which is basically a consecutive memory having ctypes.py_object * size as the total size.

How self.memory[i] is related to self.memory?

My understanding is that self.memory has no indexing and it is an object representing one single memory chunk.

self.memory can also be assigned directly using self.memory = (ctypes.py_object * size)() — cup
– cup, Commented Mar 25, 2022 at 7:01

user459872 · Accepted Answer · 2022-03-25 06:51:31Z

1

self.memory here is an array of NULL PyObject* pointers.

>>> import ctypes
>>> array_type = ctypes.py_object * 3
>>> array = array_type()
>>> array[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: PyObject is NULL

Which means self.array can contains three elements of type Pyobject(Any Python object). So this is valid.

>>> import ctypes
>>> array_type = ctypes.py_object * 3
>>> array = array_type()
>>> for i in range(3):
...     array[i] = f"{i}: test string"
...
>>> array._objects
{'0': '0: test string', '1': '1: test string', '2': '2: test string'}

answered Mar 25, 2022 at 6:51

user459872

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5 Comments

meallhour Over a year ago

can you please provide more details on what you mean by array of NULL PyObject* pointers. `?

user459872 Over a year ago

@meallhour ctypes already provides an Array class(result of ctypes.py_object * 3) which implements getitem. That's why you are able to index the array elements.

user459872 Over a year ago

Also when you instantiate a ctypes.py_object it will always be a NULL PyObject* pointer unless you pass a value(ie, ctypes.py_object(10))

meallhour Over a year ago

thanks for all your inputs. What is complexity for array_type = ctypes.py_object * 3? Is it O(1)

meallhour Over a year ago

can you please provide your inputs to the question? stackoverflow.com/questions/71624787

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