CODEWITHSUNDEEP
pythonpointersreference
honkingForMoreSleeptime
honkingForMoreSleeptime

Reputation: 113

Python 2.x - References and pointers

I have a question. How can I get "reference-pointer effect" in Python 2.x?

I have a class, containing 2 dictionaries - 1 with character representation and 1 with integer representation (retrieved with ord(character)). Main problem is I will print them a lot of times, so converting them in-the-fly is a bad idea (I think). However, switching between them would be useful. 

class Character_Dict(object):
    def __init__(self, key_length):
        self.char_dict = {}
        self.ASCII_dict = {}
        self.key_length = key_length
        self.dictionary = <<here should be a dictionary in usage>>

I could just assign wanted one to self.dictionary, but when assigning for example self.char_dict any change in self.dictionary would not apply to self.char_dict which in my case is a point of whole this construction.

Is there any mechanism in Python 2.x which would allow to do things like that?

[EDIT 1]: Dictionaries contains lists of symbols encrypted with usage of n-th byte of some key: 

n [list of character encrypted with n-th byte of key]

0 ['\xc5', '\x9a', '\xa5', '\x8d', '\xc8', '\xc8', '\x92', '\x9b', '\x82', '\x92', '\x86']
1 ['a', 'm', '.', 'a', '%', ',', ' ', '*', '$', ' ', '(']
2 ['\x18', '~', '4', '4', '?', ',', ',', '0', '9', ',', '\xe7']
3 ['\xe8', '\xe2', '\xe8', '\xec', ':', '\xe6', '\xe6', '0', '\xe6', '\xf3', '\xa9']
...
255 ['\x12', 'S', '\xcc', '_', '\xc0', 'S', '\x01', 'S', 'S', 'S']

[EDIT 2]: My encryption key has 256 bytes. Message which was encrypted by usage of that key is 2688 bytes long. That means that encryption key was repeated 10.5 times.

Think about changing the 3rd letter which was encrypted with usage of 10th letter of key. That's (3-1)*256+10 byte. Instead of reading that letter, I can simply read whole stream and use my class

fileXOR = open('2011061.xor', 'r')
key_length = 256

number_of_bytes = os.path.getsize('2011061.xor')
print number_of_bytes
amount_of_key_repetition = number_of_bytes/key_length.__float__()
print "Key has been repeated", amount_of_key_repetition, "times"

character_dict = Character_Dict(key_length)
for counter_x in range(0, key_length):
    character_dict.dictionary[counter_x] = []
print character_dict

for current_byte in range(0, number_of_bytes):
    read_character = fileXOR.read(1)
    character_dict.dictionary[current_byte % key_length].append(read_character)

fileXOR.close()

and I can access my character simply by:

character_dict.dictionary[10][2]

Now, imagine that I need change character_dict.dictionary[10][2]. In constructor I had assigned self.char_dict to self.dictionary. Changing object_name.dictionary will not modify object_name.char_dict (AFAIK).

I want object_name.dictionary to be sometimes a ASCII representation, and sometimes an integer representation. That would reduce a lot of code and simplify any changes made into ciphertext.

Upvotes: 0

Views: 131

Answers (2)

honkingForMoreSleeptime
honkingForMoreSleeptime

Reputation: 113

So, I totally was in misconception of assigning. I was assured that every time you are assigning to objX there is created a copy of this objX. The source of this assumption was simply:

a = 8
b = a
b = 20
print a
>>> 8

I feel ashamed now. At least I understand how memory in Python is managed :)

Upvotes: 0

johntellsall
johntellsall

Reputation: 15170

In Python, the weakref module lets you store references to objects. If the source object gets deleted or garbage collected, this can be detected by calling the weakref.

Example from documentation:

>>> import weakref
>>> class Object:
...     pass
...
>>> o = Object()
>>> r = weakref.ref(o)
>>> o2 = r()
>>> o is o2
True

If the referent no longer exists, calling the reference object returns None:

>>> del o, o2
>>> print r()
None

Upvotes: 1

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