CODEWITHSUNDEEP
pythonclasspython-2.7object
Muhamed Huseinbašić
Muhamed Huseinbašić

Reputation: 580

Object's unique identifier (translate from hexadecimal into decimal)

I'm learning Python (classes at this momment). On this site (bottom of the page) first exercise (under 13.7. Exercises) says:

Create and print a Point object, and then use id to print the object's unique identifier. Translate the hexadecimal form into decimal and confirm that they match.

I need help with this because I'm not quite sure I understand what I have to do.

If I do:

class Point:
    pass

print Point()
print id(Point)

I get this output:

<__main__.Point instance at 0xb71c496c>
3072094252

So, should I do this first part of exercise like this or? And what now? I assume that this second line is decimal number (Am I right?). But what with the first line? How to translate it into decimal?

Upvotes: 3

Views: 2069

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1123420

Translate the second number into hexadecimal with the hex() function then test if it is present in the first:

p = Point()
hexadecimal_id = hex(id(p))
present = hexadecimal_id in repr(p)

Note that I first store a reference to the Point() instance; otherwise you'd get a new one with potentially a new id() value.

Also, don't confuse the class with the instance; the class is an object in its own right, and as such has a id() value too.

To go the other way, you'd have to parse out the hexadecimal string; if you are going to assume it is the part after the last space that's doable as:

hexadecimal_id = repr(p).rpartition(' ')[-1][:-1]
present = int(hexadecimal_id, 16) == id(p)

Here, the str.rpartition() method splits on the last space, and we take whatever comes after it with [-1] (last element), then shorted that result by one character to remove the > character at the end.

Once you have the hexadecimal number you can interpret it as an integer with the int() function, specifying the base as 16.

Demo:

>>> class Point:
...     pass
... 
>>> p = Point()
>>> id(p)
4300021632
>>> hex(id(p))
'0x1004d1f80'
>>> p
<__main__.Point instance at 0x1004d1f80>
>>> hex(id(p)) in repr(p)
True
>>> # the other direction
... 
>>> repr(p).rpartition(' ')[-1][:-1]
'0x1004d1f80'
>>> hexadecimal_id = repr(p).rpartition(' ')[-1][:-1]
>>> int(hexadecimal_id, 16)
4300021632
>>> int(hexadecimal_id, 16) == id(p)
True

Upvotes: 6

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