{"id":813,"date":"2020-10-26T09:35:24","date_gmt":"2020-10-26T09:35:24","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=813"},"modified":"2025-03-27T13:19:12","modified_gmt":"2025-03-27T13:19:12","slug":"python-references","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-references\/","title":{"rendered":"Python References"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll have a good understanding of Python references and referencing counting.<\/p>\n\n\n\n

Introduction to Python references #<\/a><\/h2>\n\n\n\n

In Python, a variable<\/a> is not a label of a value as you may think. Instead, A variable references an object that holds a value. In other words, variables are references.<\/p>\n\n\n\n

The following example assigns a number with the value of 100<\/code> to a variable:<\/p>\n\n\n

counter = 100<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Behind the scene, Python creates a new integer object (int<\/code>) in the memory and binds the counter<\/code> variable to that memory address:<\/p>\n\n\n
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\"\"<\/figure>\n<\/div>\n\n\n
When you access the counter<\/code> variable, Python looks up the object referenced by the counter<\/code> and returns the value of that object:<\/p>\n\n\n
print(counter) # 100<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
So variables are references that point to the objects in the memory.<\/p>\n\n\n\n
For example, the following returns the memory address of the integer object referenced by the counter<\/code> variable:<\/p>\n\n\n
counter = 100<\/span>\nprint(id(counter)) <\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
140717671523072<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The id()<\/code> function returns the memory address of an object referenced by a variable as a base-10 number.<\/p>\n\n\n\n
counter = 100<\/span>\n\nprint(id(counter)) \nprint(hex(id(counter))) <\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
140717671523072<\/span>\n0x7ffb62d32300<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Reference counting #<\/a><\/h2>\n\n\n\n
An object in the memory address can have one or more references. For example:<\/p>\n\n\n
counter = 100<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The integer object with the value of 100 has one reference which is the counter<\/code> variable. If you assign the counter<\/code> to another variable e.g., max<\/code>:<\/p>\n\n\n
counter = 100<\/span>\nmax = counter<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, both counter<\/code> and max<\/code> variables reference the same integer object. The integer object with the value 100 has two references:<\/p>\n\n\n\n
\"Python<\/figure>\n\n\n\n
If you assign a different value to the max<\/code> variable:<\/p>\n\n\n
max = 999<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
…the integer object with value 100 has one reference, which is the counter<\/code> variable:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
And the number of references of the integer object with a value of 100 will be zero if you assign a different value to the counter<\/code> variable:<\/p>\n\n\n
counter = 1<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
\"\"<\/figure>\n\n\n\n
Once an object doesn’t have any reference, Python Memory Manager will destroy that object and reclaim the memory.<\/p>\n\n\n\n
Counting references #<\/a><\/h2>\n\n\n\n
To get the number of references of an object, you use the from_address()<\/code> method of the ctypes<\/code> module.<\/p>\n\n\n
ctypes.c_long.from_address(address).value<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To use this method, you need to pass the memory address of the object that you want to count the references. Also, the address needs to be an integer number.<\/p>\n\n\n\n
The following defines a function called ref_count()<\/code> that uses the from_address()<\/code> method:<\/p>\n\n\n
import<\/span> ctypes\n\n\ndef<\/span> ref_count<\/span>(address)<\/span>:<\/span>\n    return<\/span> ctypes.c_long.from_address(address).value<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, you can use a shorter ref_count()<\/code> function instead of using the long syntax like above.<\/p>\n\n\n\n
This example defines a list of three integers:<\/p>\n\n\n
numbers = [1<\/span>, 2<\/span>, 3<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To get the memory address of the numbers<\/code> list, you use the id()<\/code> function as follows:<\/p>\n\n\n
numbers_id = id(numbers)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The following shows the number of references of the list referenced by the numbers<\/code> variable:<\/p>\n\n\n
print(ref_count(numbers_id))  # 1<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
It returns one because only the numbers<\/code> variable references the list.<\/p>\n\n\n\n
This assigns the numbers<\/code> variable to a new variable:<\/p>\n\n\n
ranks = numbers<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The number of references of the list should be two now because it is referenced by both numbers<\/code> and ranks<\/code> variables:<\/p>\n\n\n
print(ref_count(numbers_id)) # 2<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you assign ranks<\/code> variable None<\/code>, the reference count of the list will reduce to one:<\/p>\n\n\n
ranks = None<\/span>\nprint(ref_count(numbers_id))  # 1<\/span>\n<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
And if you assign the numbers<\/code> variable None<\/code>, the number of references of the list will be zero:<\/p>\n\n\n
numbers = None<\/span>\nprint(ref_count(numbers_id))  # 0<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Put it all together:<\/p>\n\n\n
import<\/span> ctypes\n\n\ndef<\/span> ref_count<\/span>(address)<\/span>:<\/span>\n    return<\/span> ctypes.c_long.from_address(address).value\n\n\nnumbers = [1<\/span>, 2<\/span>, 3<\/span>]\nnumbers_id = id(numbers)\n\nprint(ref_count(numbers_id))  # 1<\/span>\n\nranks = numbers\nprint(ref_count(numbers_id))  # 2<\/span>\n\nranks = None<\/span>\nprint(ref_count(numbers_id))  # 1<\/span>\n\nnumbers = None<\/span>\nprint(ref_count(numbers_id))  # 0<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Try it<\/a><\/p>\n\n\n\n
Output:<\/p>\n\n\n
1\n2\n1\n0<\/code><\/span><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
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  • Python variables are references to objects located in the memory<\/li>\n\n\n\n
  • Use the id()<\/code> function to get the memory address of the object referenced by a variable.<\/li>\n<\/ul>\n
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Try it<\/a><\/p>\n\n\n\n
To convert this memory address to a hexadecimal string, you use the hex()<\/a><\/code> function:<\/p>\n\n\n
Try it<\/a><\/p>\n\n\n\n
To find the memory address of an object referenced by a variable, you pass the variable to the built-in id()<\/a><\/code> function.<\/p>\n\n\n\n