The Curious Case of Matching Memory Locations in Python

So I attended a Python meetup yesterday which was quite informative and one of the talks which I found fascinating was about Cleaning the trash in Python by Rivas. He starts his talk by explaining what is trash in programming sense, and goes on to compare memory allocation for objects and arrays in C and Python.

So he mentions that when we create a value 100 and reuse it, Python will point it to the older reference of 100 as it already exists. He explains about finding memory locations of objects using the id method and how they point to the same memory location. What exactly does the id method do?

id: Return identity of an object

It returns the memory location of an object. This can be used to check if two objects point to the same location. So we will go right ahead and see a few quirks of Python using this.

In [1]: a = 100

In [2]: b = 100

In [3]: id(a)
Out[3]: 139944507028096

In [4]: id(b)
Out[4]: 139944507028096

All is well and good in Python land, being memory efficient and all when we initialize something with 100. Let's proceed:

In [5]: c = 257

In [6]: d = 257

In [7]: id(c)
Out[7]: 139944437174032

In [8]: id(d)
Out[8]: 139944437170992

So what happened exactly? Let's try another example:

In [9]: e = 256

In [10]: f = 256

In [11]: id(e)
Out[11]: 139944507033088

In [12]: id(f)
Out[12]: 139944507033088

Uh...oh! Any guesses what happened here?

Explanation: What Python does here is that it stores all integers from -5 to 256 in memory because they are most frequently used and so it doesn't create new memory locations for those objects, therefore when you access a number in that range, Python just fetches it from memory and returns the value.

Let me blow your mind even more with the following code:

In [13]: id(257)
Out[13]: 139944437173872

In [14]: print(id(257), id(257), id(257) == id(257))
139944437171184 139944437171184 True

Woah, what kind of sorcery is this? You must be like, "Hey, you just told me that numbers after 256 get a different address every time, so what happened here?"

Explanation: The reason is that when Python encounters constants in a single statement, it adds them to a dictionary and looks up in the dictionary for every constant. If it finds it in the dictionary, then it reuses the existing memory location. This is not the case in In[5] and In[6] because they are two separate statements.

Let's take it up a notch and try exploring more:

In [15]: def func():
    ...:     a = 257
    ...:     b = 257
    ...:     return id(a) == id(b)
    ...:

In [16]: func()
Out[16]: True

We found that for numbers above 256, Python stores them in new memory locations unless they are used in the same line/statement. However in the above example, they are separate statements and still return True when we compare the memory locations. So why the odd behaviour?

Explanation: The reason behind this is that within the same compiler scope like in a function or a class, everything is stored in a constants dictionary (consts in actual code) and hence any time a constant is encountered, it is first checked in the constants dictionary and if it's present then it doesn't store it in a new memory location and if not then goes ahead and stores it. Python's idea of optimization during compile time. I had this doubt for a long time which was later cleared by Prasanth Raghu, whom I met in one of the future meetups who also had been working with the CPython code for quite some time. Have a look at the following links to get a better understanding of the flow.

• The program starts in the compiler_function: link
• Reaches VISIT_SEQ_IN_SCOPE function call: link
• Inside the if condition of VISIT_SEQ_IN_SCOPE calls compiler_visit_stmt: link
• Evaluates to Assign_kind: link
• Reaches VISIT function call: link
• Function definition of VISIT: link
• Goes to compiler_visit_expr: link
• The statement is a Num_kind so it calls ADDOP_O: link. consts is used here which comes from the following links:
  • Definition of *u_consts: link
  • Store it in tmp: link
  • The consts dictionary: link
• This internally calls compiler_addop_o: link
• Calls the compiler_add_o function: link
• Fetch from dictionary and return if exists: link

Hope the flow is clear, and do let me know if something was confusing.

References

• Video of the talk by Rivas on YouTube
• Python Docs relating to integer objects in memory.
• Understanding ## in C