Object Mutability in Python

Internal state

changing the data inside the object is called modifying the internal state of the object, the state(data) is changed, but memory address has not changed


an object whose internal state can be changed

  • Lists, Sets, Dictionarys, User-defined Classes


an object whose internal state can not be changed

  • Numbers(int, float, Booleans), String, Tuples, Frozen Sets, User-defined Classes
  • variable re-assignment change the reference not the internal value



Keyword “Arguments” and Spread

“arguments” is another keyword JavaScript engine create during the creation of execution context, arguments contains a list of all the values of all the parameters that passed to a function.

argument is an array-like object, it doesn’t has all the features of JavaScript array

“arguments” will become deprecated in the future, the new thing is called spread



Variables in Python

variables are memory references, not equal to the object but reference(alias) the object at memory space.

Find out memory address referenced: using id()

reference counting

after we created a object in memory, python keep track of the number of the references we have to the object, as soon as the count goes to 0, python memory manager destroy object and reclaim the memory space

Find out reference count:

  • using sys.getrefcount()
  • using ctypes.c_long.from_address()

circular references

in the circumstance of circular reference, the reference count could not goes to 0(memory leak), need garbage collector to identify it

garbage collection

can be control programmatically using the gc module, turned on by default, beware to turn it off, for python < 3.4, if even one of the objects in the circular reference has a destructor, the destruction order may be important, but the GC does not know what order should be, so the circular reference objects will be marked as uncollectable and cause memory leak

dynamically typing

python variable name has no references to any type, when we use type(), python looks up the object which is referenced and return the type of the object

variable equality

  • identity operator(var_a is var_b) compare the memory address
  • equality operator(var_a == var_b) compare the object state

Keyword “this”

JavaScript engine every time an execution context is created, that is every time a function is RUN, it gives a variable called “this” pointed to a different object depends on how the function is invoked(where the function is and how it been called)

Weird part



By Value v.s. By Reference

by value (primitive type)

copying the value into two separate spots in memory

by reference (all objects)

no copy of object is created, instead, two names or more point to same address