Import sys foo = # 2 references, 1 from the foo var and 1 from getrefcount print ( sys. You can always check the number of current references using sys.getrefcount function. That is another reason to keep functions small and simple.
#Garbage collection code#
In Python, the most popular block of code is a function this is where most of the garbage collection happens. To remove something from memory, you need to either assign a new value to a variable or exit from a block of code. It's important to understand that until your program stays in a block, Python interpreter assumes that all variables inside it are in use. In other words, it only destroys the names. When Python interpreter exits from a block, it destroys local variables and their references that were created inside the block. Variables, which are defined inside blocks (e.g., in a function or class) have a local scope (i.e., they are local to its block). You can get it by calling the globals() function. To keep them alive, all globals are stored inside a dictionary. Thus, the reference count of objects, which are referred by global variables, never drops to zero. Usually, such variables live until the end of the Python's process. Variables, which are declared outside of functions, classes, and blocks, are called globals. If another variable references an item in a list, the item won't be deallocated. For example, when a list is deleted, the reference count for all its items is decreased. Thus other objects may be deallocated in turn. If an object contains references to other objects, then their reference count is automatically decremented too. If the reference counting field reaches zero, CPython automatically calls the object-specific memory deallocation function. appending an object to a list (object's reference count will be increased).Examples, where the reference count increases: See Objects, Types and Reference Counts section, for a detailed explanation. To keep track of references, every object (even integer) has an extra field called reference count that is increased or decreased when a pointer to the object is created or deleted. This code creates two references to a single object:Īn assignment statement itself (everything on the left) never copies or creates new data. A single object can have many references (variable names). For example, the assignment statement just adds a new reference to the right-hand side. Reference counting is a simple technique in which objects are deallocated when there is no reference to them in a program.Įvery variable in Python is a reference (a pointer) to an object and not the actual value itself. The reference counting module is fundamental to Python and can't be disabled, whereas the cyclic GC is optional and can be triggered manually. That is why Python has a supplemental algorithm called generational cyclic GC. The reference counting algorithm is incredibly efficient and straightforward, but it cannot detect reference cycles. Standard CPython's garbage collector has two components, the reference counting collector and the generational garbage collector, known as gc module. Garbage collections algorithms track which objects can be deallocated and pick an optimal time to deallocate them. Removing objects prematurely will result in a program crash. Python needs to know when your object is no longer needed. Unlike allocation, automatic deallocation is tricky. Python does it when you need to create a new object. Even simple programs that import third-party libraries can allocate millions of objects during the program lifetime. Since most objects are small, custom memory allocator saves a lot of time on memory allocations. If you are interested in Python's memory model, you can read my article on memory management. If a long-running Python process takes more memory over time, it does not necessarily mean that you have memory leaks. In some cases, all allocated memory could be released only when a Python process terminates. The amount of memory that Python holds depends on the usage patterns. Instead, it has a dedicated object allocator for objects smaller than 512 bytes, which keeps some chunks of already allocated memory for further use in the future. Unlike many other languages, Python does not necessarily release the memory back to the Operating System. However, understanding how GC works can help you write better and faster Python programs. When objects are no longer needed, Python automatically reclaims memory from them. Usually, you do not need to worry about memory management. This article describes garbage collection (GC) in Python 3.7.
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