Extension type

“Extension type” can refer to either a Cython class defined with cdef class or @cclass, or more generally to any Python type that is ultimately implemented as a native C struct (including the built-in types like int or dict).

Dynamic allocation or Heap allocation

A C variable allocated with malloc (in C) or new (in C++) is allocated dynamically/heap allocated. Its lifetime is until the user deletes it explicitly (with free in C or del in C++). This can happen in a different function than the allocation.

Global Interpreter Lock or GIL

A lock inside the Python interpreter to ensure that only one Python thread is run at once. This lock is purely to ensure that race conditions do not corrupt internal Python state. Python objects cannot be manipulated unless the GIL is held. It is most relevant to Cython when writing code that should be run in parallel. If you are not aiming to write parallel code then there is usually no benefit to releasing the GIL in Cython. You should not use the GIL as a general locking mechanism in your code since many operations on Python objects can lead to it being released and to control being passed to another thread. Also see the CPython project’s glossary entry.


A pointer is a variable that stores the address of another variable (i.e. direct address of the memory location). They allow for dynamic memory allocation and deallocation. They can be used to build dynamic data structures. Read more.

Python object

When using Python, the contents of every variable is a Python object (including Cython extension types). Key features of Python objects are that they are passed by reference and that their lifetime is managed automatically so that they are destroyed when no more references exist to them. In Cython, they are distinct from C types, which are passed by value and whose lifetime is managed depending on whether they are allocated on the stack or heap. To explicitly declare a Python object variable in Cython use cdef object abc. Internally in C, they are referred to as PyObject*.

Stack allocation

A C variable declared within a function as cdef SomeType a is said to be allocated on the stack. It exists for the duration of the function only.

Typed memoryview

A useful Cython type for getting quick access to blocks of memory. A memoryview alone does not actually own any memory. However, it can be initialized with a Python object that supports the buffer protocol (typically “array” types, for example a Numpy array). The memoryview keeps a reference to that Python object alive and provides quick access to the memory without needing to go through the Python API of the object and its __getitem__() / __setitem__() methods. For more information, see Typed Memoryviews.