Python C API¶
CPython comes with a C API called the “Python C API”. The most common type is
PyObject* and functions are prefixed with
_Py for private
functions but you must not use them!).
Historical design choices¶
CPython was created in 1991 by Guido van Rossum. Some design choices made sense in 1991 but don’t make sense anymore in 2015. For example, the GIL was a simple and safe choice to implement multithreading in CPython. But in 2015, smartphones have 2 or 4 cores, and desktop PC have between 4 and 8 cores. The GIL restricts peek performances on multithreaded applications, even when it’s possible to release the GIL.
CPython uses a Global Interpreter Lock called “GIL” to avoid concurrent accesses to CPython internal structures (shared resources like global variables) to ensure that Python internals remain consistent.
See also Kill the GIL.
Reference counting and garbage collector¶
The C structure of all Python objects inherit from the
which contains the field
Py_ssize_t ob_refcnt;. This is a simple counter
1 when the object is created, increased each time that a
variable has a strong reference to the object, and decreased each time that a
strong reference is removed. The object is removed when the counter reached
In some cases, two objects are linked together. For example, A has a strong
reference to B which has a strong reference to A. Even if A and B are no more
referenced outside, these objects are not destroyed because their reference
counter is still equal to
1. A garbage collector is responsible to
find and break reference cycles.
See also the PEP 442: Safe object finalization implemented in Python 3.4 which helps to break reference cycles.
Popular projects using the Python C API¶