What is the default Python implementation?

The default Python implementation is CPython, the reference implementation written primarily in C and Python. When people download Python from python.org, they normally receive CPython unless they deliberately choose another implementation.

CPython translates Python source code into bytecode and runs that bytecode on its virtual machine. It also includes the standard library and the tools most programmers associate with a normal Python installation. Its reference status means that Python language behavior is commonly defined and tested first against CPython.

Other implementations serve specialized purposes. Jython runs on the Java Virtual Machine, IronPython targets the .NET ecosystem, and PyPy uses a just-in-time compiler to improve performance for some workloads. These are genuine Python implementations, but they are not the default distribution used by most programmers.

CPython’s design has influenced both its strengths and limitations. Its extensive C interface makes native extensions practical, while its traditional global interpreter lock limits simultaneous execution of Python bytecode across threads in many builds. Even so, CPython remains the dominant implementation and the baseline against which alternatives are compared.