Open-source implementation for .NET and Mono written in C#, licensed under Apache License 2.0. It relies on DLR (Dynamic Language Runtime). It supports only version 2.7, version 3 is currently being developped.
Differences with CPython:
Tight integration with .NET Framework.
Strings are Unicode by default.
Does not support extensions for CPython written in C.
Does not suffer from Global Interpreter Lock.
Performance is usually lower, though it depends on tests.
Allows for classical OO programming with multiple inheritance using pure Python syntax, parsed by CPython’s native parser
Hierarchical URL based module system allowing module distribution via PyPi
Multi-level sourcemaps and optional annotation of target code with source references
Compact downloads, kB’s rather than MB’s
Operator overloading can be switched on and off locally to facilitate readable numerical math
Code size and speed
Integration with HTML