Saying that they are "compiled in C" is a bit confusing statement. Let me rephrase it: CPython (the most widespread implementation of the Python interpreter, to which you are referring) is a C program, and those are C functions built in the interpreter, not Python code that the interpreter magically chooses to compile to C.

When you feed a .py file to Python, it parses it and converts each function into bytecode, i.e. a compact binary form of the operations that the function does . This is fed to the Python virtual machine, a piece of the interpreter which reads the bytecode and does what it tells it to do.

When you call functions written in Python nothing special happens: the VM will just jump to interpret their bytecode; but when you call a function that is built in inside the interpreter, the interpreter calls the corresponding C function that it has built in (this happens all the time, for example most operations on primitive types are actually performed by code compiled directly inside the interpreter).

Function written in C in CPython are generally faster, both because they are written in a language that is tailored for high performance, compiled to native code (no need for an interpreter, runs directly on the CPU) and has simpler, less dynamic semantics (for example, in Python each access to a member is a complex hashtable lookup, in C it can boil down to a single assembly instruction).

It's generally interpreted, so how does some of the code get compiled while another is interpreted - and in a different language? Why not just compile the whole thing?

You seem to think that the it's the interpreter to decide what code is compiled and what is interpreted; that's a wrong premise. The compiled parts are code that it's written directly in C by the CPython developers, and is compiled inside the interpreter (other code written in C or other compiled languages can be called by Python if it's compiled in Python extensions or via ctypes). All the CPython interpreter can do when you run it is to interpret Python and call ready-made C functions.

Saying that they are "compiled in C" is a bit confusing statement. Let me rephrase it: CPython (the most widespread implementation of the Python interpreter, to which you are referring) is a C program, and those are C functions built in the interpreter, not Python code that the interpreter magically chooses to compile to C.

When you feed a .py file to Python, it parses it and converts each function into bytecode, i.e. a compact binary form of the operations that the function does . This is fed to the Python virtual machine, a piece of the interpreter which reads the bytecode and does what it tells it to do.

When you call functions written in Python nothing special happens: the VM will just jump to interpret their bytecode; but when you call a function that is built in inside the interpreter, the interpreter calls the corresponding C function that it has built in (this happens all the time, for example most operations on primitive types are actually performed by code compiled directly inside the interpreter).

Function written in C in CPython are generally faster, both because they are written in a language that is tailored for high performance, compiled to native code (no need for an interpreter, runs directly on the CPU) and has simpler, less dynamic semantics (for example, in Python each access to a member is a complex hashtable lookup, in C it can boil down to a single assembly instruction).

It's generally interpreted, so how does some of the code get compiled while another is interpreted - and in a different language? Why not just compile the whole thing?

You seem to think that the it's the interpreter to decide what code is compiled and what is interpreted; that's a wrong premise. The compiled parts are code that it's written directly in C by the CPython developers, and is compiled inside the interpreter when the interpreter is built (other code written in C or other compiled languages can be called by Python if it's compiled in Python extensions or via ctypes). All the CPython interpreter can do when you run it is just to interpret Python and call ready-made C functions.