Since most modern OS are written in C, compilers/interpreters for modern high-level languages are also written in C. Python is not an exception - its most popular/"traditional" implementation is called CPython and is written in C.
What was C written in?
C started with the BCPL language, Ken Thomson had access to a compiler for it that ran on their General Electrics 635 main frame. Unhappy with the language, Thomson used BCPL to write a compiler for the B language, an evolutionary step beyond BCPL that removed some of the technical problems in BCPL.
Are all programming languages written in C?
2. Development of New Languages
Every language uses C language in variable capacity. For example, Python uses C for creating standard libraries, whereas the syntaxes and control structures of languages like C++, PHP and Perl are based on C.