Greenspun's tenth rule of programming states

Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug ridden, slow implementation of half of Common Lisp.

Observe that the Python interpreter is written in C.

In fact, most popular computer languages can be thought of as a poorly implemented Common Lisp. There is a reason for this. Church's lambda calculus is a great foundation for reasoning about programming language semantics. Lisp can be seen as a realization of a lambda calculus interpreter. By reasoning about a language's semantics in Lisp, we're essentially reasoning about the semantics in a variation of lambda calculus.