Agreed! Optimizing Python code is indeed really hard, and the lack of const and ability to describe capture semantics don't help.

To be fair, the equivalent in C++ is:

  #include <iostream>
  int main() {
    int i;
  
    i = 1;
    auto incr_by_1 = [&](int x) {return x + i;};
    i = 4;
    auto incr_by_4 = [&](int x) {return x + i;};
    std::cout << incr_by_1(0) << " and " << incr_by_4(0) << std::endl;
    return 0;
  }

which prints "4 and 4". Replace the first [&] with [i] and it prints "1 and 4".

A Python implementation also can't replace incr1 by a single CPU increment instruction because it doesn't know the type of x.

That's still a far cry from being unable to give semantics for Python code without running it.