You simply can't in Python, not only due to unwind-first exception handling, but also because one module is represented by multiple objects in memory: one obje

You simply can't in Python, not only due to unwind-first exception handling, but also because one module is represented by multiple objects in memory: one object for each other module that imports it. That makes it impossible to redefine anything (you'll always miss some of the copies of whatever you tried to redefine). Importing things from modules piecemeal makes it even harder to find them. The importlib.reload function only applies to one copy of a module. A comprehensive solution would be incredibly complicated and would still have limitations. In contrast, a Lisp package exists in exactly one place. No matter where you get it from, two references to the same package are always eq (having "is" equality in Python terms). Therefore, if you redefine something in a package, you redefine it for everything that uses the package. It also helps that imports in Lisp apply to the symbols, not the things they refer to. So importing something doesn't hide a copy of it anywhere. The most naive redefinition technique will always work, with no edge cases. The inability for class redefinition to affect already existing objects in Python is another severe limitation. Even if you could redefine anything reliably in Python, you'd still have this problem.