Python is actually different in that the whole "pass by value" vs. "pass by reference" distinction doesn't really apply. At the C level, yes, every Python object is "passed by reference" in the sense that pointers to its C structure are what get passed around. But at the Python level, Python "variables" aren't labels for memory locations, as they are in other languages; they're namespace bindings. And a function can't access the caller's namespace at all unless the global or nonlocal keywords are explicitly used. So in many ways Python's function call semantics are more like "pass by value" than "pass by reference"; but they're really not quite the same as either one.