In CPython, a list comprehension looks like this internally: dis.dis(lambda y: [x + 1 for x in y]) 1 0 LOAD_CONST 1 (<code object

In CPython, a list comprehension looks like this internally: dis.dis(lambda y: [x + 1 for x in y]) 1 0 LOAD_CONST 1 (<code object <listcomp> at 0x106f441e0, file "<ipython-input-2-1f82f6afa2d5>", line 1>) 2 LOAD_CONST 2 ('<lambda>.<locals>.<listcomp>') 4 MAKE_FUNCTION 0 6 LOAD_FAST 0 (y) 8 GET_ITER 10 CALL_FUNCTION 1 12 RETURN_VALUE Disassembly of <code object <listcomp> at 0x106f441e0, file "<ipython-input-2-1f82f6afa2d5>", line 1>: 1 0 BUILD_LIST 0 2 LOAD_FAST 0 (.0) >> 4 FOR_ITER 12 (to 18) 6 STORE_FAST 1 (x) 8 LOAD_FAST 1 (x) 10 LOAD_CONST 0 (1) 12 BINARY_ADD 14 LIST_APPEND 2 16 JUMP_ABSOLUTE 4 >> 18 RETURN_VALUE You get very similar disassembled code to this: def foo(y): def bar(): l = [] a = l.append for x in y: a(x + 1) return l return bar() Obviously you can't use the `LIST_APPEND` primitive in a for loop. Also, note that CPython doesn't try to preallocate the list. `l = [None] * len(y)` and then iterating over a range might beat a comprehension for long lists. The performance story with numba and numpy is basically that you have a boundary between Python and their internal representation of data, and crossing that boundary is slow. For instance, in numpy, even if you specify a dtype, `np.array` must check and cast every element to that type. Likewise, `tolist` must construct new python objects for every value in the array. Once you're dealing with ndarrays, though, operations are fast as the array has a single type and the code is heavily optimized. But iterating over ndarrays from python tends to be worse than native Python containers. Numba is a mixed bag. It's no worse than numpy at interpreting ndarrays, and bad with Python containers. It has to inspect arguments to call a JITted function, and this can be far more surprising than numpy. But if you can combine many JITted functions, you can combine a lot of logic and completely avoid introspection in a way you can't do with numpy alone. The difficulty of doing that is simply that it's having to reimplement many of the numpy features in an early-binding language.