Both work differently. The list comprehension version takes advantage of the special bytecode LIST_APPEND which calls PyList_Append directly for us. Hence it avoids an attribute lookup to list.append and a function call at the Python level. >>> def func_lc(): [x**2 for x in y] … >>> dis.dis(func_lc) 2 0 LOAD_CONST 1 (<code object <listcomp> at … Read more
Yes, your understanding is correct. There is basically (very basically) a giant switch statement inside the CPython interpreter that says “if the current opcode is so and so, do this and that”. http://hg.python.org/cpython/file/3.3/Python/ceval.c#l790 Other implementations, like Pypy, have JIT compilation, i.e. they translate Python to machine codes on the fly.
The interesting bit is that it depends on the order in which the integers are first created. For example instead of shuffle create a random sequence with random.randint: from timeit import timeit import random a = [random.randint(0, 10**6) for _ in range(10**6)] for _ in range(5): print(timeit(lambda: list(a), number=10)) This is as fast as copying … Read more
Remember that the Python int type does not have a limited range like C int has; the only limit is the available memory. Memory goes to storing the value, the current size of the integer storage (the storage size is variable to support arbitrary sizes), and the standard Python object bookkeeping (a reference to the … Read more
In order to optimize set(list_2), the interpreter needs to prove that list_2 (and all of its elements) does not change between iterations. This is a hard problem in the general case, and it would not surprise me if the interpreter does not even attempt to tackle it. On the other hand a set literal cannot … Read more
[*] TL;DR: This “empty __class__ cell” error will happen when the metaclass tries to call a method in the defined class (or instantiate it) before it is done with its __new__ and __init__,and the called method uses super. The error will also happen if one writes a call to super() in a function defined outside … Read more
See the Rationale section from PEP 424 that introduced __length_hint__ and offers insight on the motivation: Being able to pre-allocate lists based on the expected size, as estimated by __length_hint__ , can be a significant optimization. CPython has been observed to run some code faster than PyPy, purely because of this optimization being present. In … Read more
Sure! The code’s here, starting with function islt and proceeding for QUITE a while;-). As Chris’s comment suggests, it’s C code. You’ll also want to read this text file for a textual explanation, results, etc etc. If you prefer reading Java code than C code, you could look at Joshua Bloch’s implementation of timsort in … Read more
When you write [None] * 10, Python knows that it will need a list of exactly 10 objects, so it allocates exactly that. When you use a list comprehension, Python doesn’t know how much it will need. So it gradually grows the list as elements are added. For each reallocation it allocates more room than … Read more
No file descriptor (FD) number is special. stdin on FD 0, stdout on FD 1 and stderr on FD 2 is just a convention. When you log in, the associated terminal device will be “connected” to these FDs. When you run a command, it inherits the descriptors unless you instruct the shell to make redirections. … Read more