sum二维数组扁平为一维数组

leetcode LCP39 无人机方阵 python代码解法，自己探索解读下

from typing import List
from collections import Counter


class Solution:

    def minimumSwitchingTimes(self, source: List[List[int]], target: List[List[int]]) -> int:
        source, target = Counter(sum(source, [])), Counter(sum(target, []))
        diff = source - target
        return sum(diff.values())

// sum实现
// Python-3.8.10Pythonbltinmodule.c简化了源码

static PyObject *
builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
{
    PyObject *result = start;
    PyObject *temp, *item, *iter;

    iter = PyObject_GetIter(iterable);
    if (iter == NULL)
        return NULL;

    if (result == NULL) {
        result = PyLong_FromLong(0);
        if (result == NULL) {
            return NULL;
        }
    }

    for(;;) {
        item = PyIter_Next(iter);
        if (item == NULL) {
            break;
        }
        
        temp = PyNumber_Add(result, item);
        result = temp;
        if (result == NULL)
            break;
    }
    return result;
}

• sum的逻辑是自然的，将序列的元素一个一个加起来(代码for段落)
• sum有个初始值start，如果不传默认为0PyLong_FromLong(0)，所以代码中将初始值设为空列表[]，否则报错，0加上列表是错误的

// Python-3.8.10Objects/abstract.c简化了源码

// PyNumber_Add实现
PyObject *
PyNumber_Add(PyObject *v, PyObject *w)
{
    PyObject *result = binary_op1(v, w, NB_SLOT(nb_add));
    return result;
}

// binary_op1实现
static PyObject *
binary_op1(PyObject *v, PyObject *w, const int op_slot)
{
    PyObject *x;
    binaryfunc slotv = NULL;
    binaryfunc slotw = NULL;

    if (v->ob_type->tp_as_number != NULL)
        slotv = NB_BINOP(v->ob_type->tp_as_number, op_slot);
    if (w->ob_type != v->ob_type &&
        w->ob_type->tp_as_number != NULL) {
        slotw = NB_BINOP(w->ob_type->tp_as_number, op_slot);
        if (slotw == slotv)
            slotw = NULL;
    }
    if (slotv) {
        x = slotv(v, w);
        if (x != Py_NotImplemented)
            return x;
    }
    if (slotw) {
        x = slotw(v, w);
        if (x != Py_NotImplemented)
            return x;
    }
    Py_RETURN_NOTIMPLEMENTED;
}

• binary_op1判断参数x, y是否实现了op_slot，对应list就是__add__