为什么+(plus)可以在Python中连接两个字符串？

我正在学习如何学习Python.

w = "This is the left side of..."
e = "a string with a right side."
print w + e

解释为什么添加两个字符串w和e+会产生更长的字符串.

即使我知道它可以工作,但我不明白为什么以及如何？请帮我.

Python用于+连接字符串,因为Python 的核心开发人员定义了该操作符.

虽然这是真的,__add__特殊的方法通常用于执行+操作,+(BINARY_ADD字节码指令)也不能打电话str.__add__,因为+对待两者的Python 2和Python 3 Python的专门弦乐器调用字符串连接功能,直接如果两个操作数+都是字符串,从而消除了需要调用特殊方法.

Python 3调用unicode_concatenate(源代码):

TARGET(BINARY_ADD) {
    PyObject *right = POP();
    PyObject *left = TOP();
    PyObject *sum;
    if (PyUnicode_CheckExact(left) &&
             PyUnicode_CheckExact(right)) {
        sum = unicode_concatenate(left, right, f, next_instr);
        /* unicode_concatenate consumed the ref to v */
    }
    else {
        sum = PyNumber_Add(left, right);
        Py_DECREF(left);
    }
    ...

case BINARY_ADD:
    w = POP();
    v = TOP();
    if (PyInt_CheckExact(v) && PyInt_CheckExact(w)) {
        /* INLINE: int + int */
        register long a, b, i;
        a = PyInt_AS_LONG(v);
        b = PyInt_AS_LONG(w);
        /* cast to avoid undefined behaviour
           on overflow */
        i = (long)((unsigned long)a + b);
        if ((i^a) < 0 && (i^b) < 0)
            goto slow_add;
        x = PyInt_FromLong(i);
    }
    else if (PyString_CheckExact(v) &&
             PyString_CheckExact(w)) {
        x = string_concatenate(v, w, f, next_instr);
        /* string_concatenate consumed the ref to v */
        goto skip_decref_vx;
    }
    else {
      slow_add:
        x = PyNumber_Add(v, w);

    ...

>>> w.__add__(e)
'This is the left side of...a string with a right side.'

>>> class STR(str):
...     def __add__(self, other):
...         print('calling __add__')
...         return super().__add__(other)
... 
>>> STR('abc') + STR('def')
calling __add__
'abcdef'