In python, if X is a list, why can X + = "ha" Run While X = x + "ha" throw an exception?

Problem

As we all know, in Python, The + operator can be used on the list, and the + operator only needs the second operand to be iterated (Original: iterable. @ Justjavac), then
+ The operation can obviously be performed on "ha.

The Code is as follows:

>>> x = []>>> x += "ha">>> x['h', 'a']>>> x = x + "ha"Traceback (most recent call last):File "<stdin>", line 1, in <module>TypeError: can only concatenate list (not "str") to list

Answer

When we use + = On the list, it is actually equivalent to calling the extend () function, rather than using
+.

• You can call extend () on an iterable object ().
  • However, when you use +, the other operand must be list ).

Why is Python so strange? It may be due to performance considerations. When you call +, a new object is created and all the content is copied. However, when you call the extend () function, you can use the existing space.

This produces another side effect: If you write x + = Y, you will see the changes in other references to the list. However, if you use X = x + y, no.

The following code illustrates this:

>>> X = ['A', 'B'] >>> y = ['C ', d'] >>>> z = x >>> x + = y >>> Z ['A', 'B', 'C ', 'D'] // Z has also changed >>> x = ['A', 'B'] >>> y = ['C ', d'] >>>> z = x >>> x = x + y >>> Z ['A', 'B'] // Original Value of the Z Function

References

Python source code
For list.

Python: + = source code:

static PyObject *list_inplace_concat(PyListObject *self, PyObject *other){    PyObject *result;    result = listextend(self, other);    if (result == NULL)        return result;    Py_DECREF(result);    Py_INCREF(self);    return (PyObject *)self;}

Python: + source code:

static PyObject *list_concat(PyListObject *a, PyObject *bb){    Py_ssize_t size;    Py_ssize_t i;    PyObject **src, **dest;    PyListObject *np;    if (!PyList_Check(bb)) {        PyErr_Format(PyExc_TypeError,                  "can only concatenate list (not \"%.200s\") to list",                  bb->ob_type->tp_name);        return NULL;    }    // etc ...

Original article: Python
-If X is list, why does x + = "ha" work, while X = x + "ha" throw an exception?

In
In python, if X is a list, why does x + = "ha" Run While X = x + "ha" throw an exception?

Translator: justjavac