{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "改变变量x的值，变量y的值也随着改变，这与Python内部的赋值机制有关。" ] }, { "cell_type": "code", "execution_count": 1, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "[1, 100, 3]\n" ] } ], "source": [ "x = [1, 2, 3]\n", "y = x\n", "x[1] = 100\n", "print (y)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "### 简单类型" ] }, { "cell_type": "code", "execution_count": 3, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "x的id： 2575200343760\n", "y的id： 2575200343760\n", "y变更值后的id： 2575142185008\n", "x的id： 2575200343888\n" ] } ], "source": [ "x = 500\n", "print('x的id：',id(x))\n", "y = x\n", "print('y的id：',id(y))\n", "y = 'foo'\n", "print('y变更值后的id：',id(y))\n", "x = 500\n", "print('x的id：',id(x))" ] }, { "cell_type": "code", "execution_count": 8, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "140729471006224\n" ] }, { "data": { "text/plain": [ "140729471006224" ] }, "execution_count": 8, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x=250\n", "print(id(x))\n", "x=250\n", "id(x)" ] }, { "cell_type": "code", "execution_count": 9, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "2575200344560" ] }, "execution_count": 9, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x = 500\n", "id(x)" ] }, { "cell_type": "code", "execution_count": 10, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "2575200344560" ] }, "execution_count": 10, "metadata": {}, "output_type": "execute_result" } ], "source": [ "y = x\n", "id(y)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "**使用is**来判断是不是指向同一个事物：" ] }, { "cell_type": "code", "execution_count": 11, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "True" ] }, "execution_count": 11, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x is y" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "现在 y 指向另一块内存：" ] }, { "cell_type": "code", "execution_count": 12, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "2575142185008" ] }, "execution_count": 12, "metadata": {}, "output_type": "execute_result" } ], "source": [ "y = 'foo'\n", "id(y)" ] }, { "cell_type": "code", "execution_count": 13, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "False" ] }, "execution_count": 13, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x is y" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "**Python**会为每个出现的对象进行赋值，哪怕它们的值是一样的（但这个值得足够大），例如：" ] }, { "cell_type": "code", "execution_count": 15, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "2575200345488" ] }, "execution_count": 15, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x = 500\n", "id(x)" ] }, { "cell_type": "code", "execution_count": 16, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "2575200345328" ] }, "execution_count": 16, "metadata": {}, "output_type": "execute_result" } ], "source": [ "y = 500\n", "id(y)" ] }, { "cell_type": "code", "execution_count": 17, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "False" ] }, "execution_count": 17, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x is y" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "为了提高内存利用效率，对于一些简单的对象，如一些数值较小的int对象，Python采用了**重用对象内存**的办法：" ] }, { "cell_type": "code", "execution_count": 18, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "2933943553392" ] }, "execution_count": 18, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x = 2000\n", "id(x)" ] }, { "cell_type": "code", "execution_count": 19, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "2933943553872" ] }, "execution_count": 19, "metadata": {}, "output_type": "execute_result" } ], "source": [ "y = 2000\n", "id(y)" ] }, { "cell_type": "code", "execution_count": 20, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "False" ] }, "execution_count": 20, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x is y" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## 容器类型" ] }, { "cell_type": "code", "execution_count": null, "metadata": {}, "outputs": [], "source": [ "x = [500, 501, 502] \n", "y = x # 变量 y 引用与 x 相同的列表对象。因此，任何通过 y 对列表所做的改变也会影响到 x，因为它们实际上指向内存中的同一个列表。\n", "y[1] = 600 # 执行 y[1] = 600。这一步直接修改了 y（同时也是 x）指向的列表中的第二个元素（索引为1），将其从 501 改为 600。此时，如果打印 x 或 y，你会看到 [500, 600, 502]。\n", "y = [700, 800] # 执行 y = [700, 800]。这行代码并没有修改原来的列表，而是让变量 y 指向了一个全新的列表 [700, 800]。这个操作不会对 x 产生任何影响，因为它只是改变了 y 所指向的对象，并没有触及到原来那个被 x 和 y 共享的列表。" ] }, { "cell_type": "code", "execution_count": 22, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "[500, 600, 502]" ] }, "execution_count": 22, "metadata": {}, "output_type": "execute_result" } ], "source": [ "x" ] } ], "metadata": { "kernelspec": { "display_name": "base", "language": "python", "name": "python3" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 3 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython3", "version": "3.13.2" }, "toc": { "base_numbering": 1, "nav_menu": {}, "number_sections": true, "sideBar": true, "skip_h1_title": false, "title_cell": "Table of Contents", "title_sidebar": "Contents", "toc_cell": false, "toc_position": {}, "toc_section_display": true, "toc_window_display": true } }, "nbformat": 4, "nbformat_minor": 4 }