按需将列表保存为 JSON答案

【问题标题】：Saving lists to JSON on demand按需将列表保存为 JSON
【发布时间】：2021-04-09 13:31:58
【问题描述】：

def listsToFile(listName, listToSave):
    #Check if file exists : if the file does not exist, create it
    if(os.path.isfile('data.json') != True):
        dataFile = open("data.json", "w")
        data = {}
        json.dump(data, dataFile)
        dataFile.close()

    with open('data.json', 'r+') as readFile: 
        readData = json.load(readFile)
        
    readData[listName] = listToSave
    with open('data.json', 'r+') as outputToFile: 
        data = json.load(outputToFile)
        data.update(readData)
        outputToFile.seek(0)
        json.dump(data, outputToFile)

我在使用上面的代码将多个 python 列表保存到 JSON 时遇到了问题。当代码在干净的data.json 文件上运行时，或者当文件尚未创建时，代码将毫无问题地执行。如果在我已经创建 data.json 文件并用必要的数据填充它之后再次运行代码，代码偶尔会失败并出现以下错误：

raise JSONDecodeError("Extra data", s, end)
json.decoder.JSONDecodeError: Extra data: line 1 column 2505 (char 2504)

以下是正确生成的 JSON 文件的示例：

{"values1": [0.0, 0.9510565162951535, 0.5877852522924732, -0.587785252292473, -0.9510565162951536, 0.0, 0.9510565162951535, 0.5877852522924732, -0.587785252292473, -0.9510565162951536], "values2": [-0.005169993459294982, 0.900960478450234, 0.688082399145687, -0.6991548418279838, -0.7067674655652273, 0.02282072685197254, 1.0032296059350259, 0.264000562571027, -0.7222227190160149, -1.4026325206242891], "values3": [19.042863922620974, 37.87458461655387, 33.45043085989223, 4.620074671576601, 4.461864779721113, 19.62458304946258, 40.0, 24.636920062805316, 4.140664880521349, -10.0], "values4": [19.042863922620974, 37.37458461655387, 32.45043085989223, 3.1200746715766012, 2.461864779721113, 17.12458304946258, 37.0, 21.136920062805316, 0.1406648805213493, -14.5], "values5": [19.042863922620974, 37.37458461655387, 32.45043085989223, 3.1200746715766012, 84.0204012020356, 17.12458304946258, 37.0, 3.475329107497103, 75.8901259294412, 45.46783947206504], "values6": [19.042863922620974, 37.37458461655387, 32.45043085989223, 25.120074671576603, 106.0204012020356, 39.12458304946258, 59.0, 25.475329107497103, 75.8901259294412, 45.46783947206504]}

以下是我运行代码并尝试将不同的列表保存到已经存在且已填充的 data.json 文件时发生的示例：

{"values1": [0.0, 0.5877852522924731, 0.9510565162951535, 0.9510565162951536, 0.5877852522924732, 1.2246467991473532e-16, -0.587785252292473, -0.9510565162951535, -0.9510565162951536, -0.5877852522924734, 0.0, 0.5877852522924731, 0.9510565162951535, 0.9510565162951536, 0.5877852522924732, 1.2246467991473532e-16, -0.587785252292473, -0.9510565162951535, -0.9510565162951536, -0.5877852522924734], "values2": [-0.10117028598579918, 0.7779790908824286, 1.200393390118824, 1.0838219894046404, 0.15757602921920028, -0.0067210289946661814, -0.5273871827688272, -1.1565635352862502, -0.8323326253928102, -1.2756839944935217, -0.05004380182838799, 0.6714864299550243, 1.0662910625661586, 0.7590821618652706, 1.0445118435048915, -0.06508749032370621, -0.40940979225502727, -0.6795514583892261, -0.8884786917027516, -0.6762588207898561], "values3": [6.858612697312811, 15.735045460801214, 20.0, 18.823023449927092, 9.471074618060788, 7.812230479799041, 2.555264794781925, -3.797286587774387, -0.52365192768276, -5.0, 7.374817122860437, 14.659830065785634, 18.646020431489255, 15.544250444310677, 18.426124042177932, 7.222926792323841, 3.746437082519922, 1.0189206909381987, -1.090537141558372, 1.052165185030268], "values4": [5.781634685737641, 13.351407441895873, 19.0, 15.543929768576376, 13.69489283405558, 10.464760506006371, 0.3539527971091214, -8.5, -3.8334192237799343, -1.8576091755881983, 8.59357984630877, 8.034254186327477, 9.959064845492588, 13.037705232284743, 12.395566807169725, 1.0706082945977524, -2.3644898895249664, -8.913833863581623, -4.831138451105485, -6.463793291134955], "values5": [5.781634685737641, 13.351407441895873, 19.0, 15.543929768576376, 13.69489283405558, 30.782546604725596, 0.3539527971091214, -8.5, -3.8334192237799343, -1.8576091755881983, 38.0, 8.034254186327477, 9.959064845492588, 27.084472735684002, 12.395566807169725, 1.0706082945977524, -2.3644898895249664, -8.913833863581623, 2.9851725802192135, -6.463793291134955], "values6": [5.781634685737641, 13.351407441895873, 19.0, 37.543929768576376, 35.69489283405558, 52.782546604725596, 22.353952797109123, 13.5, -3.8334192237799343, -1.8576091755881983, 38.0, 8.034254186327477, 9.959064845492588, 27.084472735684002, 12.395566807169725, 1.0706082945977524, -2.3644898895249664, -8.913833863581623, 2.9851725802192135, -6.463793291134955]}955]}

如您所见，在第二个 JSON 文件的最后，有一个垃圾值 955 并添加了 ]}。

这些列表不是一次性添加的。在生成每个列表时，它们会被一一添加。我使用 JSON 是因为我以后可以按名称访问所有列表。

【问题讨论】：

这似乎是由于更新后的数据比原始数据短，因此不会覆盖整个文件。写入新数据后在文件上调用.truncate() 将确保没有任何内容超出该点。
感谢您的洞察力，事后看来，添加较短列表时存在一些剩余数据是有道理的。到目前为止它工作正常，没有任何问题。

标签： python json list

【解决方案1】：

问题在于 seek 调用 - 它会覆盖字符，但不能保证文件内容是有效的 JSON

如果您打算用新的 JSON 覆盖文件内容，则以写入模式打开文件

    with open('data.json', 'r+') as readFile: 
        readData = json.load(readFile)
        
    readData[listName] = listToSave
    data.update(readData)
    with open('data.json', 'w') as outputToFile: 
        json.dump(data, outputToFile)

【讨论】：