列表理解

Question

我无法使切片正常工作。我有一个看起来像这样的字符串列表：

Dim jsonURL As String = "https://prod-13.canadacentral.logic.azure.com******************************************"
Dim reader As StreamReader
Dim errorMsg As String = Nothing
Dim CurrentFurn As String
Try
    Dim request As HttpWebRequest = CType(WebRequest.Create(jsonURL), HttpWebRequest)
    request.Credentials = New NetworkCredential(USER,PASSWORD")

    Dim response As HttpWebResponse = CType(request.GetResponse(), HttpWebResponse)
    reader = New StreamReader(response.GetResponseStream())
    Dim jsonStr As String = reader.ReadToEnd()

    Dim TCICFurnaceMiner = JsonConvert.DeserializeObject(Of List(Of JSON_data))(jsonStr)

    If CurrentMachine = "F1" Then
        CurrentFurn = "Furnace 1 (lbs per hour 2400)"
    ElseIf CurrentMachine = "F2" Then
        CurrentFurn = "Furnace 2 (lbs per hour 4200)"
    End If

    Dim ProdList = From ProdData In TCICFurnaceMiner Where ProdData.Machine.Contains(CurrentFurn) Select ProdData

    chrtFurnace.ChartAreas(0).AxisX.LabelStyle.Angle = -45
    chrtFurnace.ChartAreas(0).AxisX.Interval = 1
    chrtFurnace.ChartAreas(0).AxisX.Minimum = 0
    chrtFurnace.ChartAreas(0).AxisX.Maximum = 24.5
    ' chrtFurnace.ChartAreas(0).AxisX.LabelStyle.Format = "HH:00"
    chrtFurnace.ChartAreas(0).AxisX.LabelStyle.IntervalType = DateTimeIntervalType.Auto
    chrtFurnace.ChartAreas(0).AxisX.Title = "Hour"
    chrtFurnace.ChartAreas(0).AxisY.Title = "Lbs"
    chrtFurnace.ChartAreas(0).AxisX.MajorGrid.Enabled = False
    chrtFurnace.ChartAreas(0).AxisY.MajorGrid.Enabled = False
    chrtFurnace.ChartAreas(0).AxisX.TitleFont = New Drawing.Font("arial", 16)
    chrtFurnace.ChartAreas(0).AxisY.TitleFont = New Drawing.Font("arial", 16)
    chrtFurnace.Series("Lbs").IsValueShownAsLabel = True

    ' Print the results of our LINQ query
    For Each ProdData In ProdList

        chrtFurnace.Series("Lbs").Points.Add(ProdData.Lbs)
        chrtFurnace.Series("Rate").Points.Add(ProdData.Rate)

    Next

Catch ex As WebException
    errorMsg = "Download failed. The response from the server was: " +
         CType(ex.Response, HttpWebResponse).StatusDescription
    Console.WriteLine("Error: " + errorMsg)
End Try

我需要将'url'保存在新列表中以进一步处理它。

这就是我正在尝试的

['subdomain', 'name', 'url']
['https://www.pedidosya.com.ar/restaurantes/buenos-aires/recoleta/empanadas-delivery?bt=RESTAURANT&page=1', 'Cümen-Cümen Empanadas Palermo', 'https://www.pedidosya.com.ar/restaurantes/buenos-aires/cumen-cumen-empanadas-palermo-menu']
['https://www.pedidosya.com.ar/restaurantes/buenos-aires/recoleta/empanadas-delivery?bt=RESTAURANT&page=1', 'Cümen-Cümen Empanadas - Barrio Norte', 'https://www.pedidosya.com.ar/restaurantes/buenos-aires/cumen-cumen-empanadas-barrio-norte-menu']

但这是我打印（）时得到的：

for row[3:3] in reader:
   menus = []
   menus.append[row]

哪个是列表的最后一部分。我需要的是：

['https://www.pedidosya.com.ar/restaurantes/buenos-aires/recoleta/empanadas-delivery?bt=RESTAURANT&page=5', 'La Pergola - Recoleta', 'https://www.pedidosya.com.ar/restaurantes/buenos-aires/la-pergola-recoleta-menu']

我已经添加了其余的代码。问题是，这不是我想像的str列表，而是type（）='_csv.reader'

这是完整的代码：

menus = ['https://www.pedidosya.com.ar/restaurantes/buenos-aires/cumen-cumen-empanadas-palermo-menu', 'https://www.pedidosya.com.ar/restaurantes/buenos-aires/cumen-cumen-empanadas-barrio-norte-menu']

Answer 1

似乎想要这样：

TypeError: a bytes-like object is required, not 'str'

我不理解通过使menus = [] for row in reader: menus.append(row[2])循环的迭代变量row[3:3]来做什么。我认为您想遍历简单的行，然后对循环中的每一行进行一些操作。

Answer 2

假设您有一个列表列表（即列表周围有一个额外的[]），而不是问题所暗示的3个孤立列表，则可以遍历列表列表并使用url元素从每个（元素2）添加到新列表。

reader = csvreader or whatever you do to define it
menu = []    
for n, i in enumerate(reader): 
    if(n != 0):
        print(i[2])
        menu.append(i[2])

我更改了代码以与csvreader对象一起使用。我们将使用enumerate一个出色的函数来代替我们忽略第一个元素的旧方法，该函数将我们属于阅读器的哪个元素视为n。因此，只要n不为零，我们将像以前一样继续。

Answer 3

问题不在于切片（尽管您也可以直接用[2]进行索引）。但是，您在循环中重新初始化菜单。因此，对于循环的每次运行，您都将覆盖之前的内容。这应该解决它：

menus = []
for row in reader:
   menus.append[row[2]]

列表理解

一种更清洁（并且更具Pythonic性）的方法是使用list comprehension：

menus = [row[2] for row in reader]

在Python中切片str的列表

3 个答案:

列表理解