关于散点图和下拉列表的回调中的短划线错误

Question

我正在学习dash库。这段代码显示了当我在数据框中选择列时的散点图。这工作没有任何问题，但回调错误发生在网页上。

在网络上，更新spas-graph.figure时出现回调错误我不明白为什么会出现这个错误。

[import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output
import plotly.graph_objects as go
import pandas as pd
df = pd.DataFrame({
'depth' : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
'upper_value' : [1, 4, 6, 2, 6, 8, 9, 10, 4, 2],
'middle_value' : [5, 3, 7, 8, 1, 2, 3, 1, 4, 8],
'down_value' : [6, 2, 1, 10, 5, 2, 3, 4, 2, 7]
})
col_list = df.columns[1:4]
app = dash.Dash(__name__)
app.layout = html.Div([
dcc.Dropdown(
id = 'select-cd',
options = [
{'label' : i, 'value' : i}
for i in col_list
]
),
dcc.Graph(id = 'spas-graph')    
])
@app.callback(
Output('spas-graph', 'figure'),
[Input('select-cd', 'value')]
)
def update_figure(selected_col):

return {
'data' : [go.Scatter(
x = df[selected_col],
y = df['depth'],
mode = 'lines + markers',
marker = {
'size' : 15,
'opacity' : 0.5,
'line' : {'width' : 0.5, 'color' : 'white'}
}
)],

'layout' : go.Layout(
xaxis={'title': 'x_scale'},
yaxis={'title': 'y_scale'},
hovermode='closest'
)
}
if __name__ == '__main__':
app.run_server(debug=True)

Answer 1

您尚未在下拉方法中定义值参数。因此，当服务器启动时，它接收到的第一个输入是None值。

你可以用两种方法来解决它：

1. 在下拉菜单中添加默认值：

2. 回调方法中的Handle None值

   import dash
   import dash_core_components as dcc
   import dash_html_components as html
   from dash.dependencies import Input, Output
   import plotly.graph_objects as go
   import pandas as pd
   df = pd.DataFrame({
   'depth' : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
   'upper_value' : [1, 4, 6, 2, 6, 8, 9, 10, 4, 2],
   'middle_value' : [5, 3, 7, 8, 1, 2, 3, 1, 4, 8],
   'down_value' : [6, 2, 1, 10, 5, 2, 3, 4, 2, 7]
   })
   col_list = df.columns[1:4]
   app = dash.Dash(__name__)
   app.layout = html.Div([
   dcc.Dropdown(
   id = 'select-cd',
   options = [
   {'label' : i, 'value' : i}
   for i in col_list
   ],
   value = col_list[0]
   ),
   dcc.Graph(id = 'spas-graph')    
   ])
   @app.callback(
   Output('spas-graph', 'figure'),
   [Input('select-cd', 'value')]
   )
   def update_figure(selected_col):
   if selected_col is None:
   selected_col = col_list[0]
   return {
   'data' : [go.Scatter(
   x = df[selected_col],
   y = df['depth'],
   mode = 'lines + markers',
   marker = {
   'size' : 15,
   'opacity' : 0.5,
   'line' : {'width' : 0.5, 'color' : 'white'}
   }
   )],
   'layout' : go.Layout(
   xaxis={'title': 'x_scale'},
   yaxis={'title': 'y_scale'},
   hovermode='closest'
   )
   }