全局变量更新者和间隔组件

Question

当我的 dash 应用程序启动时，一个全局变量由一个计算量大的函数创建 (dat)。我希望 dat 定期更新，而不是通过任何用户操作。我知道由于内存共享的方式，普通的 python 全局变量不适合在破折号中使用，所以我使用隐藏的 div 来存储定期更新的数据。

我不希望在每个用户启动应用程序时更新数据，而是希望在后台进行更新，并最好使用更新后的数据更新所有用户的应用程序。如果必须使用页面刷新来更新应用程序，那也没关系，但更新后的数据将可供所有用户使用。

我在下面包含了一个示例，它执行我想要执行的任务，问题是如果它是字符串或 numpy 数组，则将数据存储在 div 中很好，但如果数据结构更复杂，例如pandas 数据框（参见注释的代码行）或 pandas 数据框字典，然后应用程序将运行，但页面不会在浏览器中加载（错误加载布局）。也许记忆是解决方案，尽管我看到的所有例子似乎都与这个有细微的不同，以至于我看不出它是如何使用的。对我来说，使用隐藏的 div 似乎有点小技巧，但情节帮助页面推荐使用

所以我的问题是如何使用比字符串或数组更复杂的数据结构在 dash 中创建和更新全局变量？

import dash
from dash.dependencies import Input, Output, Event
import dash_html_components as html
import dash_core_components as dcc
from datetime import datetime
import numpy as np
import pandas as pd


app = dash.Dash(__name__)

def compute_expensive_data():
    t=datetime.now()
    dat = np.array([t.minute, t.second])
    #d = {'time' : pd.Series(np.array([t.minute, t.second]), index=['minute', 'second'])}
    #dat = pd.DataFrame(d)

    return  dat

dat = compute_expensive_data()
print(dat)

app.layout = html.Div([
        html.H3('Original Time: Minute = ' + str(dat[0]) + ': Second = ' + str(dat[1])),
        #html.H3('Original Time: Minute = ' + str(dat['time']['minute']) + ': Second = ' + str(dat['time']['second'])),
        html.Div(id='title-line-children'),
        dcc.RadioItems(
            id='time-dropdown',
            options=[
                {'label': 'Minute', 'value': 'minute'}, {'label': 'Second', 'value': 'second'},
            ],
            value='minute'
        ), 

        # Hidden div inside the app that stores the intermediate value
        html.Div(id='intermediate-value', style={'display': 'none'}, children = dat),

        dcc.Interval(
            id='interval-component',
            interval=20*1000 # 20 seconds in milliseconds
        )

    ])

@app.callback(
    Output('title-line-children', 'children'),
    [Input('time-dropdown', 'value'), Input('intermediate-value', 'children')])
def render(value,dat1):
    if value == 'minute':
        printStr = str(dat1[0])
        #printStr = str(dat1['time']['minute'])
        outStr = 'Minute = ' + printStr
    elif value == 'second':
        printStr = str(dat1[1])
        #printStr = str(dat1['time']['second'])
        outStr = 'Second = ' + str(dat1[1])

    return outStr

@app.callback(Output('intermediate-value', 'children'),
              events=[Event('interval-component', 'interval')])
def update_global_var():
    return compute_expensive_data()

if __name__ == '__main__':
    app.run_server(debug=True)

Answer 1

好的，所以答案很简单。我只需要 jsonify 我的数据框。那么它基本上只是一个可以保存在隐藏div中的字符串。

import dash
from dash.dependencies import Input, Output, Event
import dash_html_components as html
import dash_core_components as dcc
from datetime import datetime
import numpy as np
import pandas as pd


app = dash.Dash(__name__)

def compute_expensive_data():
    t=datetime.now()
    d = {'time' : pd.Series(np.array([t.minute, t.second]), index=['minute', 'second'])}
    dat = pd.DataFrame(d).to_json()

    return  dat

dat = compute_expensive_data()
print(dat)

app.layout = html.Div([
        html.H3('Original Time: Minute = ' + str(pd.read_json(dat)['time']['minute']) + ': Second = ' + str(pd.read_json(dat)['time']['second'])),
        html.Div(id='title-line-children'),
        dcc.RadioItems(
            id='time-dropdown',
            options=[
                {'label': 'Minute', 'value': 'minute'}, {'label': 'Second', 'value': 'second'},
            ],
            value='minute'
        ), 

        # Hidden div inside the app that stores the intermediate value
        html.Div(id='intermediate-value', style={'display': 'none'}, children = dat),

        dcc.Interval(
            id='interval-component',
            interval=20*1000 # 20 seconds in milliseconds
        )

    ])

@app.callback(
    Output('title-line-children', 'children'),
    [Input('time-dropdown', 'value'), Input('intermediate-value', 'children')])
def render(value,dat1):
    if value == 'minute':
        printStr = str(pd.read_json(dat1)['time']['minute'])
        outStr = 'Minute = ' + printStr
    elif value == 'second':
        printStr = str(pd.read_json(dat1)['time']['second'])
        outStr = 'Second = ' + printStr

    return outStr

@app.callback(Output('intermediate-value', 'children'),
              events=[Event('interval-component', 'interval')])
def update_global_var():
    return compute_expensive_data()

if __name__ == '__main__':
    app.run_server(debug=True)

Answer 2

答案已更新至 Dash 1.8：

import dash
from dash.dependencies import Input, Output
import dash_html_components as html
import dash_core_components as dcc
from datetime import datetime
import numpy as np
import pandas as pd

app = dash.Dash(__name__)

def compute_expensive_data():
    t=datetime.now()
    d = {'time' : pd.Series(np.array([t.minute, t.second]), index=['minute', 'second'])}
    dat = pd.DataFrame(d).to_json()
    return  dat

dat = compute_expensive_data()
print(dat)

app.layout = html.Div([
        html.H3('Original Time: Minute = ' + str(pd.read_json(dat)['time']['minute']) + ': Second = ' + str(pd.read_json(dat)['time']['second'])),
        html.Div(id='title-line-children'),
        dcc.RadioItems(
            id='time-dropdown',
            options=[
                {'label': 'Minute', 'value': 'minute'}, {'label': 'Second', 'value': 'second'},
            ],
            value='minute'
        ), 
        # Hidden div inside the app that stores the intermediate value
        html.Div(id='intermediate-value', style={'display': 'none'}, children = dat),
        dcc.Interval(
            id='interval-component',
            interval=2*1000, # 20 seconds in milliseconds
            n_intervals=0
        )
    ])

@app.callback(
    Output('title-line-children', 'children'),
    [Input('time-dropdown', 'value'), Input('intermediate-value', 'children')])
def render(value,dat1):
    if value == 'minute':
        printStr = str(pd.read_json(dat1)['time']['minute'])
        outStr = 'Minute = ' + printStr
    elif value == 'second':
        printStr = str(pd.read_json(dat1)['time']['second'])
        outStr = 'Second = ' + printStr
    return outStr

@app.callback(Output('intermediate-value', 'children'),
              [Input('interval-component', 'n_intervals')])
def update_global_var(n):
    return compute_expensive_data()

if __name__ == '__main__':
    app.run_server(debug=False)