title: I dont understand How does python Dash @Callback know to execute the def function
body: <p>I want understand how the @callback function knows how to execute the def update_graph, because I dont see any link where I use a variable lets say in the Input country_selector or value in the callback AND in the def function at the same time so callback knows that I want the def function to be executed. Can anyone give me simple answer for that?</p>
  <pre><code>@app.callback(
          Output('timeseries', 'figure'),
          [Input('country_selector', 'value')]
          
               )
  
  def update_graph(selected_dropdown_value):
      
      trace = []  
      for countriesAndTerritories in selected_dropdown_value:
          #Erstelle Balkeindiagramm iterativ
          trace.append(go.Bar( 
                              x = df.month,
                              y= df[df[&quot;countriesAndTerritories&quot;] == countriesAndTerritories] [&quot;cases&quot;],
                              name = countriesAndTerritories
                              ))
          
          data = trace
  </code></pre>