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Browse buttons for file selection and progress bar

Hello,

I want to create web app in dash, to do some data analysis of two files,
EDIT :1st question:
1. i know that i can use dcc.Update() to open file , but how can i store it to variable ?

  1. i want to show the progress of this file danalysis, do you have any example of status bars? example:
    1 of X rows has been analyzed, and bar with percentage shown.

Thanks in advice
K.