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To start debugging your code, click on the left side of the line number - you will put a breakpoint there (indicated by a red dot).

If you start the server in the debug mode and open the page in your browser, so the code execution gets to that breakpoint, your interface will display a lot of new information:

Flask in debugger mode

The most important ones are:

  • Yellow border around the active breakpoint (in our case, we have only one breakpoint). It shows where the code execution stopped.
  • On the left side, you can see a list of all the currently available variables. We only have local variables in our example, but if we had some global variables, they would be displayed as well.
  • Underneath you have the “Watch” section - you can add variables there, and VS Code will print their current value. It’s useful if you know in which variable you have a bug, and you want to see how it’s changing during the execution of your program.
  • Then you have a “Call stack” and a list of active breakpoints.
  • At the top of the main window, you have a debug toolbar. It contains the standard set of buttons to:

    • Continue the execution until the next breakpoint
    • Execute the next line
    • Step inside or outside of a function
    • Restart the whole debugging section
    • And stop the debugger
  • At the bottom of the window, next to the terminal, you have a “DEBUG CONSOLE” tab. You can use it to execute any Python code.