Why are some variables and comments in my IPython notebook red?

Question

This is because the indentation is screwed up. For consistency the codemirror parser will make a red line if it's not indented 4 spaces (or if it's indented with tabs, depending on codemirror version). The parser has some edge case that indeed highlight only a few of theses lines in red, I won't go into details, but if you indent 4 spaces it will work.

You can either:

• select the block of code and press tab
• use alt-click'n'drag vertically to place multiple cursors in front of your code, and press space enough time to make the correct indent 4 space.

Setting the indent to 2 is possible, but complex and not recommended. 4 space is the python norm.

This is the official solution from the Jupyter Notebook documentation:

1. Open a Jupyter Notebook
2. Select a Code Cell

3. Open your browser's JavaScript console and run the following snippet:

   var cell = Jupyter.notebook.get_selected_cell();
   var config = cell.config;
   var patch = {
         CodeCell:{
           cm_config:{indentUnit:2}
         }
       }
   config.update(patch)
   

4. Reload the Notebook page

This fix is permanent.

To reverse the change repeat the process running this snippet:

var cell = Jupyter.notebook.get_selected_cell();
var config = cell.config;
var patch = {
      CodeCell:{
        cm_config:{indentUnit: null} # only change here.
      }
    }
config.update(patch)