code: def tabs_or_spaces(physical_line, indent_char):
      
      indent = indent_match(physical_line).group(1)
      for offset, char in enumerate(indent):
          if char != indent_char:
              return offset, "E101 indentation contains mixed spaces and tabs"
docs: Never mix tabs and spaces.
  
      The most popular way of indenting Python is with spaces only.  The
      second-most popular way is with tabs only.  Code indented with a mixture
      of tabs and spaces should be converted to using spaces exclusively.  When
      invoking the Python command line interpreter with the -t option, it issues
      warnings about code that illegally mixes tabs and spaces.  When using -tt
      these warnings become errors.  These options are highly recommended!