Another victim of PEP 8.
Frenzy Frenzy, I agree with Peter Jones that inconsistency is the issue here. That said, I presume that you were being consistent and that npp silently changed your tabs to spaces, per its default behavior (yea for npp for following the rules; boo for the PEP 8 authors for making that rule).
It shouldn’t have been an issue if you were starting a module from scratch, but would definitely lead to problems if you were editing existing code that already contained real tabs. Once the code is hosed, you have a few options in npp for cleaning it up:
View the white space characters, per Alan Kilborn’s post, and manually fix them
Use the TAB to space option Peter cites - or, ideally, it’s inverse operations, Space to TAB (All) and Space to TAB (Leading)
Use Search > Replace with the Search Mode > Extended option to manually find or replace tab characters using their special \t representation
The automatic space-to-tab converters are good, but bulk operations inevitably (and understandably) overgeneralize. While time consuming, the third option gives you more control over the process, with some level of automation.
FWIW, I’m +1 with you for using tabs. I’m always amused (or perhaps bemused) that right at the top of PEP 8 is a section called "A Foolish Consistency is the Hobgoblin of Little Minds", followed immediately by the first, and most foolish, of its recommendations - to use spaces instead of tabs (which many in the Python community follow consistently, admittedly with good intentions).
I’ll refrain from commenting on the merits of tabs; there’s plenty on the web about that already. Suffice it to say, there’s a large community of Python programmers that agree with your approach.