Using excepto/menos for "except."

Question

In answering this question, I used "excepto por".  The lesson doesn't cover this option, but I have seen it in written Spanish.Except for the white one, all the t-shirts are cool.

Answer

Hola Richard
It's not always possible to add por when using excepto. The use of the preposition is mostly possible if the element that comes after excepto has somehow a "causal" meaning. For example, you can say:
Me gustó mucho la fiesta excepto por la banda de música, que era muy mala.
I liked the party a lot except for the music band, which was awful. 
Meaning "except the fact that the band was bad, that was the reason why I didn't like it." 
So, it's not really interchangeable all the time, although that may be the case in English. 
Saludos