Question about idiomatic use of _ and private stuff.

[Steven W. Orr]

I understand that two leading underscores in a class attribute make the 
attribute private. But I often see things that are coded up with one 
underscore. Unless I'm missing something, there's a idiom going on here.

Why do people sometimes use one leading underscore?

TIA

-- 
Time flies like the wind. Fruit flies like a banana. Stranger things have  .0.
happened but none stranger than this. Does your driver's license say Organ ..0
Donor?Black holes are where God divided by zero. Listen to me! We are all- 000
individuals! What if this weren't a hypothetical question?
steveo at syslang.net