Life-time of temporary variables in list comprehensions

[beginner]

Hi All,

If I have a list comprehension:

ab=["A","B"]
c = "ABC"
[1.0 if c=='A' else c='B' for c in ab]
print c

>>"B"

My test shows that if c is not defined before the list comprehension,
it will be created in the list comprehension; if it is defined before
the list comprehension, the value will be overwritten. In other words,
temp variables are not local to list comprehensions.

My question is why is this and is there any way to make c local to
list comp?

Thanks,
Geoffrey