[Tutor] fibonacci [caching] [What's the precedence of "not foo in bar"?]

[Danny Yoo]

On Fri, 14 Feb 2003, Gregor Lingl wrote:

> And what about this?
>
> def fib(n, cache={0:0, 1:1}):
>      if not n in cache:
>          cache[n] = fib(n-2) + fib(n-1)
>      return cache[n]


Hi Gregor,


I feel a little uncomfortable with the 'not n in cache' part: the reason
is because it takes a little effort deciding if Python will interpret it
as:

    (not n) in cache              ## Version 1

or

    not (n in cache)              ## Version 2


There is a distinct difference between the meanings!  Which way will it
go?  Let's test it by deliberately writing a fork in the road.


    text_expression = not "foo" in {0 : 'zero', "foo" : "bar"}

This expression is specifically tailored to show which fork Python takes.
What do you think this test expression evaluates to?




Anyway, because of the potential confusion, we'll often use:

    x not in L

rather than

    not x in L

because it's just not really productive to play these precedence games,
even though it can be fun.  *grin*