efficiency question

[David Harvey]

Hi,

Suppose I write

if x in ("abc", "def", "xyz"):
doStuff()

elif x in ("pqr", "tuv", "123"):
doOtherStuff()

elif ...

etc.

When is python building the tuples? Does it need to build the tuple
every time it comes through this code? Or does it somehow recognise
that they are constant and cache them?

In other words, is anything gained (efficiency-wise) by first putting
say

tuple1 = ("abc", "def", "xyz")
tuple2 = ("pqr", "tuv", "123")

somewhere where I know it's executed once, and then writing

if x in tuple1:
doStuff()

elif x in tuple2:
doOtherStuff()

elif ...

????

(The tuples I have in mind are of course much longer than three
elements)

Many thanks

David

[Scott David Daniels]

David Harvey wrote:

if x in ("abc", "def", "xyz"):
doStuff()
elif x in ("pqr", "tuv", "123"):
doOtherStuff()
elif ...

If the code really looks like this:
# one-time-only code
big_dispatch_table = {}
for function, keys in [
(doStuff, ["abc", "def", "xyz"]),
(doOtherStuff, ["pqr", "tuv", "123"]),
... ]:
for key in keys:
big_dispatch_table[key] = function
Then your actual code goes something like:

big_dispatch_table[x]()

--Scott David Daniels
sc***********@acm.org