[Tutor] Tuple indexing

[Peter Otten]

Ian D wrote:

> Hi
> 
> 
> 
> I have seen some examples that seem to use a tuple with a method called
> index()
> 
> 
> The original code I was looking at had this sort of thing:
> 
> 
> 

Hi Ian! Please don't use that much whitespace. It makes your post hard and 
unpleasant to read. Thank you.


> SENSORS =  ('sensor1', 'sensor2')
> 
> 
> 
>                 pin_index = SENSORS.index("sensor1")
> 
> 
> so the result is that pin_index the is equal to 0
> 
> 
> I then read that a Tuple has no attribute index on this site
> http://www.diveintopython.net/native_data_types/tuples.html

Dive into Python is quite old.

>>>> t.index("example")
> Traceback (innermost last): File "<interactive input>", line 1, in ?
> AttributeError: 'tuple' object has no attribute 'index'
> 
> 
> 
> The example seems to work with 2.7 and 3.3 for me.

The tuple.index() method has been added in Python 2.6:

https://docs.python.org/2.6/whatsnew/2.6.html

"""
Tuples now have index() and count() methods matching the list type’s index() 
and count() methods:

>>> t = (0,1,2,3,4,0,1,2)
>>> t.index(3)
3
>>> t.count(0)
2
(Contributed by Raymond Hettinger)
"""

> But I don't find much documentation on indexing Tuples using this method.
> I am just wondering if there is more specific documentation on using
> Tuples this way

Many Python coders don't use tuples to look up an item index; the 
traditional application is to pass multiple values around, e. g.

def extent(thing):
    x = calculate_width(thing)
    y = calculate_height(thing)
    return x, y

width, height = extent(picture)

portrait = width < height

In the example you have to know the index beforehand, by reading the code or 
its documentation rather than going through all items for a matching item. 

When you want to treat all items in a tuple uniformly in most cases using a 
tuple is a premature optimisation; use a list or set unless you can name a 
compelling reason not to.

Your sensors example could be solved with a dict:

sensors = {"sensor1": 0, "sensor2": 1}

pin_index = sensors["sensor1"]

This approach will still work well for huge numbers of sensors (constant 
time or O(1)), unlike tuple/list.index() where the average lookup time grows 
with the number of items in the tuple or list (O(n)).