Python ordered dictionary: Why is [] notation required to change dictionary values using a for loop?

Question

I need to change the values of a Ordered Dictionary. I used a for loop but the values weren't changing. I discovered this was because I was assigning to the variable name in the loop rather than directly with the bracket [] notation. Why can't I refer to the values in the loop by the loop variable name?

Tried:

idx = 0
for name,val in settings.items():
    idx += 1
    val = idx
    print name,val

Result: OrderedDict([('Accel X Bias', None), ('Mag X Bias', None)])

Expected: OrderedDict([('Accel X Bias', 1), ('Mag X Bias', 2)])

Full code:

import collections
settings = collections.OrderedDict([('Accel X Bias', None), ('Mag X Bias', None)])
idx = 0

print "\nIn loop, values are changed:"
for name,val in settings.items():
    idx += 1
    val = idx
    print name,val

print "\nAfter Loop, values didn't change:\n",settings

for name,val in settings.items():
    idx += 1
    val = idx
    settings[name] = idx
print "\nAfter Loop, bracket notation used, values changed successfully:\n",settings

This is probably some basic programming principle so I'm interested in 'why' so I don't make more mistakes like these. Thanks!

Answer 1

In the loop, name and val are bound to each of the objects in the mapping in turn. Simply rebinding the names will not modify the original iterable.

Answer 2

Variable names in Python are references that "point to" values. Here's how that looks before you try val = idx:

And here's what it looks like after:

If you did settings['Accel X Bias'] = idx instead, you'd get: