A basic dictionary question

[Peter Otten]

David Aldrich wrote:

> Hi
> 
> I am fairly new to Python.  I am writing some code that uses a dictionary
> to store definitions of hardware registers.  Here is a small part of it:
> 
> import sys
> 
> register = {
>     'address'  : 0x3001c,
>     'fields' : {
>         'FieldA' : {
>             'range' : (31,20),
>         },
>         'FieldB' : {
>             'range' : (19,16),
>         },
>     },
>     'width' : 32
> };
> 
> def main():
>     fields = register['fields']
>     for field, range_dir in fields:         <== This line fails
>         range_dir = field['range']
>         x,y = range_dir['range']
>         print(x, y)
> 
> if __name__ == '__main__':
>     main()
> 
> I want the code to print the range of bits of each field defined in the
> dictionary.
> 
> The output is:
> 
> Traceback (most recent call last):
>   File "testdir.py", line 32, in <module>
>     main()
>   File "testdir.py", line 26, in main
>     for field, range_dir in fields:
> ValueError: too many values to unpack (expected 2)
> 
> Please will someone explain what I am doing wrong?

for key in some_dict:
    ...

iterates over the keys of the dictionary, for (key, value) pairs you need

for key, value in some_dict.items():
    ...

 
> Also I would like to ask how I could print the ranges in the order they
> are defined.  Should I use a different dictionary class or could I add a
> field to the dictionary/list to achieve this?

Have a look at collections.OrderedDict:

https://docs.python.org/dev/library/collections.html#collections.OrderedDict

If you don't care about fast access by key you can also use a list of 
(key, value) pairs.