Newbie question: Dictionary of lists

[Marc Christiansen]

Kossmann, <BKossmann at dthr.ab.ca> wrote:

> Hello, Python gurus!

Much too much honor! I'm by far not a guru. :-)

> Trouble is, when I implement my FOR loop as below, *all* of the dictionary's
> January elements are updated to the value of the last January item in the
> history file.  I've searched high and low for a solution, but I'm stumped.  
[...]

> # Null list required to add an element if it's not the first element
>    if not history.has_key(dictionaryKey):
>       history[dictionaryKey] = nullList

This line is your problem. Since everything in python is a reference
(this bites everyone at least ones), for every dictionaryKey you put a
reference to nullList into the dictonary and hence modify nullList in
your code. Try

 history[dictionaryKey] = nullList[:]

This will make a copy of nullList and put a reference to this copy in
the dictionary.

I'm sure others will explain it better.

Ciao
  Marc