Newbie question: Dictionary of lists

[Remco Gerlich]

Kossmann, Bill wrote in comp.lang.python:
> 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.

This is a problem with references. Remember that *everything* is a reference
in Python. 


> # TEST.PY ============================================================
> 
> import string
> 
> history = {}
> history['DEFAULT'] = [31,29,31,30,31,30,31,31,30,31,30,31]
> 
> # Null elements - one for each month of the year (Jan ... Dec)
> nullList = ['', '', '', '', '', '', '', '', '', '', '', '']
> 
> # Short test file (16000 lines in production)
> histFile = open("2000.txt", "r")
> for histLine in histFile.readlines():
>    histRcd = string.split(histLine, ",")
> 
> # Key is our cost centre code
>    dictionaryKey = histRcd[0]+histRcd[1]+histRcd[2]+histRcd[3]
> 
> # Null list required to add an element if it's not the first element
>    if not history.has_key(dictionaryKey):
>       history[dictionaryKey] = nullList

Note: history[dictionaryKey] now points to the same list as nullList.
Every key you do this for, history[key] points to that same list.

> # Obviously must evaluate for other months, but January will do for testing
>    if (histRcd[4][:3] == 'JAN'):
>       history[dictionaryKey][0] = float(string.strip(histRcd[5]))

And now you change that list. So now nullList is changed,
history[dictionaryKey] is changed, everywhere you point to that list, it's
changed. They're all just different names for the same list.

[snip rest]

Try initializing it with 
history[dictionaryKey] = nullList[:]

The [:] makes a slice that's equal to the whole list - except that it's
a *copy* of the list. That way, all the lists are seperate.

-- 
Remco Gerlich,  scarblac at pino.selwerd.nl
  6:07pm  up 85 days, 11 min,  6 users,  load average: 0.08, 0.02, 0.01