List of dictionaries, in a dictionary - in Python

Question

I have a case where I need to construct following structure programmatically (yes I am aware of .setdefault and defaultdict but I can not get what I want)

I basically need a dictionary, with a dictionary of dictionaries created within the loop. At the beginning the structure is completely blank.

structure sample (please note, I want to create an array that has this structure in the code!)

RULE = {
     'hard_failure': {
        4514 : {
           'f_expr' = 'ABC',
           'c_expr' = 'XF0',
     }
    }
   }

pseudo code that needs to create this:

...
self.rules = {}
for row in rows:
     a = 'hard_failure'
     b = row[0] # 4514
     c = row[1] # ABC
     d = row[2] # XF0
     # Universe collapse right after
     self.rules = ????
...

The code above is obviously not working since I dont know how to do it!

Answer 1

Example, that you've posted is not a valid python code, I could only imagine that you're trying to do something like this:

self.rules[a] = [{b:{'f_expr': c, 'c_expr': d}}]

this way self.rules is a dictionary of a list of a dictionary of a dictionary. I bet there is more sane way to do this.

Answer 2

Your example code doesn't seem to be valid Python. It's not clear if the second level element is supposed to be a list or a dictionary.

However, if you're doing what I think you're doing, and it's a dictionary, you could use a tuple as a key in the top-level dictionary instead of nesting dictionaries:

>>> a = 'hard_failure'
>>> b = 4514
>>> c = "ABC"
>>> d = "XF0"
>>> rules = {}
>>> rules[(a,b)] = {'f_expr' : a,'c_expr' : d}
>>> rules
{('hard_failure', 4514): {'c_expr': 'XF0', 'f_expr': 'hard_failure'}}

Answer 3

My favorite way to deal with nested dictionaries & lists of dictionaries is to use PyYAML. See this response for details.

Answer 4

rules = {}
failure = 'hard_failure'
rules[failure] = []
for row in rows:
  #this is what people are referring to below.  You left out the addition of the    dictionary structure to the list.
  rules[failure][row[0]] = {} 
  rules[failure][row[0]]['type 1'] = row[1]
  rules[failure][row[0]]['type 2'] = row[2]

This is what I created based on how I understood the questions. I wasn't sure what to call the 'f_expr' and 'c_expr' since you never mention where you get those but I assume they are already know column names in a resultset or structure of some sort.

Just keep adding to the structure as you go.

Answer 5

Well, I apologize for the confusion, I never claimed that code actually compiled, hence (pseudo). Arthur Thomas put me on the right track, here is slightly modified version. (Yes, now its a simply nested dictionary, 3 levels down)

    RULE_k = 'hard_failure'
    self.rules = {}
    for row in rows:
           self.rules_compiled.setdefault(RULE_k, {})
           self.rules_compiled[RULE_k][row[1]] = {}
           self.rules_compiled[RULE_k][row[1]]['f_expr'] = row[0]
           self.rules_compiled[RULE_k][row[1]]['c_expr'] = row[1]