Question

I am attempting to use a module called interface.py which defines a list of conditions and a few functions to check arguments against those conditions. There are many thousands of conditions however, and so I want to use a dictionary instead of a list to prevent needing to look at all of them. To do this I’m using the following code:

def listToDictionary(list):
    """This function takes a list of conditions and converts it to a dictionary
    that uses the name of the condition as a key."""

    d = {}
    for condition in list:
        if condition.name.lower() not in d:
            d[condition.name.lower()] = []
        d[condition.name.lower()].append(condition)
    return d

conditionList = listToDictionary(conditions.list) #the condition list comes from another module

Further into the file are the actual interface functions that take arguments to compare with the list of conditions – these functions are written assuming that conditionList will be a dictionary.

Unfortunately this isn’t working. Giving error details is difficult because this code is being imported by a django page and I am trying to avoid talking about django so this question stays uncomplicated. Essentially the pages including this code will not load, and if I change it back to just using a list everything works fine.

My suspicion is that the problem has to do with how Python treats import statements. I need the listToDictionary conversion to run as soon as interface.py is imported, otherwise the interface functions will expect a dictionary and get a list instead. Is there any way to ensure that this is happening?

Answer 1

An educated guess: the list in conditions.list is not yet fully constructed when your module is being imported. As a result, you get a dictionary that is missing some entries or even empty, which is causing problems later. Try deferring the construction of the dict, like this:

conditionTable = None     # shouldn't call it list if it's a dict

def get_cond_table():
    global conditionTable
    if conditionTable is None:
        conditionTable = listToDictionary(conditions.list)
    return conditionTable

Instead of referring to conditionList in your functions, refer to get_cond_table().