Python - Using defaultdict to make dictionary of custom objects

Question

I have created the following class. Package, website and comments are all strings and distroDict is a (string, list) dictionary.

class TableEntry(object):

    def __init__(self, package, website, distroDict, comments):
        self.package = package
        self.website = website
        self.distroDict = distroDict
        self.comments = comments

I want to use defaultdict(TableEntry) to make a (string, TableEntry) custom dictionary.

tableDict = defaultdict(TableEntry)
entry = TableEntry(package, website, distroDict, comments)
tableDict[package].append(entry)

I want the package to be the key and the entry object to be the value. I can use values within the entry object but if I try to append it to tableDict I receive the following error.

Traceback (most recent call last):
  File "wiki.py", line 151, in <module>
    printMetaData(lines, f)
  File "wiki.py", line 73, in printMetaData
    tableDict[package].append(entry)
TypeError: __init__() takes exactly 5 arguments (1 given)

I have also tried the following:

tableDict[package].append(TableEntry(package, website, distroDict, comments))

And receive the same error essentially:

Traceback (most recent call last):
  File "wiki.py", line 150, in <module>
    printMetaData(lines, f)
  File "wiki.py", line 73, in printMetaData
    tableDict[package].append(TableEntry(package, website, distroDict, comments))
TypeError: __init__() takes exactly 5 arguments (1 given)

Isn't this a list of table entries, so don't you want of `defaultdict(list)` that you are `append`ing `TableEntry`s to? Default dict expects a zero parameter function for creating it's default value, so you can't actually do `defaultdict(TableEntry)`. — AChampion, Dec 17 '15 at 03:27
@AChampion wouldnt `defaultdict(list)` give me a dict with { string, [list] }? Whereas I am looking for a dict with { string, TableEntry }. I need to map a String to an object than can hold multiple pieces of information and a string, list dict will not suffice. If I cannot do `defaultdict(TableEntry)` how else could I accomplish this? — javakid1993, Dec 17 '15 at 16:10
`defaultdist(list)` will give you a a { string: [] } dictionary but as you are appending a `TableEntry`s you needed a collection, e.g. a dictionary of { string: [TableEntry, TableEntry, ...]}. However, if you just want a dictionary of `{string: TableEntry}` why do you need a `defaultdict`, wouldn't `tableDict = {}; tableDict[package] = TableEntry(...)` not work? — AChampion, Dec 18 '15 at 02:41

score 9 · Accepted Answer · edited May 23 '17 at 12:10

When you try to get something from a defaultdict it returns an instance of the default_factory you instantiated it with, which in your case is TableEntry that has no method append. But that's actually not why you are getting the error. This happens because when an instance is returned it is also evaluated, and since TableEntry doesn't have the arguments it needs, the Python interpreter will complain. But even if TableEntry was given arguments in the defaultdict call, you would probably get a TypeError because defaultdict needs to be called with a callable (and not an instance of a class, as it would be).

In summary, you cant instantiate a defaultdict with an non-callable unless you use subclassing. Instead you should do something like this:

class TableEntry(object):
    def add(self, package, website, distroDict, comments):
        self.package = package
        self.website = website
        self.distroDict = distroDict
        self.comments = comments

tableDict = defaultdict(TableEntry)
entry = ("package", "website", "distroDict", "comments")
tableDict["package"].add(*entry)

Then you can do e.g. tableDict["package"]["website"] to get the string you wrote to self.website.

Peter Wu · Answer 2 · 2021-10-24T00:41:31.310

Another approach is to make the arguments optional for the __init__ method, and assign the entry object to tableDict directly.

class TableEntry:
    def __init__(self, package=None, website=None, distroDict=None, comments=None):
        self.package = package
        self.website = website
        self.distroDict = distroDict
        self.comments = comments

tableDict = defaultdict(TableEntry)
tableDict["package"] = TableEntry("package", "website", "distroDict", "comments")

Python - Using defaultdict to make dictionary of custom objects

2 Answers2

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