From the comments, I’m assuming you want a dictionary that fits the following conditions: Is initialized with set of keys with an empty list value for each Has defaultdict behavior that can initialize an empty list for non-existing keys @Aaron_lab has the right method, but there’s a slightly cleaner way: d = defaultdict(list,{ k:[] for … Read more
Instance lay-out conflict is a fancy way of saying that you’re trying to inherit from multiple–in this case built-in–types that cannot cooperate with each other quite well. More technically these classes have conflict in functionality of their common attributes. In this case both OrderedDict and defaultdict are two dictionary-like types with their own unique __setitem__ … Read more
>>> sorted(cityPopulation.iteritems(),key=lambda (k,v): v[0],reverse=True) #1990 [(‘C’, [30, 10, 20]), (‘B’, [20, 30, 10]), (‘A’, [10, 20, 30])] >>> sorted(cityPopulation.iteritems(),key=lambda (k,v): v[2],reverse=True) #2010 [(‘A’, [10, 20, 30]), (‘C’, [30, 10, 20]), (‘B’, [20, 30, 10])] Note in python 3 you can’t automagically unpack lambda arguments so you would have to change the code sorted(cityPopulation.items(), key=lambda k_v: … Read more
defaultdict docs say for default_factory: If the default_factory attribute is None, this raises a KeyError exception with the key as argument. What if you just set your defaultdict’s default_factory to None? E.g., >>> d = defaultdict(int) >>> d[‘a’] += 1 >>> d defaultdict(<type ‘int’>, {‘a’: 1}) >>> d.default_factory = None >>> d[‘b’] += 2 Traceback … Read more
Usually, a Python dictionary throws a KeyError if you try to get an item with a key that is not currently in the dictionary. The defaultdict in contrast will simply create any items that you try to access (provided of course they do not exist yet). To create such a “default” item, it calls the … Read more
All that defaultdict requires is a callable object that will return what should be used as a default value when called with no parameters. If you were to call the int constructor, it would return 0 and if you were to call lambda: 0, it would return 0. Same with the lists. The only difference … Read more
In addition to Martijn’s explanation: A module-level function is a function which is defined at module level, that means it is not an instance method of a class, it’s not nested within another function, and it is a “real” function with a name, not a lambda function. So, to pickle your defaultdict, create it with … Read more
If you are using Python 3.2+, use can use str.format_map(). For bond, bond: from collections import defaultdict ‘{bond}, {james} {bond}’.format_map(defaultdict(str, bond=’bond’)) Result: ‘bond, bond’ For bond, {james} bond: class SafeDict(dict): def __missing__(self, key): return ‘{‘ + key + ‘}’ ‘{bond}, {james} {bond}’.format_map(SafeDict(bond=’bond’)) Result: ‘bond, {james} bond’ In Python 2.6/2.7 For bond, bond: from collections import … Read more
Short answer (as per Montaro’s answer below) defaultdict(lambda:1) Long answer on how defaultdicts work ht = {} ht = defaultdict(lambda:0, ht) defaultdicts are different from dict in that when you try to access a regular dict with a key that does not exists, it raises a KeyError. defaultdict, however, doesn’t raise an error: it creates … Read more
I think the first line means that when I call x[k] for a nonexistent key k (such as a statement like v=x[k]), the key-value pair (k,0) will be automatically added to the dictionary, as if the statement x[k]=0 is first executed. That’s right. This is more idiomatically written x = defaultdict(int) In the case of … Read more