Can’t pickle defaultdict

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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 module-level function instead of a lambda function:

def dd():
    return defaultdict(int)

dict1 = defaultdict(dd) # dd is a module-level function

than you can pickle it

tmp = pickle.dumps(dict1) # no exception
new = pickle.loads(tmp)

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