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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def all_default_fields():
 """Helper to retrieve all fields which has default value. :returns: List of default fields. :rtype: list """

    default_fields = []
    for item in dir(fields):
        if not item.startswith("__"):
            var = getattr(definitions, item)
            if isinstance(var, dict):
                if var.get('replace_null', False):
                    default_fields.append(var)
    return default_fields