附加到Python中的嵌套列表或dict

Question

我经常发现自己通过逐行读取文件来填充列表和字符串.

假设我正在读一份人和他们喜欢的食物清单:

ANNE      CHEESE  
ANNE      POTATO 
JOE       PEAS    
JOE       CHIPS   
JOE       FISH
BERNARD   LENTILS

到Python字典:

{
 "ANNE"   : ["CHEESE", "POTATO"], 
 "JOE"    : ["PEAS",   "CHIPS",   "FISH"],
 "BERNARD": ["LENTILS"]
}

我使用的一般模式是逐行读取文件,在每种情况下,在尝试追加之前检查密钥是否已经存在.今天我决定概括一下并编写一个safe_append函数,在附加到列表或设置字典键之前创建相关对象:

def safe_append(list_object, list_key, list_value, value_dict_key= None):
    # Add empty dict if it does not already exist
    if list_key not in list_object:
        if value_dict_key is not None:
            list_object[list_key] = {}
        else: 
            list_object[list_key] = []
    # Append/set value
    if value_dict_key is not None:
        list_object[list_key][value_dict_key] = list_value
    else: 
        list_object[list_key].append(list_value)
    # Return object (for chaining)
    return list_object 

# Usage: dict in dict
x = {}
safe_append(x, "a","b",value_dict_key = "c")
>>> {"a":{"c":"b"}}
# Usage: list in dict
x = []
safe_append(x, "a","b")
>>> {"a":["b"]}

这看起来相当笨拙和丑陋.我的问题:有没有更好/更pythonic的方式来做到这一点？

Answer 1

更好的方法是使用defaultdict:

>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> d["hello"].append(1)
>>> d["hello"].append(2)
>>> dict(d)
{'hello':[1, 2]}