如何处理Python字典中丢失的键

Question

我有一个包含键和值的字典列表。不幸的是，某些键对某些词典不可用。例如。1969 年不可用。当我运行代码时，输​​出卡住了，因为没有“年份”键。我怎样才能让程序继续到 1970 等等？

我是编程新手，不知道如何进行。

testdict = [{"brand": "ford", "model": "Mustang", "year": 1964},
            {"brand": "ford", "model": "Mustang", "year": 1965},
            {"brand": "ford", "model": "Mustang", "year": 1966},
            {"brand": "ford", "model": "Mustang", "year": 1967},
            {"brand": "ford", "model": "Mustang", "year": 1968},
            {"brand": "ford", "model": "Mustang"},
            {"brand": "ford", "model": "Mustang", "year": 1970},
            {"brand": "ford", "model": "Mustang", "year": 1971},
            {"brand": "ford", "model": "Mustang", "year": 1972},
            {"brand": "ford", "model": "Mustang", "year": 1973},
            {"brand": "ford", "model": "Mustang", "year": 1974},]

for x in testdict:
    print(x["brand"], x["year"])

我得到这个输出：

福特 1964

福特 1965

福特 1966

福特 1967

福特 1968

KeyError Traceback（最近一次调用最后一次） in () 1 for x in testdict: ----> 2 print(x["brand"], x["year"])

关键错误：'年'

如何跳过字典中不存在的值？

Answer 1

get 有用：

{ "exist": True }["notExist"]
# => KeyError
{ "exist": True }.get("notExist")
# => Null
{ "exist": True }.get("notExist", 17)
# => 17

for x in testdict:
    print(x["brand"], x.get("year", "N/A"))

另外，in：

"notExist" in { "exist": True }
# => False

for x in testdict:
    if "year" in x:
        print(x["brand"], x["year"])

您还可以使用 捕获异常except，遵循 EAFP 原则（请求宽恕比许可更容易）：

for x in testdict:
    try:
        print(x["brand"], x["year"])
    except KeyError:
        pass # didn't want to print that anyway

Answer 2

我们可用的选择之一是将代码放在 try- except 块中。对于你的情况，它应该是这样的

for data in testlist:
    try:
        print(data['brand'],data['year'])
    except KeyError as err:
        # Do something to handle the error

或者，如果您只想在字典中找不到键时返回特定值，则可以使用 dict 类的 get 成员方法：

dictionary.get('Ask for a key',default='Return this in case key not found')