TypeError: ‘NoneType’ 对象在 Python 中不可迭代

TypeError: ‘NoneType’ object is not iterable in Python

当我们尝试迭代一个None值时，会出现 Python“TypeError: ‘NoneType’ object is not iterable”。要解决该错误，找出变量在何处被赋值None并更正赋值或检查变量是否None在迭代之前未存储。

typeerror nonetype 对象不可迭代

下面是错误如何发生的示例。

主程序


my_list = None

# ⛔️ TypeError: 'NoneType' object is not iterable
for el in my_list:
    print(el)

我们正在尝试迭代一个None值，但该值None不可迭代，这会导致错误。

最常见的None价值来源是：

有一个不返回任何东西的函数（None隐式返回）。
明确地将变量设置为None.
将变量分配给调用不返回任何内容的内置函数的结果。
具有仅在满足特定条件时才返回值的函数。

不显式返回值的函数 return None。

主程序


# 👇️ this function returns None
def get_list():
    print(['a', 'b', 'c'])


# 👇️ None
my_list = get_list()

# ⛔️ TypeError: 'NoneType' object is not iterable
for el in my_list:
    print(el)

您可以使用return语句从函数返回值。

主程序


def get_list():
    return ['a', 'b', 'c']  # 👈️ return value


# 👇️ ['a', 'b', 'c']
my_list = get_list()

for el in my_list:
    print(el) # 👉️ a, b, c

如果您需要在迭代之前if检查变量是否未存储
值，请使用语句。None

主程序


my_list = None

if my_list is not None:
    for i in my_list:
        print(i)
else:
    # 👇️ this runs
    print('variable stores a None value')

或者，您可以提供一个空列表作为后备。

主程序


my_list = None

for i in my_list or []:
    print(i)

请注意，有许多内置函数（例如sort()）可以就地改变原始对象并返回None。

Make sure you aren’t storing the result of calling one in a variable.

Another common cause of the error is having a function that returns a value only
if a condition is met.

main.py


def get_list(a):
    if len(a) > 3:
        return a


# 👇️ None
my_list = get_list(['a', 'b'])

# ⛔️ TypeError: 'NoneType' object is not iterable
for i in my_list:
    print(i)

The if statement in the get_list function is only run if the passed in
argument has a length greater than 3.

In all other cases, the function doesn’t return anything and ends up implicitly returning None.

To solve the error, you either have to check if the function didn’t return
None, or return a default value if the condition is not met.

main.py


def get_list(a):
    if len(a) > 3:
        return a

    return [] # 👈️ return empty list if condition not met


# 👇️ []
my_list = get_list(['a', 'b'])

for i in my_list:
    print(i)

Now the function is guaranteed to return a value regardless if the condition is
met.

If you need to check if an object is iterable, use a try/except statement.

main.py


my_str = 'hello'

try:
    my_iterator = iter(my_str)

    for i in my_iterator:
        print(i) # 👉️ h, e, l, l, o
except TypeError as te:
    print(te)

The iter() function
raises a TypeError if the passed in value doesn’t support the __iter__()
method or the sequence protocol (the __getitem__() method).

If we pass a non-iterable object like a None value to the iter() function,
the except block is run.

main.py


my_list = None

try:
    my_iterator = iter(my_list)

    for i in my_iterator:
        print(i)
except TypeError as te:
    print(te)  # 👉️ 'NoneType' object is not iterable

Examples of iterables include all sequence types (list, str, tuple) and
some non-sequence types like dict, file objects and other objects that define
an __iter__() or a __getitem__() method.

Conclusion #

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