NameError：名称“reduce”未在 Python 中定义

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NameError：名称“reduce”未在 Python 中定义

NameError: name ‘reduce’ is not defined in Python

Python“NameError: name ‘reduce’ is not defined”发生在我们使用该
reduce()函数而不先导入它时。functools要解决该错误，请在使用之前
从模块中导入函数- from functools import reduce。

nameerror name reduce 未定义

下面是错误如何发生的示例。

主程序
def do_math(accumulator, current):
    return accumulator + current

# ⛔️ NameError: name 'reduce' is not defined
print(reduce(do_math, [1, 2, 3], 0))  # 👉️ 6

为了解决这个错误，我们必须从
模块中导入reduce函数。functools

主程序
from functools import reduce


def do_math(accumulator, current):
    return accumulator + current


print(reduce(do_math, [1, 2, 3], 0))  # 👉️ 6

reduce
函数采用以下 3 个参数：

姓名 描述
功能 一个带有 2 个参数的函数 – 累加值和可迭代的值。
可迭代的 iterable 中的每个元素都将作为参数传递给函数。
初始值设定项 一个可选的初始化值，在计算中放置在可迭代项之前。

你会经常看到reduce()函数被传递了一个lambda：

主程序
from functools import reduce


# 👇️ 6
print(
    reduce(
        lambda accumulator, current: accumulator + current,
        [1, 2, 3],
        0
    )
)

例子中的lambda函数以累计值和当前值作为参数，返回两者之和。

如果我们为参数提供一个值initializer，它会被放在计算中可迭代项的前面。

主程序
from functools import reduce


def do_math(accumulator, current):
    print(accumulator) # 👉️ is 0 on first iteration
    return accumulator + current


# 👇️ 6
print(
    reduce(
        do_math, [1, 2, 3],
        0  # 👈️ initializer set to 0
    )
)

在示例中，我们传递0了初始化参数，因此 的值
accumulator将出现0在第一次迭代中。

The value of the accumulator would get set to the first element in the
iterable if we didn’t pass a value for the initializer.

main.py
def do_math(accumulator, current):
    print(accumulator)  # 👉️ is 1 on first iteration
    return accumulator + current


# 👇️ 6
print(
    reduce(
        do_math, [1, 2, 3],
    )
)

If the iterable is empty and the initializer is provided, the initializer
is returned.

main.py
from functools import reduce


def do_math(accumulator, current):
    return accumulator + current


# 👇️ 100
print(
    reduce(
        do_math,
        [],
        100  # 👈️ initializer set to 100
    )
)

If the initializer is not provided and the iterable contains only 1 item,
the first item is returned.

main.py
from functools import reduce


def do_math(accumulator, current):
    return accumulator + current


# 👇️ 123
print(
    reduce(
        do_math,
        [123],
    )
)

The list in the example only contains a single element and we didn’t provide a
value for the initializer, so the reduce() function returned the list
element.

Conclusion #

The Python “NameError: name ‘reduce’ is not defined” occurs when we use the
reduce() function without importing it first. To solve the error, import the
function from the functools module before using it –
from functools import reduce.

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