python - 如何在 Python 的嵌套函数中正确引用全局变量

Question

假设我有以下简单情况:

import pandas as pd

def multiply(row):
    global results
    results.append(row[0] * row[1])

def main():
    results = []
    df = pd.DataFrame([{'a': 1, 'b': 2}, {'a': 3, 'b': 4}, {'a': 5, 'b': 6}])
    df.apply(multiply, axis=1)
    print(results)

if __name__ == '__main__':
    main()

这导致以下回溯:

Traceback (most recent call last):

  File "<ipython-input-2-58ca95c5b364>", line 1, in <module>
    main()

  File "<ipython-input-1-9bb1bda9e141>", line 11, in main
    df.apply(multiply, axis=1)

  File "C:\Users\bbritten\AppData\Local\Continuum\Anaconda3\lib\site-packages\pandas\core\frame.py", line 4262, in apply
    ignore_failures=ignore_failures)

  File "C:\Users\bbritten\AppData\Local\Continuum\Anaconda3\lib\site-packages\pandas\core\frame.py", line 4358, in _apply_standard
    results[i] = func(v)

  File "<ipython-input-1-9bb1bda9e141>", line 5, in multiply
    results.append(row[0] * row[1])

NameError: ("name 'results' is not defined", 'occurred at index 0')

我知道我可以将 results = [] 移动到 if 语句来让这个例子工作，但是有没有办法保持我现在的结构并使其发挥作用？

Answer 1

您必须在函数之外声明结果，例如:

import pandas as pd

results = []

def multiply(row):
    # the rest of your code...

更新

另请注意，python 中的 list 是可变的，因此您无需在函数开头使用 global 指定它。示例

def multiply(row):
    # global results -> This is not necessary!
    results.append(row[0] * row[1])