Question

能否请您告诉我为什么在代码中找不到num：

from collections import defaultdict
import re

result = defaultdict(list)

for l in my_list:
    k = None
    for v in l:
        if v in keywords:
            k = v
        if re.match(r'[0-9,.]+$', v):
            num = v
    if k is not None:
        result[k].append(num)

错误：

> --------------------------------------------------------------------------- NameError                                 Traceback (most recent call
> last) <ipython-input-84-31a1ed6e427e> in <module>
>      12             num = v
>      13     if k is not None:
> ---> 14         result[k].append(num)
>      15 
> 
> NameError: name 'num' is not defined

我无法理解此错误。

Answer 1

在您的代码中，num = v并不总是执行，只有在if条件为True时才运行。首先用一个零值初始化num，它将解决错误。

from collections import defaultdict
import re

result = defaultdict(list)
num = 0
for l in my_list:
    k = None
    for v in l:
        if v in keywords:
            k = v
        if re.match(r'[0-9,.]+$', v):
            num = v
    if k is not None:
        result[k].append(num)

Answer 2

仅当您的陈述为真时才创建变量：

if re.match(r'[0-9,.]+$', v):
        num = v

如果语句不正确，则不会创建该变量。

Answer 3

很简单：

from collections import defaultdict
import re

result = defaultdict(list)

for l in my_list:
    k = None
    for v in l:
        if v in keywords:
            k = v
        if re.match(r'[0-9,.]+$', v): # Here is an if statement, and num only gets defined if the condition meets it
            num = v
    if k is not None:
        result[k].append(num)

如果我评论过的if语句从未满足，则num永远不会被定义。

Answer 4

我已在下面的代码中添加了注释。 “ if条件”从未为真，因此变量num从未初始化。

     if re.match(r'[0-9,.]+$', v): # this if condition never got true
         num = v     # the initialization never executed

检查“ re.match（）” documentation也是一个好主意。\

为什么“如果条件”永远不成立，是因为在代码中您从未在字符类中包含“ $”。

if re.match(r'[0-9,.]+$', v): # the $ should be inside [0-9,.,$]
                num = v

'$' is usually a metacharacter, but inside a character class it’s stripped of its special nature。我尝试了自己的字符串。

import re
String = "0,1,2,3,4,5,6,7,8,9,.,$"

if re.match(r"[0-9,.,$]", String):# $ should be inside character class
    print("matched") # this print statement executes output is `matched`
else:
    print("not matched")

我的代码输出显示matched

尝试附加值时出现NameError

4 个答案: