Python：正则表达式 findall 返回一个列表，为什么尝试访问列表元素 [0] 会返回错误？
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Python: Regex findall returns a list, why does trying to access the list element [0] return an error?
提问by Louis93
Taken from the documentation, the following is a snippet showing how the regex method findall works, and confirms that it does return a list.
以下摘自文档的片段，显示了正则表达式方法 findall 的工作原理，并确认它确实返回了一个列表。
re.findall(r"\w+ly", text)
['carefully', 'quickly']
However the following code fragment generates an out of bounds error (IndexError: list index out of range)when trying to access the zeroth element of the list returned by findall.
但是，以下代码片段IndexError: list index out of range在尝试访问 findall 返回的列表的第零个元素时会生成越界错误 ( )。
Relevant Code Fragment:
相关代码片段：
population = re.findall(",([0-9]*),",line)
x = population[0]
thelist.append([city,x])
Why does this happen?
为什么会发生这种情况？
For some more background, here's how that fragment fits into my entire script:
有关更多背景信息，以下是该片段如何适合我的整个脚本：
import re
thelist = list()
with open('Raw.txt','r') as f:
for line in f:
if line[1].isdigit():
city = re.findall("\"(.*?)\s*\(",line)
population = re.findall(",([0-9]*),",line)
x = population[0]
thelist.append([city,x])
with open('Sorted.txt','w') as g:
for item in thelist:
string = item[0], ', '.join(map(str, item[1:]))
print string
EDIT: Read comment below for some background on why this happened. My quick fix was:
编辑：阅读下面的评论，了解为什么会发生这种情况的一些背景。我的快速修复是：
if population:
x = population[0]
thelist.append([city,x])
采纳答案by nneonneo
re.findallwill return an empty list if there are no matches:
re.findall如果没有匹配项，将返回一个空列表：
>>> re.findall(r'\w+ly', 'this does not work')
[]
回答by wim
re.findallcan return you an empty list in the case where there was no match. If you try to access [][0]you will see that IndexError.
re.findall在没有匹配的情况下可以返回一个空列表。如果您尝试访问，[][0]您将看到IndexError。
To take into account no matches, you should use something along the lines of:
要考虑没有匹配项，您应该使用以下内容：
match = re.findall(...)
if match:
# potato potato
回答by James Shrum
I had this same issue. The solution seems very simple, and I don't know why I wasn't considering it.
我有同样的问题。解决方案看起来很简单，我不知道为什么我没有考虑它。
if match:
instead of
代替
if match[0]:
