如何从混合元素的嵌套列表中提取IPv4及保留'-'元素

问题背景 #

给定一个混合了'-'分隔符和[IPv6, IPv4]子列表的不规则嵌套列表，需要提取所有子列表中的IPv4地址（子列表第2个元素），同时保留原列表中的'-'元素。原输入列表如下：

['-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90']]

期望输出结果：

['-','172.29.1.90','172.29.1.90','172.29.1.90','172.29.1.90','172.29.1.90','172.29.1.90','-','172.29.1.90','172.29.1.90','172.29.1.90','172.29.1.90','172.29.1.90','172.29.1.90','-','172.29.1.90','172.29.1.90','172.29.1.90','172.29.1.90','-','172.29.1.90']

常规列表推导式[ip[1] for ip in host_ip_list]会因'-'不是列表类型而触发索引错误，以下是可行的解决方案：

解决方案 #

方法1：带条件判断的列表推导式 #

利用Python的三元表达式在列表推导式中区分元素类型：

host_ip_list = ['-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90']]

filtered_host_ip_list = [ip[1] if isinstance(ip, list) else ip for ip in host_ip_list]
print(filtered_host_ip_list)

逻辑说明：遍历原列表每个元素，若元素是列表则提取其索引为1的IPv4地址，否则直接保留原元素（即'-'）。

方法2：普通循环遍历 #

如果需要更直观的代码逻辑，可使用for循环逐个处理元素：

host_ip_list = ['-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], ['fe80::a108:b52e:cfba:a646', '172.29.1.90'], '-', ['fe80::a108:b52e:cfba:a646', '172.29.1.90']]

filtered_host_ip_list = []
for item in host_ip_list:
    if isinstance(item, list):
        filtered_host_ip_list.append(item[1])
    else:
        filtered_host_ip_list.append(item)
print(filtered_host_ip_list)

逻辑说明：初始化空列表，遍历原列表时判断元素类型，按规则添加到结果列表中。

内容的提问来源于Stack Exchange，提问作者OverflowStack