python csv read 第一行 python csv读取一行

1. 基础语句

1.1 文件的读取

 如果需要读取一行数据如下表1所示，那么需要读取域名下面的数据，便使用如下代码：

with open('A.csv','rb') as csvfile:
    reader = csv.reader(csvfile)
    rows = [row for row in reader]

 其中每一个row就是一行['121.241.244.92', 'known attacker', '"blocklist.de (+dataplane.org,greensnow.co,rulez.sk,rutgers.edu)"']，对应的row[0]就是121.241.244.92。
那么读取某一列就很容易了，取出每一row里面的第0个元素组成colum，便是下面的语句可以取得第0列 column0 = [row[0] for row in reader] 。而取得某一行便相对复杂一些，代码如下：

import csv
with open('A.csv','rb') as csvfile:
    reader = csv.reader(csvfile)
    for i,rows in enumerate(reader):
        if i == 2:
            row = rows

 上面的代码中，运用到了enumerate函数，enumerate() 函数用于将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列，同时列出数据和数据下标，一般用在 for 循环当中。即对于一个列表中每一个元素添加一个索引。通过索引选取对于的行。
seasons = ['Spring', 'Summer', 'Fall', 'Winter']
list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]

reader列表
[['121.241.244.92', 'known attacker', '"blocklist.de (+dataplane.org,greensnow.co,rulez.sk,rutgers.edu)"'],
['121.32.150.82', 'known attacker', '"blocklist.de (+dataplane.org,greensnow.co)"'],
['121.33.237.102', 'known attacker', '"blocklist.de (+dataplane.org,greensnow.co,rutgers.edu)"']]

表1

域名
121.241.244.92	known attacker	"blocklist.de (+dataplane.org,greensnow.co,rulez.sk,rutgers.edu)"
121.32.150.82	known attacker	"blocklist.de (+dataplane.org,greensnow.co)"
121.33.237.102	known attacker	"blocklist.de (+dataplane.org,greensnow.co,rutgers.edu)"

1.2 文件的写入

 文件的写入就相对简单很多，只需记得"a+"是追加写入文件、"W"是覆盖写入即可，直接贴出下面的代码：

with open("test.csv", "a+") as csvfile:
    writer = csv.writer(csvfile)
    writer.writerow(rows)

2. 在工程中的运用

 情景：面对一份上百万的trails.csv文件，如下图所示，现在我需要把第一列的中只含有ip或者是IP:port的那一行读取出来，并且放入新的文件中。

在这就需要使用正则表达式对ip进行匹配，匹配成功输入文件中。匹配IPV4及端口号的正则 \A\d+\.[0-9.]+\Z|\A\d+\.[0-9.]+\:[0-8]+\Z。这里使python教程用到search语句进行匹配，代码如下：

with open("trails.csv", "r", encoding = "utf-8") as f:
    reader = csv.reader(f)
    for rows in reader:
        m = re.search(r"\A\d+\.[0-9.]+\Z|\A\d+\.[0-9.]+\:[0-8]+\Z", rows[0])
        if m:
            with open("test.csv", "a+") as csvfile:
                writer = csv.writer(csvfile)
                writer.writerow(rows)