python匹配两个文件中相同的内容

data_small.txt中内容如下： 343 0 5258 1 3973 2 data_big.txt中内容如下： 343 2009-05-30T17:01:58Z 39.04183745 -94.5914053833 9191 343 2009-05-28T23:40:31Z 39.0523183095 -94.6074986458 8904 23 2009-05-28T23:40:31Z 39.0523183095 -94.6074986458 8904 56 2009-05-27T18:59:50Z 39.0424168 -94.59061145 9188 5258 2009-05-15T00:09:42Z 38.9920234667 -94.5920920333 10927 5258 2009-05-27T18:59:50Z 39.0424168 -94.59061145 9188 545 2009-05-15T00:09:42Z 38.9920234667 -94.5920920333 10927 3973 2009-05-14T20:47:20Z 39.0142536 -94.5928215833 12305 3973 2009-05-14T20:43:05Z 39.0146281324 -94.5907831192 9627

需求：将data_big中有data_small第一列所对应的那一行重新写入新的text文件。 即得到new_data.txt如下： 343 2009-05-30T17:01:58Z 39.04183745 -94.5914053833 9191 343 2009-05-28T23:40:31Z 39.0523183095 -94.6074986458 8904 5258 2009-05-15T00:09:42Z 38.9920234667 -94.5920920333 10927 5258 2009-05-27T18:59:50Z 39.0424168 -94.59061145 9188 3973 2009-05-14T20:47:20Z 39.0142536 -94.5928215833 12305 3973 2009-05-14T20:43:05Z 39.0146281324 -94.5907831192 9627

代码：

''' 根据data_small筛选数据集，得到新的小数据集。 '''
fid = open('data_new', 'w')
with open(r'./data_small.txt', mode='r', encoding='utf8') as rf1, open(r'./data_big.txt', mode='r', encoding='utf8') as rf2:
content1 = rf1.readlines(-1)  # 读取所有行
content2 = rf2.readlines(-1)
for i in content1:
x_1 = i.split()
for j in content2:
x_2 = j.split()
if x_1[0] == x_2[0]:  # 如果相同写入新的文件
fid.write(j)
else:
pass
fid.close()

以上代码时间复杂度太高，可用以下代码：

fid = open('data_new_1', 'w')
with open(r'./data_small.txt', mode='r', encoding='utf8') as rf1, open(r'./data_big.txt', mode='r', encoding='utf8') as rf2:
content1 = rf1.readlines(-1)  # 读取所有行
user_id = [line.split()[0] for line in content1]
content2 = rf2.readlines(-1)
for j in content2:
x_2 = j.split()
if x_2[0] in user_id:  
fid.write(j)
fid.close()

tips：

r只读，r+读写，文件不存在报错 w只写，w+读写，若文件不存在可创建，新写入内容会覆盖之前内容 a附加写，不可读，a+附加读写，若文件不存在可创建，可追加写，不覆盖

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