面試題-python 如何讀取一個大於 10G 的txt檔案？

悠悠發表於2021-03-03

前言

用python 讀取一個大於10G 的檔案，自己電腦只有8G記憶體，一執行就報記憶體溢位：MemoryError
python 如何用open函式讀取大檔案呢？

讀取大檔案

首先可以自己先製作一個大於10G的txt檔案

a = '''
2021-02-02 21:33:31,678 [django.request:93] [base:get_response] [WARNING]- Not Found: /http:/123.125.114.144/
2021-02-02 21:33:31,679 [django.server:124] [basehttp:log_message] [WARNING]- "HEAD http://123.125.114.144/ HTTP/1.1" 404 1678
2021-02-02 22:14:04,121 [django.server:124] [basehttp:log_message] [INFO]- code 400, message Bad request version ('HTTP')
2021-02-02 22:14:04,122 [django.server:124] [basehttp:log_message] [WARNING]- "GET ../../mnt/custom/ProductDefinition HTTP" 400 -
2021-02-02 22:16:21,052 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/login HTTP/1.1" 301 0
2021-02-02 22:16:21,123 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/login/ HTTP/1.1" 200 3876
2021-02-02 22:16:21,192 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/img/main_bg.png HTTP/1.1" 200 2801
2021-02-02 22:16:21,196 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/iconfont/style.css HTTP/1.1" 200 1638
2021-02-02 22:16:21,229 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/img/bg.jpg HTTP/1.1" 200 135990
2021-02-02 22:16:21,307 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/iconfont/fonts/icomoon.ttf?u4m6fy HTTP/1.1" 200 6900
2021-02-02 22:16:23,525 [django.server:124] [basehttp:log_message] [INFO]- "POST /api/login/ HTTP/1.1" 302 0
2021-02-02 22:16:23,618 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/index/ HTTP/1.1" 200 18447
2021-02-02 22:16:23,709 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/js/commons.js HTTP/1.1" 200 13209
2021-02-02 22:16:23,712 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/css/admin.css HTTP/1.1" 200 19660
2021-02-02 22:16:23,712 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/css/common.css HTTP/1.1" 200 1004
2021-02-02 22:16:23,714 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/js/app.js HTTP/1.1" 200 20844
2021-02-02 22:16:26,509 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/report_list/1/ HTTP/1.1" 200 14649
2021-02-02 22:16:51,496 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/test_list/1/ HTTP/1.1" 200 24874
2021-02-02 22:16:51,721 [django.server:124] [basehttp:log_message] [INFO]- "POST /api/add_case/ HTTP/1.1" 200 0
2021-02-02 22:16:59,707 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/test_list/1/ HTTP/1.1" 200 24874
2021-02-03 22:16:59,909 [django.server:124] [basehttp:log_message] [INFO]- "POST /api/add_case/ HTTP/1.1" 200 0
2021-02-03 22:17:01,306 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/edit_case/1/ HTTP/1.1" 200 36504
2021-02-03 22:17:06,265 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/add_project/ HTTP/1.1" 200 17737
2021-02-03 22:17:07,825 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/project_list/1/ HTTP/1.1" 200 29789
2021-02-03 22:17:13,116 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/add_config/ HTTP/1.1" 200 24816
2021-02-03 22:17:19,671 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/config_list/1/ HTTP/1.1" 200 19532
'''
while True:
    with open("xxx.log", "a", encoding="utf-8") as fp:
         fp.write(a)

迴圈寫入到 xxx.log 檔案，執行 3-5 分鐘，pycharm 開啟檢視檔案大小大於 10G

於是我用open函式直接讀取

f = open("xxx.log", 'r')
print(f.read())
f.close()

丟擲記憶體溢位異常:MemoryError

Traceback (most recent call last):
  File "D:/2021kecheng06/demo/txt.py", line 35, in <module>
    print(f.read())
MemoryError

執行的時候可以看下自己電腦的記憶體已經佔了100%， cpu高達91% ，不掛掉才怪了！

這種錯誤的原因在於，read()方法執行操作是一次性的都讀入記憶體中，顯然檔案大於記憶體就會報錯。

read() 的幾種方法

1.read() 方法可以帶引數 n， n 是每次讀取的大小長度，也就是可以每次讀一部分，這樣就不會導致記憶體溢位

f = open("xxx.log", 'r')
print(f.read(2048))
f.close()

執行結果

2019-10-24 21:33:31,678 [django.request:93] [base:get_response] [WARNING]- Not Found: /http:/123.125.114.144/
2019-10-24 21:33:31,679 [django.server:124] [basehttp:log_message] [WARNING]- "HEAD http://123.125.114.144/ HTTP/1.1" 404 1678
2019-10-24 22:14:04,121 [django.server:124] [basehttp:log_message] [INFO]- code 400, message Bad request version ('HTTP')
2019-10-24 22:14:04,122 [django.server:124] [basehttp:log_message] [WARNING]- "GET ../../mnt/custom/ProductDefinition HTTP" 400 -
2019-10-24 22:16:21,052 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/login HTTP/1.1" 301 0
2019-10-24 22:16:21,123 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/login/ HTTP/1.1" 200 3876
2019-10-24 22:16:21,192 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/img/main_bg.png HTTP/1.1" 200 2801
2019-10-24 22:16:21,196 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/iconfont/style.css HTTP/1.1" 200 1638
2019-10-24 22:16:21,229 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/img/bg.jpg HTTP/1.1" 200 135990
2019-10-24 22:16:21,307 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/iconfont/fonts/icomoon.ttf?u4m6fy HTTP/1.1" 200 6900
2019-10-24 22:16:23,525 [django.server:124] [basehttp:log_message] [INFO]- "POST /api/login/ HTTP/1.1" 302 0
2019-10-24 22:16:23,618 [django.server:124] [basehttp:log_message] [INFO]- "GET /api/index/ HTTP/1.1" 200 18447
2019-10-24 22:16:23,709 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/js/commons.js HTTP/1.1" 200 13209
2019-10-24 22:16:23,712 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/css/admin.css HTTP/1.1" 200 19660
2019-10-24 22:16:23,712 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/css/common.css HTTP/1.1" 200 1004
2019-10-24 22:16:23,714 [django.server:124] [basehttp:log_message] [INFO]- "GET /static/assets/js/app.js HTTP/1.1" 200 20844
2019-10-24 22:16:26,509 [django.server:124] [basehttp:log_message] [I

這樣就只讀取了2048個字元，全部讀取的話，迴圈讀就行

f = open("xxx.log", 'r')
while True:
    block = f.read(2048)
    print(block)
    if not block:
        break
f.close()

2.readline()：每次讀取一行，這個方法也不會報錯

f = open("xxx.log", 'r')

while True:
    line = f.readline()
    print(line, end="")
    if not line:
        break
f.close()

3.readlines()：讀取全部的行，生成一個list，通過list來對檔案進行處理，顯然這種方式依然會造成：MemoyError

真正 Pythonic 的方法

真正 Pythonci 的方法，使用 with 結構開啟檔案，fp 是一個可迭代物件，可以用 for 遍歷讀取每行的檔案內容

with open("xxx.log", 'r') as fp:
    for line in fp:
        print(line, end="")

yield 生成器讀取大檔案

前面一篇講yield 生成器的時候提到讀取大檔案，函式返回一個可迭代物件，用next()方法讀取檔案內容

def read_file(fpath):
    BLOCK_SIZE = 1024
    with open(fpath, 'rb') as f:
        while True:
            block = f.read(BLOCK_SIZE)
            if block:
                yield block
            else:
                return
if __name__ == '__main__':
    a = read_file("xxx.log")
    print(a)            # generator objec
    print(next(a))      # bytes型別
    print(next(a).decode("utf-8"))   # str