• Open
• 開啟及閱讀檔案
  • readline()
  • readlines()
  • str.splitlines()
• 開啟與關閉檔案的語法糖
• 開啟及寫入/更新檔案
• 刪除檔案

這篇文章是閱讀Asabeneh的30 Days Of Python: Day 19 - File Handling後的學習筆記與心得。

這個章節的應用跟Node.js比較有關，因為在網頁上跑的JavaScript通常是不能跟本地端的檔案互動的，要透過瀏覽器，但又只能在基於Chromium的瀏覽器使用。

這邊我也是拆成兩天寫?，覺得後面的東西愈來愈複雜，跟前端的距離愈來愈遠...開始真正的學習新知了。

• Day 21 會寫如何跟檔案互動
• Day 22 則是處理不同附檔名檔案的例子

Open

使用內建函式open來開啟檔案讀寫：

語法：open(file, mode="r")

mode參數可以使用的值有：

開啟及閱讀檔案

為了方便說明，我開了一個資料夾叫test_python，並在裡面創了兩個檔案main.py以及crazy_friday.txt。透過執行main.py，去開啟並閱讀crazy_friday.txt的內容：

f = open('./crazy_friday.txt')
print(f);
# <_io.TextIOWrapper name='./crazy_friday.txt' mode='r' encoding='cp950'>

這樣會拿到一個物件，但還看不到檔案內容；要再透過這個物件的read方法回傳出字串內容：

? 如果有開啟檔案，就一定要再透過close方法關閉檔案，釋放記憶體。

f = open('./crazy_friday.txt')

txt = f.read()
print(txt)
f.close()

• read(size=-1)(預設)會印出檔案中全部文字內容：

TGIF. Let's party all the night!

Oh no! It seems all the beer in the town has been sold out.

Sad, I guess no party today...

若給size參數值，則能限定要印出多少字：

txt = f.read(5)
print(txt) # TGIF.

readline()

使用readline(size=-1)這個方法只會回傳文本中的第一行：

line = f.readline()
print(line) # TGIF. Let's party all the night!

像read一樣，可以給size參數值，指定要印到第幾個字元。

readlines()

使用readlines(hint=-1)這個方法回依據文本中的斷行(\n)，回傳一個 list：

lines = f.readlines()
print(lines) # ["TGIF. Let's party all the night!\n", '\n', 'Oh no! It seems all the beer in the town has been sold out.\n', '\n', 'Sad, I guess no party today...']

若給hint參數值，則會印到所有段落中所含總字元數不超出該值的段落：

lines = f.readlines(34)
print(lines) # ["TGIF. Let's party all the night!\n", '\n', 'Oh no! It seems all the beer in the town has been sold out.\n']

str.splitlines()

另一個可以傳回依據段落分割的 list 的方法是接著 read 使用 splitlines：

f = open('./crazy_friday.txt')

lines = f.read().splitlines()

print(lines) # ["TGIF. Let's party all the night!", '', 'Oh no! It seems all the beer in the town has been sold out.', '', 'Sad, I guess no party today...']

f.close()

開啟與關閉檔案的語法糖

前面提到如果有開檔，就一定要關檔，Python中提供了with這個 keyword 讓程式能在該 clause 執行後自動關閉檔案：

with open('./crazy_friday.txt') as f:
    line = f.readline(17)
    print(line) # TGIF. Let's party

# 下面是一般的寫法
f = open('./crazy_friday.txt')
lines = f.readlines(17)
print(lines) # ["TGIF. Let's party all the night!\n"]
f.close()

在沒有關檔的情況下繼續讀的話會：

with open('./crazy_friday.txt') as f:
    line = f.readline(17)
    lines = f.readlines(17)
    
    print(line) # TGIF. Let's party
    print(lines) # [' all the night!\n', '\n', 'Oh no! It seems all the beer in the town has been sold out.\n']

• 可以看到realines變成從readline讀完的地方繼續讀。

開啟及寫入/更新檔案

透過給 open(file, mode="r") 第二個參數做到：

• mode="a"：會在檔案結尾新增字串，如果檔案不存在，就會創造一個新的檔案：

with open('./crazy_friday.txt', 'a') as f:
    f.write('This text has to be appended at the end.')

with open('./crazy_friday.txt') as f:
    txt = f.read()
    print(txt)

"""
TGIF. Let's party all the night!

Oh no! It seems all the beer in the town has been sold out.

Sad, I guess no party today...This text has to be appended at the end.
"""

• mode="w"：會覆寫現有的檔案內容，如果檔案不存在，則會創造一個新的檔案：

with open('./crazy_friday.txt', 'w') as f:
    f.write('This text will be written in a newly created file')

with open('./crazy_friday.txt') as f:
    txt = f.read()
    print(txt) # This text will be written in a newly created file

刪除檔案

透過 os 模組(module)可以從檔案目錄中移除檔案，如果該檔案不存在，會拋出 FileNotFoundError：

import os
os.remove('./crazy_friday.txt')

可以加上一個判斷式，避免程式中斷：

import os
if os.path.exists("./crazy_friday.txt"):
    os.remove("./crazy_friday.txt")
else:
    print("The file does not exist")