為什麼要用檔案

python的列表等資料結構很好用，然而這些程式碼中的資料都是耦合較高的。

時常需要用到外部資料，也就離不開檔案、資料庫...

怎麼使用檔案

關於檔案，python的基本輸入機制是基於行的。

open() BIF 用來開啟檔案。

close() BIF 用來關閉檔案。

檔案常用操作示例

sketch.py

#open the data file, and read the first two lines.
data = open('sketch.txt')
print(data.readline(),end = '')
print(data.readline(),end = '')
data.close()
 

data = open('sketch.txt')
data.seek(0)    #return to the head of the file.
for each_line in data:
    if not each_line.find(':')==-1:
        (role, word) = each_line.split(':',1)
        print(role, end = '')
        print(' said: ',end = '')
        print(word, end ='')
data.close()

程式碼段3

#<handle exception,suggested method>
try:
    data = open('sketch.txt')
    for each_line in data:
        try:
            (role, word) = each_line.split(':',1)
            print(role, end = '')
            print(' said: ',end = '')
            print(word, end ='')
        except ValueError:
            pass

    data.close()
except IOError:
    print('The data file is missing!')
 

程式碼段1是讀入檔案並輸出前兩行的示例。

關於異常處理

後面兩段程式碼，主要用於處理一個不太特殊的“特殊情況”：非標準格式輸入造成的程式錯誤。

解決的方法 要麼給出額外的程式碼邏輯去處理，要麼通過異常處理方法解決。

前者的思路很清晰，每次做一件事前先判斷是否存在問題，但問題在於：

當需要解決的錯誤多了，程式碼很臃腫，程式碼本來的功能不突出。而且需要人工考慮各種各樣的錯誤，這本身往往並不可行。

後者允許錯誤發生，在錯誤發生時捕捉並進行處理。

兩種處理方法見程式碼段2、3。

1.find(str)方法可以查詢str在目標字串中出現的位置下標， 如果沒有出現過該str，則返回-1.

2.split(mark)用於分割字串，和java的String.split()方法效果類似。

3.try/except類似與java的try/catch 程式碼段3針對檔案不存在，檔案行分割錯誤進行了異常處理。

進一步改進

以上程式碼需要進一步改進

1.如果發生了異常 關閉檔案語句不會被執行，這可能造成一系列嚴重問題->finally語句

2.還可以讓try/except更簡潔一點 ->with語句，自帶except功能 

（稍後總結）

附：sketch.txt內容。

Man: Is this the right room for an argument?
Other Man: I've told you once.
Man: No you haven't!
Other Man: Yes I have.
Man: When?
Other Man: Just now.
Man: No you didn't!
Other Man: Yes I did!
Man: You didn't!
Other Man: I'm telling you, I did!
Man: You did not!
Other Man: Oh I'm sorry, is this a five minute argument, or the full half hour?
Man: Ah! (taking out his wallet and paying) Just the five minutes.
Other Man: Just the five minutes. Thank you.
Other Man: Anyway, I did.
Man: You most certainly did not!
Other Man: Now let's get one thing quite clear: I most definitely told you!
Man: Oh no you didn't!
Other Man: Oh yes I did!
Man: Oh no you didn't!
Other Man: Oh yes I did!
Man: Oh look, this isn't an argument!
(pause)
Other Man: Yes it is!
Man: No it isn't!
(pause)
Man: It's just contradiction!
Other Man: No it isn't!
Man: It IS!
Other Man: It is NOT!
Man: You just contradicted me!
Other Man: No I didn't!
Man: You DID!
Other Man: No no no!
Man: You did just then!
Other Man: Nonsense!
Man: (exasperated) Oh, this is futile!!
(pause)
Other Man: No it isn't!
Man: Yes it is!