Python文件读写不起作用

Question

I am making a game for Python and in it there is code to write to a file the answer for when I am running it myself to prevent myself having to actually play the game. I have already coded for writing and reading a file which works fine, but for the cheat .txt file, printing its contents returns only [] .

Here's a shortened example of what is happening.

file = open("E:\\ICT and Computer Science\\Python\\GCSE\\cheat.txt", "a+")
text = file.readlines()
print(text)
[]
file.close()



file = open("E:\\ICT and Computer Science\\Python\\GCSE\\cheat.txt", "r+")
text = file.readlines()
print(text)
['xcfghujiosdfnonoooooowhello']

It appears now on a network machine that the a+ does not work, but r+. I fully understand the functionality of each mode, but can anyone suggest why it cannot read (or write, which returns the argument's length) when in a+ mode?

Note a+ is the required mode as it needs to be appended to the file.

Edit: as I am typing file.write() , the small box assisting you apply arguments reads 'See source or doc'.

Answer 1

Take a look at opening modes (python uses same modes as C fopen ) http://www.manpagez.com/man/3/fopen/

 ``r''   Open text file for reading.  The stream is positioned at the
         beginning of the file.

 ``r+''  Open for reading and writing.  The stream is positioned at the
         beginning of the file.

 ``w''   Truncate to zero length or create text file for writing.  The
         stream is positioned at the beginning of the file.

 ``w+''  Open for reading and writing.  The file is created if it does not
         exist, otherwise it is truncated.  The stream is positioned at
         the beginning of the file.

 ``a''   Open for writing.  The file is created if it does not exist.  The
         stream is positioned at the end of the file.  Subsequent writes
         to the file will always end up at the then current end of file,
         irrespective of any intervening fseek(3) or similar.

 ``a+''  Open for reading and writing.  The file is created if it does not
         exist.  The stream is positioned at the end of the file.  Subse-
         quent writes to the file will always end up at the then current
         end of file, irrespective of any intervening fseek(3) or similar.

You can clearly see in descrpition of 'a+' mode that stream is positioned at the end of the file. So at this point if you perform reading it wil lcontinue from current position (end of the file), and thus your output.

To have proper output in this case you can use file.seek() function like this:

with open("E:\\ICT and Computer Science\\Python\\GCSE\\cheat.txt", "a+") as file:
    file.seek(0)
    text = file.readlines()
    print(text)

['actual output']

Answer 2

This is happening because of the file descriptor (fd), is at the end of the file, If you need to move the fd to the start of the file,

import os
file = open("E:\\ICT and Computer Science\\Python\\GCSE\\cheat.txt", "a+")
os.lseek(file, 0, 0)
text = file.readlines()
print(text)
['xcfghujiosdfnonoooooowhello']

Answer 3

in case of a+ both reading and writing you can have two functions seaparately for reading before file.readlines() you should have file.seek(0) for positioning of file descriptor at start of file

code :

file = open("E:\\ICT and Computer Science\\Python\\GCSE\\cheat.txt", "a+")
file.seek(0)
text = file.readlines()
print(text)
file.close()

it will work perfectly