Python-打开文本文件进行编辑，然后使用fileinput.input（）对其进行修改

Question

I have a python script used to edit a text file . Firstly, the first line of the text file is removed. After that, a line is added to the end of the text file .
I noticed a weird phenomenon, but I cannot explain the reason of this behaviour:

This script works as expected (removes the first line and adds a line at the end of the file):

import fileinput

# remove first line of text file
i = 0
for line in fileinput.input('test.txt', inplace=True):
    i += 1    
    if i != 1:
        print line.strip()

# add a line at the end of the file
f = open('test.txt', 'a+') # <= line that is moved
f.write('test5')
f.close()

But in the following script, as the text file is opened before removing, the removal occurs but the content isn't added (with the write() method):

import fileinput

# file opened before removing
f = open('test.txt', 'a+') # <= line that is moved

# remove first line of text file
i = 0
for line in fileinput.input('test.txt', inplace=True):
    i += 1    
    if i != 1:
        print line.strip()

# add a line at the end of the file
f.write('test5')
f.close()

Note that in the second example, open() is placed a the beginning, whereas in the first it is called after removing the last line of the text file .

What's the explanation of the behaviour?

Answer 1

When using fileinput with the inplace parameter, the modified content is saved in a backup file. The backup file is renamed to the original file when the output file is closed. In your example, you do not close the fileinput file explicitly, relying on the self-triggered closing, which is not documented and might be unreliable.

The behaviour you describe in the first example is best explained if we assume that opening the same file again triggers fileinput.close() . In your second example, the renaming only happens after f.close() is executed, thus overwriting the other changes (adding "test5").

So apparently you should explicitly call fileinput.close() in order to have full control over when your changes are written to disk. (It is generally recommended to release external resources explicitly as soon as they are not needed anymore.)

EDIT:

After more profound testing, this is what I think is happening in your second example:

• You open a stream with mode a+ to the text file and bind it to the variable f .
• You use fileinput to alter the same file. Under the hood, a new file is created, which is afterwards renamed to what the file was called originally. Note: this doesn't actually change the original file – rather, the original file is made inaccessible, as its former name now points to a new file.
• However, the stream f still points to the original file (which has no file name anymore). You can still write to this file and close it properly, but you cannot see it anymore (since it has no filename anymore).

Please note that I'm not an expert in this kind of low-level operations; the details might be wrong and the terminology certainly is. Also, the behaviour might be different across OS and Python implementations. However, it might still help you understand why things go different from what you expected.

In conclusion I'd say that you shouldn't be doing what you do in your second example. Why don't you just read the file into memory, alter it, and then write it back to disk? Actual in-place (on-disk) altering of files is no fun in Python, as it's too high-level a language.