Python不使用truncate擦除文件内容

Question

I am having problems erasing file contents. I have a file with 'ABC' in it. I open it up and truncate() to clear everything from the file and then write new contents to it. But whatever I write just gets appended to what was previously there.

>>> handle=open('test.txt', 'r+')
>>> stuff = handle.read()
>>> stuff
'ABC'
>>> handle.truncate()
>>> handle.write('DEF'+stuff)
>>> handle.close()
>>> handle=open('test.txt', 'r+')
>>> handle.read()
'ABCDEFABC'

I think I should be getting 'DEFABC' but instead I get 'ABC' with 'DEFABC' appended to the end.

Answer 1

After handle.read() , you are at the end of the file, so there's nothing to truncate from here. Issue handle.seek(0) after handle.read() . Then write your data to the file, then truncate if necessary.

Answer 2

From the documentation of truncate , the default value is the current position. You need to pass 0 .

Truncate the file's size. If the optional size argument is present, the file is truncated to (at most) that size. The size defaults to the current position. The current file position is not changed. Note that if a specified size exceeds the file's current size, the result is platform-dependent: possibilities include that the file may remain unchanged, increase to the specified size as if zero-filled, or increase to the specified size with undefined new content. Availability: Windows, many Unix variants.

Answer 3

From the docs:

file.truncate([size])

Truncate the file's size. If the optional size argument is present, the file is truncated to (at most) that size. The size defaults to the current position.

You call handle.truncate() after calling handle.read() , at which time the current position is the end of the file, so Python truncates the file to its current size, which does nothing. You need to pass 0 as the argument to truncate : handle.truncate(0) .