为什么python文件描述符“ w +”用于读取时会擦除文件内容？

Question

contents of foo.txt

Hello World!

Two functions :

def read1(file):
    with open(file, 'r+') as fp:
        print fp.read()

def read2(file):
    with open(file, 'w+') as fp:
        print fp.read()

the function read1('foo.txt') outputs: Hello World!

the function read2('foo.txt') outputs:

It essentially outputs nothing. But why?

Not only that, the contents of foo.txt has been erased as well.

I thought that w+ means you truncate the file, meaning you write over it. But I did not even use fp.write() .

Answer 1

"Truncate the file" does not mean writing over existing contents (but leaving that content present until it's overwritten); rather, it means entirely removing everything past the point at which the file is truncated. In this case, that point is the very beginning of the file.

If you want to open the file and be able to read existing contents before performing an in-place write over any subset, r+ is the appropriate mode.

---

Quoting a definition of "truncate" from WordNet 3.0, to "truncate" something is to cut it off, or cut it short:

truncate

3: make shorter as if by cutting off; "truncate a word"; "Erosion has truncated the ridges of the mountains" [syn: truncate, cut short]

Thus, the name matches well with the behavior at hand.