来自文件的Python 3 read（）不适用于1个字节的文件

Question

I'm trying to read a simple value from a file like this:

import pdb
import string

def getOrderID():
    f = open( 'testid.txt', 'w+')
    tmp = f.read()
    print( 'tmp: ', tmp, len(tmp) )
    if tmp.isdigit():
        newid = int(newid) + 1
    else:
        newid = 1

    print( 'Newid: ', newid )

    f.seek(0)
    f.write(str(newid))
    f.close()

    return newid

print( getOrderID() )

tmp is always an empty string, size 0

newid is always 1. Why?

Answer 1

Description of the w+ mode: 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.

That's why the string is empty and newid is 1.