如果文件不存在，Python 中的 open() 不会创建文件

Question

What is the best way to open a file as read/write if it exists, or if it does not, then create it and open it as read/write? From what I read, file = open('myfile.dat', 'rw') should do this, right?

It is not working for me (Python 2.6.2) and I'm wondering if it is a version problem, or not supposed to work like that or what.

The bottom line is, I just need a solution for the problem. I am curious about the other stuff, but all I need is a nice way to do the opening part.

The enclosing directory was writeable by user and group, not other (I'm on a Linux system... so permissions 775 in other words), and the exact error was:

IOError: no such file or directory.

Answer 1

您应该在w+模式下使用open ：

file = open('myfile.dat', 'w+')

Answer 2

The advantage of the following approach is that the file is properly closed at the block's end, even if an exception is raised on the way. It's equivalent to try-finally , but much shorter.

with open("file.dat","a+") as f:
    f.write(...)
    ...

a+ Opens a file for both appending and reading. The file pointer is at the end of the file if the file exists. The file opens in the append mode. If the file does not exist, it creates a new file for reading and writing. - Python file modes

seek() method sets the file's current position.

f.seek(pos [, (0|1|2)])
pos .. position of the r/w pointer
[] .. optionally
() .. one of ->
  0 .. absolute position
  1 .. relative position to current
  2 .. relative position from end

Only "rwab+" characters are allowed; there must be exactly one of "rwa" - see Stack Overflow question Python file modes detail .

Answer 3

Good practice is to use the following:

import os

writepath = 'some/path/to/file.txt'

mode = 'a' if os.path.exists(writepath) else 'w'
with open(writepath, mode) as f:
    f.write('Hello, world!\n')

Answer 4

Change "rw" to "w+"

Or use 'a+' for appending (not erasing existing content)

Answer 5

>>> import os
>>> if os.path.exists("myfile.dat"):
...     f = file("myfile.dat", "r+")
... else:
...     f = file("myfile.dat", "w")

r+ means read/write

Answer 6

'''
w  write mode
r  read mode
a  append mode

w+  create file if it doesn't exist and open it in write mode
r+  open for reading and writing. Does not create file.
a+  create file if it doesn't exist and open it in append mode
'''

example:

file_name = 'my_file.txt'
f = open(file_name, 'w+')  # open file in write mode
f.write('python rules')
f.close()

[FYI am using Python version 3.6.2]

Answer 7

Since python 3.4 you should use pathlib to "touch" files.
It is a much more elegant solution than the proposed ones in this thread.

from pathlib import Path

filename = Path('myfile.txt')
filename.touch(exist_ok=True)  # will create file, if it exists will do nothing
file = open(filename)

Same thing with directories:

filename.mkdir(parents=True, exist_ok=True)

Answer 8

My answer:

file_path = 'myfile.dat'
try:
    fp = open(file_path)
except IOError:
    # If not exists, create the file
    fp = open(file_path, 'w+')

Answer 9

Use:

import os

f_loc = r"C:\Users\Russell\Desktop\myfile.dat"

# Create the file if it does not exist
if not os.path.exists(f_loc):
    open(f_loc, 'w').close()

# Open the file for appending and reading
with open(f_loc, 'a+') as f:
    #Do stuff

Note: Files have to be closed after you open them, and the with context manager is a nice way of letting Python take care of this for you.

Answer 10

I think it's r+ , not rw . I'm just a starter, and that's what I've seen in the documentation.

Answer 11

open('myfile.dat', 'a') works for me, just fine.

in py3k your code raises ValueError :

>>> open('myfile.dat', 'rw')
Traceback (most recent call last):
  File "<pyshell#34>", line 1, in <module>
    open('myfile.dat', 'rw')
ValueError: must have exactly one of read/write/append mode

in python-2.6 it raises IOError .

Answer 12

What do you want to do with file? Only writing to it or both read and write?

'w' , 'a' will allow write and will create the file if it doesn't exist.

If you need to read from a file, the file has to be exist before open it. You can test its existence before opening it or use a try/except.

Answer 13

For Python 3+, I will do:

import os

os.makedirs('path/to/the/directory', exist_ok=True)

with open('path/to/the/directory/filename', 'w') as f:
    f.write(...)

So, the problem is with open cannot create a file before the target directory exists. We need to create it and then w mode is enough in this case.

Answer 14

将 w+ 用于写入文件，如果存在则截断，r+ 用于读取文件，如果文件不存在则创建一个但不写入（并返回 null）或 a+ 用于创建新文件或附加到现有文件。

Answer 15

If you want to open it to read and write, I'm assuming you don't want to truncate it as you open it and you want to be able to read the file right after opening it. So this is the solution I'm using:

file = open('myfile.dat', 'a+')
file.seek(0, 0)

Answer 16

So You want to write data to a file, but only if it doesn't already exist?.

This problem is easily solved by using the little-known x mode to open() instead of the usual w mode. For example:

 >>> with open('somefile', 'wt') as f:
 ...     f.write('Hello\n')
...
>>> with open('somefile', 'xt') as f:
...     f.write('Hello\n')
...
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
FileExistsError: [Errno 17] File exists: 'somefile'
  >>>

If the file is binary mode, use mode xb instead of xt.

Answer 17

If you just need a file, Use:

file = open('myfile.dat', 'w+')

usually there might be reminder to declear that the file doesn't exists and you make a new one. Use:

try:
    file = open('myfile.dat')
except:
    print("the file doesn't exists, make a new one")
    file = open('myfile.dat', 'w+')

Answer 18

import os, platform
os.chdir('c:\\Users\\MS\\Desktop')

try :
    file = open("Learn Python.txt","a")
    print('this file is exist')
except:
    print('this file is not exist')
file.write('\n''Hello Ashok')

fhead = open('Learn Python.txt')

for line in fhead:

    words = line.split()
print(words)