我也很困惑……但也许我的答案会帮助到别人。 我假设您希望利用“w+”模式来创建不存在的文件。

实际上，如果文件不存在，只能创建w, w+， a, a+。

但是如果你需要读取文件的数据(当有数据的文件确实存在的情况下)，你不能用w+开箱即用，因为它会截断文件。哎呀，你不是那个意思!

所以，你最好的朋友可能是file.seek(0)的+:

with open('somefile.txt', 'a+') as f:
    f.seek(0)
    for line in f:
        print(f.readline())

文件被截断了，所以你可以调用read()(不会引发异常，不像使用'w'打开时那样)，但你会得到一个空字符串。

R代表读

W代表写

如果文件存在，R +为读写不删除原始内容，否则引发异常

W +表示删除原始内容，如果文件存在则读写，否则创建文件

例如,

>>> with open("file1.txt", "w") as f:
...   f.write("ab\n")
... 
>>> with open("file1.txt", "w+") as f:
...   f.write("c")
... 

$ cat file1.txt 
c$

>>> with open("file2.txt", "r+") as f:
...   f.write("ab\n")
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory: 'file2.txt'

>>> with open("file2.txt", "w") as f:
...   f.write("ab\n")
... 
>>> with open("file2.txt", "r+") as f:
...   f.write("c")
... 

$ cat file2.txt 
cb
$

Python中的所有文件模式

r for reading r+ opens for reading and writing (cannot truncate a file) w for writing w+ for writing and reading (can truncate a file) rb for reading a binary file. The file pointer is placed at the beginning of the file. rb+ reading or writing a binary file wb+ writing a binary file a+ opens for appending ab+ Opens a file for both appending and reading in binary. The file pointer is at the end of the file if the file exists. The file opens in the append mode. x open for exclusive creation, failing if the file already exists (Python 3)

假设你用一个with语句打开文件，就像你应该做的那样。然后你可以这样从你的文件中读取:

with open('somefile.txt', 'w+') as f:
    # Note that f has now been truncated to 0 bytes, so you'll only
    # be able to read data that you write after this point
    f.write('somedata\n')
    f.seek(0)  # Important: return to the top of the file before reading, otherwise you'll just read an empty string
    data = f.read() # Returns 'somedata\n'

注意f.seek(0)——如果您忘记了这一点，f.read()调用将尝试从文件末尾读取，并返回一个空字符串。

如h4z3所述， 在实际应用中， 有时你的数据太大，不能直接加载所有内容，或者你有一个生成器，或者实时传入的数据，你可以使用w+存储在一个文件中，然后读取。