医生说,

模式'r+', 'w+'和'a+'打开文件进行更新(注意'w+'会截断文件)。在区分二进制文件和文本文件的系统上,在模式后追加'b'以二进制模式打开文件;在没有这种区别的系统中,添加“b”没有任何效果。

这里

w+:打开文件进行读写操作。如果文件存在,则覆盖现有文件。如果该文件不存在,则创建一个新文件进行读写。

但是,如何用w+读取打开的文件?


当前回答

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
$

其他回答

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。

实际上,其他关于r+模的答案都有问题。

测试。在文件内容中:

hello1
ok2
byebye3

py脚本是:

with open("test.in", 'r+')as f:
    f.readline()
    f.write("addition")

执行它和测试。In的内容将更改为:

hello1
ok2
byebye3
addition

然而,当我们将脚本修改为:

with open("test.in", 'r+')as f:
    f.write("addition")

测试。也可以这样回答:

additionk2
byebye3

因此,如果我们不执行读取操作,r+模式将允许我们从头覆盖内容。如果我们做一些读操作,f.r ewrite()只会追加到文件中。

顺便说一下,如果我们在f.e re (write_content)之前执行f.e re (0,0), write_content将从(0,0)位置开始覆盖它们。

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。

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。

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)