在 python 中第二次读取文件时出现问题

Question

I am a bit confused as to why the following code snippet returns the correct output while reading the same file for second time:

textCont = "Hello World"
print("Original content of the file")
print(open(filename).read())
textFile = open(filename, "w")
textFile.write(textCont)
print("New file content:")
textFile = open(filename)
print(textFile.read())
textFile.close()

where filename is a file that contains some existing data. This file is to be read, over-written and then read again.

In the above case, the same variable has been used for opening the file in write mode, and then in read mode. This works fine and gives a proper output when it is read for the second time (showing the content which has overwritten the previous one)

But the following version of the code doesn't work:

textCont = "Hello World"
print("Original content of the file")
print(open(filename).read())
textFile = open(filename, "w")
textFile.write(textCont)
print("New file content:")
textFile_1 = open(filename)
print(textFile_1.read())
textFile.close()
textFile_1.close()

When read is done for the second time using variable other than the one used for opening the file in write mode, then it returns an empty string.

I know the fact that when the same file is read for second time, it returns an empty string. But then why the code in the first case returns the correct output?

Can anyone please provide a suitable explanation of this?

Answer 1

The issue in the second case got resolved, thanks to @Stefan.

The solution is to close the file just after writing but before reading it again.

print(open(filename).read())
textFile = open(filename, "w")
textFile.write(textCont)
textFile.close() // close the file from write mode before reading it again
print("your file content:")
textFile_1 = open(filename)
print(textFile_1.read())
textFile_1.close()

Answer 2

The first thing to understand is that when you do textFile.write there is no certainty that the write to the file will happen immediately. This is to make the writing more efficient by putting the data first in a buffer which will be written to the file only when it is full or flush is called or the file is closed (which internally does a flush).

(Technically, flush is not enough to ensure that the data is written to the file see os.fsync )

So the reason is does not work with two different variables name is because the write operation never really happens before the line textFile_1.close()

The reason it "works" with the same variable name is that when you rebind textFile = open(filename) the file that was previously bound to textFile is now not referenced anywhere and thus the garbage collector deletes it. When a file handle is deleted the data is written to the file and thus you can read it afterward.

Also, you should use the with open idiom when dealing with files: link

See below, the FileDebug wrapper shows you when the file is being deleted and thus when the data is written.

class FileDebug:
    def __init__(self, f, name):
        self.f = f
        self.name = name

    def close(self):
        print(f"Closing file (var: {self.name})")
        self.f.close()

    def write(self, *args, **kwargs):
        self.f.write(*args, **kwargs)

    def read(self, *args, **kwargs):
        return self.f.read(*args, **kwargs)

    def __del__(self):
        print(f"Del completed (var: {self.name})")

filename = "text.txt"
def different_name():
    textCont = "Hello World"
    print("Original content of the file")
    print(FileDebug(open(filename), "No name").read())
    textFile = FileDebug(open(filename, "w"), "textFile")
    textFile.write(textCont)
    print("New file content:")
    textFile_1 = FileDebug(open(filename), "textFile_1")
    print(textFile_1.read())
    textFile.close()
    textFile_1.close()

def same_name():
    textCont = "Hello World"
    print("Original content of the file")
    print(FileDebug(open(filename), "No name").read())
    textFile = FileDebug(open(filename, "w"), "textFile")
    textFile.write(textCont)
    print("New file content:")
    textFile = FileDebug(open(filename), "textFile")
    print(textFile.read())
    textFile.close()


different_name()
"""
Original content of the file
Del completed (var: No name)

New file content:

Closing file (var: textFile)
Closing file (var: textFile_1)
Del completed (var: textFile)
Del completed (var: textFile_1)
"""

#same_name()
"""
Original content of the file
Del completed (var: No name)

New file content:
Del completed (var: textFile) # the file is closed and the data is written
Hello World
Closing file (var: textFile)
Del completed (var: textFile)
"""