Python-何时写入文件

Question

Learning python now. I have the following program.

1. Why doesn't the program print anything after the last line? It looks like "target" doesn't have any value that was written. (even if I open the actual file, there are no values why is that?

2. I tried adding that line above the "target.close" thinking the the file doesn't get written on until that line. That did not work either. so what is the purpose of "target.close"?

3. how is that "target.truncate()" gets effect right away. After that command, and the script pauses on an input, if I open the file, I can see all the data it had has been erased away.

from sys import argv
script, filename = argv

print (f"We are going to erase {filename}")
print ("If you don't want that, press CTRL + C")
print ("if you want that, press ENTER")
input("?  ")

print("Opening the file.......")
target = open(filename,"w+")

print("Truncating the file....")
target.truncate()
print("Finished Truncating")

print("Gimme 3 lines...")

Line1 = input("Line 1:  ")
Line2 = input("Line 2:  ")
Line3 = input("Line 3:  ")

print("Writing these lines to the file")

target.write(Line1 + "\n")
target.write(Line2 + "\n")
target.write(Line3 + "\n")


print ("Finally, we close it")
target.close

input("Do you want to read the file now?")
print(target.read())

Answer 1

target.close

is missing the () call parenthesis. That is why nothing is written.

Then if you want to read the file, you will need to reopen it:

print(open(filename).read())

Answer 2

Solution

target.close is missing a parenthesis, ie it should be target.close() .

But looking at your intention, it seems that you want to do target.flush() because you are also attempting target.read() soon after - you'll be unable to read from the file if you closed it.

Why does it happen

By default a certain amount of data that is written to the file is actually stored into a buffer - in memory - before it is actually written to the file. If you want to update the file immediately, you'll need to call the flush method, ie target.flush() Calling target.close() will automatically flush the data that has been buffered, hence why target.close() also updates the file similar to target.flush() .