If you're opening a file using the 'with&#

问题:

For opening files, I'm used to the apparently older syntax:

f = open("sub_ranks.txt","r+")
for line in f:
    ...
f.close()

I've been told to use this syntax instead a couple of times now..

with open("sub_ranks.txt", "r+") as f:
    for line in f:
        ...

Is a file object "close" statement still needed in the second example, when the "with" statement is being used?

And if so, is there any concrete reason to use the "with" statement for file reading? In this case, it's (slightly) more verbose.

回答1:

The answer to your immediate question is "No". The with block ensures that the file will be closed when control leaves the block, for whatever reason that happens, including exceptions (well, excluding someone yanking the power cord to your computer and some other rare events).

So it's good practice to use a with block.

Now arguably, having opened a file only for reading and then failing to close it is not that much of a problem. When garbage collection comes around (whenever that may be), that file will be closed, too, if there are no references to it anymore; at the latest that will happen when your program exits. In fact, several code samples in the official docs neglect closing a file that has been opened only for read access. When writing a file or when using the "read plus" mode like in your example, you definitely need to close the file. There are many questions her on SO dealing with incomplete/corrupted files because of a failure to close them properly.

回答2:

From the python docs, I see that with is a syntactic sugar for the try/finally blocks. So,

Is a file object "close" statement still needed in the second example, when the "with" statement is being used?

No.

From the Python docs:

The ‘with‘ statement clarifies code that previously would use try...finally blocks to ensure that clean-up code is executed. In this section, I’ll discuss the statement as it will commonly be used. In the next section, I’ll examine the implementation details and show how to write objects for use with this statement.

The ‘with‘ statement is a control-flow structure whose basic structure is:

with expression [as variable]: with-block

The expression is evaluated, and it should result in an object that supports the context management protocol (that is, has enter() and exit() methods).

Here's another article that makes it clear.

回答3:

No.

Say you want to print the hostname like so:

with open("/etc/hostname","r") as f: print f.read() 

It will open the file, do its job then close the file.