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Python – When to use file vs open

Posted by: admin November 1, 2017 Leave a comment

Questions:

What’s the difference between file and open in Python? When should I use which one? (Say I’m in 2.5)

Answers:

You should always use open().

As the documentation states:

When opening a file, it’s preferable
to use open() instead of invoking this
constructor directly. file is more
suited to type testing (for example,
writing “isinstance(f, file)”).

Also, file() has been removed since Python 3.0.

Questions:
Answers:

Two reasons: The python philosophy of “There ought to be one way to do it” and file is going away.

file is the actual type (using e.g. file('myfile.txt') is calling its constructor). open is a factory function that will return a file object.

In python 3.0 file is going to move from being a built-in to being implemented by multiple classes in the io library (somewhat similar to Java with buffered readers, etc.)

Questions:
Answers:

file() is a type, like an int or a list. open() is a function for opening files, and will return a file object.

This is an example of when you should use open:

f = open(filename, 'r')
for line in f:
    process(line)
f.close()

This is an example of when you should use file:

class LoggingFile(file):
    def write(self, data):
        sys.stderr.write("Wrote %d bytes\n" % len(data))
        super(LoggingFile, self).write(data)

As you can see, there’s a good reason for both to exist, and a clear use-case for both.

Questions:
Answers:

Functionally, the two are the same; open will call file anyway, so currently the difference is a matter of style. The Python docs recommend using open.

When opening a file, it’s preferable to use open() instead of invoking the file constructor directly.

The reason is that in future versions they is not guaranteed to be the same (open will become a factory function, which returns objects of different types depending on the path it’s opening).

Questions:
Answers:

Only ever use open() for opening files. file() is actually being removed in 3.0, and it’s deprecated at the moment. They’ve had a sort of strange relationship, but file() is going now, so there’s no need to worry anymore.

The following is from the Python 2.6 docs. [bracket stuff] added by me.

When opening a file, it’s preferable to use open() instead of invoking this [file()] constructor directly. file is more suited to type testing (for example, writing isinstance(f, file)

Questions:
Answers:

According to Mr Van Rossum, although open() is currently an alias for file() you should use open() because this might change in the future.