Python 2.7 tempfile.NamedTemporaryFile returns object in type 'instance' not 'file'. Why?

Question

Some libraries checking whether type of input f is file or not. And Python 2.7 library tempfile returns object in type file for

type(tempfile.TemporaryFile())  # type is file

And for

type(tempfile.NamedTemporaryFile())  # type is instance

Is there some reason or it is just a bug?

Answer 1

Read the documentation for NamedTemporaryFile:

The returned object is always a file-like object whose file attribute is the underlying true file object. This file-like object can be used in a with statement, just like a normal file.

Answer 2

Looking at the implementation , both TemporaryFile and NamedTemporaryFile are actually factory functions. Depending on the OS, TemporaryFile can simply returns a low-level file handle (on non-posix compliant systems and cygwin, TemporaryFile is NamedTemporaryFile ). NamedTemporaryFile always returns an instance of _TemporaryFileWrapper , which is an old-style class (hence the 'instance' ) that wraps a low-level file handle.

I would not say that this is a bug (though, maybe qwirky due to the naming convention not following PEP8) as everything behaves according to the documentation -- After all, the returned values are file-like .

Answer 3

The problem I had with NamedTemporaryFile is that pylint complained that the result was not an iterable. Probably this is a bug in pylint.

#!/usr/bin/python

'''
Test program for pylint complaint about file-like object returned from
tempfile functions.
'''

import tempfile

cmdOut = tempfile.NamedTemporaryFile(prefix="tmp-scc-cmd-stderr")
print "Return value from NamedTemporaryFile() is %s" % type(cmdOut)

print >>cmdOut, "foo\nbar"              # send two lines of output

cmdOut.seek(0)                          # go back to beginning to read output
for line in cmdOut:
    print line.rstrip()

The reported type is 'instance'.

% python ~/perl/tempfile-pylint.py
Return value from NamedTemporaryFile() is <type 'instance'>
foo
bar

However pylint complains that

E: 14,12: Non-iterable value cmdOut is used in an iterating context (not-an-iterable)

I agree this isn't a bug in tempfile, but does create a bit of confusion in cases like this.