We Keep Coding

Does Python GC close files too?

Consider the following piece of Python (2.x) code:
for line in open('foo').readlines():
print line.rstrip()
I assume that since the open file remains unreferenced it has to be closed automatically. I have read about garbage collector in Python which frees memory allocated by unused objects. Is GC general enough to handle the files too?

UPDATE
For current versions of python, the clear recommendation is to close files explicitly or use a with statement. No indication anymore that the GC will close the file for you. So now the answer should be: Maybe, but no guarantee. Always use close() or a with statement.
In the Python 3.8 docs the text has been updated to:
If you’re not using the with keyword, then you should call f.close() to close the file and immediately free up any system resources used by it.
Warning: Calling f.write() without using the with keyword or calling f.close() might result in the arguments of f.write() not being completely written to the disk, even if the program exits successfully.
Old Answer:
Taken from the Python 3.6 docs:
If you’re not using the with keyword, then you should call f.close() to close the file and immediately free up any system resources used by it. If you don’t explicitly close a file, Python’s garbage collector will eventually destroy the object and close the open file for you, but the file may stay open for a while. Another risk is that different Python implementations will do this clean-up at different times.
So yes, the file will be closed automatically, but in order to be in control of the process you should do so yourself or use a with statement:
with open('foo') as foo_file:
for line in foo_file.readlines():
print line.rstrip()
foo_file will be clsoed once the with block ends
In the Python 2.7 docs, the wording was different:
When you’re done with a file, call f.close() to close it and free up
any system resources taken up by the open file. After calling
f.close(), attempts to use the file object will automatically fail.
so I assume that you should not depend on the garbage collector automatically closing files for you and just do it manually/use with

I often use open without with so I ran a little test. For the test I use Python 3.9 so I'm not speaking of earlier versions but for 3.9 at least, we do not need the with to have a clean file close.
bash
inotifywait -m "testfile"
python3.9
lines=[line for line in open("testfile")]
sleep(5)
for line in lines:
print(line)
Watch the inotifywait window and run the python script. Before the sleep the final event will be CLOSE_NOWRITE,CLOSE and there will be no other events from that file through the run of the python script.

It depends on what you do, check out this description how it works.
In general I would recommend to use the context manager of the file:
with open("foo", "r") as f:
for line in f.readlines():
# ....
which is similar to (for basic understanding):
file_context_manager = open("foo", "r").__enter__()
for line in file_context_manager.readlines():
# ....
file_context_manager.__exit__()
The first version is a lot more readable, and the withstatement calls the exit method automatically (plus a bit more context handling).
The file will be closed automatically when the scope of the with statement is left.

How can I get python3 file.close to release the file lock?

Python3 file.close is apparently not letting go of the file, so it can't be used by Win7 commands.
To wit:
import os, time
hfile = "Positions.htm"
hf = open(hfile, "w")
hf.write(str(buf))
hf.close
time.sleep(2) # give it enough time to do the close
os.system(hfile) # run the html file in the default browser
This results in an error message from Win7, saying that it cannot access the file because it is currently in use. However, it is easily accessed after the python program terminates.
Yes, I know that similar questions have been asked here, but I have not seen one that gave a general answer.

You just forgot to call close()
hf.close # wrong
hf.close() # right
You can see that hm.close just gives you a bound method without calling it:
>>> hm.close
<built-in method close of _io.TextIOWrapper object at 0x7ffe8ec74b40>
As Padriac Cunning ham pointed out, you don't need to do this if you just use with syntax:
with open(hfile, 'w') as hf:
hf.write(str(buf))
# Automatically closed

How to create a temporary file that can be read by a subprocess?

I'm writing a Python script that needs to write some data to a temporary file, then create a subprocess running a C++ program that will read the temporary file. I'm trying to use NamedTemporaryFile for this, but according to the docs,
Whether the name can be used to open the file a second time, while the named temporary file is still open, varies across platforms (it can be so used on Unix; it cannot on Windows NT or later).
And indeed, on Windows if I flush the temporary file after writing, but don't close it until I want it to go away, the subprocess isn't able to open it for reading.
I'm working around this by creating the file with delete=False, closing it before spawning the subprocess, and then manually deleting it once I'm done:
fileTemp = tempfile.NamedTemporaryFile(delete = False)
try:
fileTemp.write(someStuff)
fileTemp.close()
# ...run the subprocess and wait for it to complete...
finally:
os.remove(fileTemp.name)
This seems inelegant. Is there a better way to do this? Perhaps a way to open up the permissions on the temporary file so the subprocess can get at it?

Since nobody else appears to be interested in leaving this information out in the open...
tempfile does expose a function, mkdtemp(), which can trivialize this problem:
try:
temp_dir = mkdtemp()
temp_file = make_a_file_in_a_dir(temp_dir)
do_your_subprocess_stuff(temp_file)
remove_your_temp_file(temp_file)
finally:
os.rmdir(temp_dir)
I leave the implementation of the intermediate functions up to the reader, as one might wish to do things like use mkstemp() to tighten up the security of the temporary file itself, or overwrite the file in-place before removing it. I don't particularly know what security restrictions one might have that are not easily planned for by perusing the source of tempfile.
Anyway, yes, using NamedTemporaryFile on Windows might be inelegant, and my solution here might also be inelegant, but you've already decided that Windows support is more important than elegant code, so you might as well go ahead and do something readable.

According to Richard Oudkerk
(...) the only reason that trying to reopen a NamedTemporaryFile fails on
Windows is because when we reopen we need to use O_TEMPORARY.
and he gives an example of how to do this in Python 3.3+
import os, tempfile
DATA = b"hello bob"
def temp_opener(name, flag, mode=0o777):
return os.open(name, flag | os.O_TEMPORARY, mode)
with tempfile.NamedTemporaryFile() as f:
f.write(DATA)
f.flush()
with open(f.name, "rb", opener=temp_opener) as f:
assert f.read() == DATA
assert not os.path.exists(f.name)
Because there's no opener parameter in the built-in open() in Python 2.x, we have to combine lower level os.open() and os.fdopen() functions to achieve the same effect:
import subprocess
import tempfile
DATA = b"hello bob"
with tempfile.NamedTemporaryFile() as f:
f.write(DATA)
f.flush()
subprocess_code = \
"""import os
f = os.fdopen(os.open(r'{FILENAME}', os.O_RDWR | os.O_BINARY | os.O_TEMPORARY), 'rb')
assert f.read() == b'{DATA}'
""".replace('\n', ';').format(FILENAME=f.name, DATA=DATA)
subprocess.check_output(['python', '-c', subprocess_code]) == DATA

You can always go low-level, though am not sure if it's clean enough for you:
fd, filename = tempfile.mkstemp()
try:
os.write(fd, someStuff)
os.close(fd)
# ...run the subprocess and wait for it to complete...
finally:
os.remove(filename)

At least if you open a temporary file using existing Python libraries, accessing it from multiple processes is not possible in case of Windows. According to MSDN you can specify a 3rd parameter (dwSharedMode) shared mode flag FILE_SHARE_READ to CreateFile() function which:
Enables subsequent open operations on a file or device to request read
access. Otherwise, other processes cannot open the file or device if
they request read access. If this flag is not specified, but the file
or device has been opened for read access, the function fails.
So, you can write a Windows specific C routine to create a custom temporary file opener function, call it from Python and then you can make your sub-process access the file without any error. But I think you should stick with your existing approach as it is the most portable version and will work on any system and thus is the most elegant implementation.
Discussion on Linux and windows file locking can be found here.
EDIT: Turns out it is possible to open & read the temporary file from multiple processes in Windows too. See Piotr Dobrogost's answer.

Using mkstemp() instead with os.fdopen() in a with statement avoids having to call close():
fd, path = tempfile.mkstemp()
try:
with os.fdopen(fd, 'wb') as fileTemp:
fileTemp.write(someStuff)
# ...run the subprocess and wait for it to complete...
finally:
os.remove(path)

I know this is a really old post, but I think it's relevant today given that the API is changing and functions like mktemp and mkstemp are being replaced by functions like TemporaryFile() and TemporaryDirectory(). I just wanted to demonstrate in the following sample how to make sure that a temp directory is still available downstream:
Instead of coding:
tmpdirname = tempfile.TemporaryDirectory()
and using tmpdirname throughout your code, you should trying to use your code in a with statement block to insure that it is available for your code calls... like this:
with tempfile.TemporaryDirectory() as tmpdirname:
[do dependent code nested so it's part of the with statement]
If you reference it outside of the with then it's likely that it won't be visible anymore.

does close() imply flush() in Python?

In Python, and in general - does a close() operation on a file object imply a flush() operation?

Yes. It uses the underlying close() function which does that for you (source).

NB: close() and flush() won't ensure that the data is actually secure on the disk. It just ensures that the OS has the data == that it isn't buffered inside the process.
You can try sync or fsync to get the data written to the disk.

Yes, in Python 3 this is finally in the official documentation, but is was already the case in Python 2 (see Martin's answer).

As a complement to this question, yes python flushes before close, however if you want to ensure data is written properly to disk this is not enough.
This is how I would write a file in a way that it's atomically updated on a UNIX/Linux server, whenever the target file exists or not. Note that some filesystem will implicitly commit data to disk on close+rename (ext3 with data=ordered (default), and ext4 initially uncovered many application flaws before adding detection of write-close-rename patterns and sync data before metadata on those[1]).
# Write destfile, using a temporary name .<name>_XXXXXXXX
base, name = os.path.split(destfile)
tmpname = os.path.join(base, '.{}_'.format(name)) # This is the tmpfile prefix
with tempfile.NamedTemporaryFile('w', prefix=tmpname, delete=False) as fd:
# Replace prefix with actual file path/name
tmpname = str(fd.name)
try:
# Write fd here... ex:
json.dumps({}, fd)
# We want to fdatasync before closing, so we need to flush before close anyway
fd.flush()
os.fdatasync(fd)
# Since we're using tmpfile, we need to also set the proper permissions
if os.path.exists(destfile):
# Copy destination file's mask
os.fchmod(fd.fileno, os.stat(destfile).st_mode)
else:
# Set mask based on current umask value
umask = os.umask(0o22)
os.umask(umask)
os.fchmod(fd.fileno, 0o666 & ~umask) # 0o777 for dirs and executable files
# Now we can close and rename the file (overwriting any existing one)
fd.close()
os.rename(tmpname, destfile)
except:
# On error, try to cleanup the temporary file
try:
os.unlink(tmpname)
except OSError:
pass
raise
IMHO it would have been nice if Python provided simple methods around this... At the same time I guess if you care about data consistency it's probably best to really understand what is going on at a low level, especially since there are many differences across various Operating Systems and Filesystems.
Also note that this does not guarantee the written data can be recovered, only that you will get a consistent copy of the data (old or new). To ensure the new data is safely written and accessible when returning, you need to use os.fsync(...) after the rename, and even then if you have unsafe caches in the write path you could still lose data. this is common on consumer-grade hardware although any system can be configured for unsafe writes which boosts performance too. At least even with unsafe caches, the method above should still guarantee whichever copy of the data you get is valid.

filehandle.close does not necessarily flush. Surprisingly, filehandle.flush doesn't help either---it still can get stuck in the OS buffers when Python is running. Observe this session where I wrote to a file, closed it and Ctrl-Z to the shell command prompt and examined the file:
$ cat xyz
ghi
$ fg
python
>>> x=open("xyz","a")
>>> x.write("morestuff\n")
>>> x.write("morestuff\n")
>>> x.write("morestuff\n")
>>> x.flush
<built-in method flush of file object at 0x7f58e0044660>
>>> x.close
<built-in method close of file object at 0x7f58e0044660>
>>>
[1]+ Stopped python
$ cat xyz
ghi
Subsequently I can reopen the file, and that necessarily syncs the file (because, in this case, I open it in the append mode). As the others have said, the sync syscall (available from the os package) should flush all buffers to disk but it has possible system-wide performance implications (it syncs all files on the system).

how to get content of a small ascii file in python?

Let's say we want to implement an equivalent of the PHP's file_get_content.
What is the best practice? (elegant and reliable)
Here are some proposition, are they correct?
using with statement:
def file_get_contents(filename):
with file(filename) as f:
s = f.read()
return s
is using standard open() safe?
def file_get_contents(filename):
return open(filename).read()
What happens to file descriptor in either solution?

In the current implementation of CPython, both will generally immediately close the file. However, Python the language makes no such guarantee for the second one - the file will eventually be closed, but the finaliser may not be called until the next gc cycle. Implementations like Jython and IronPython will work like this, so it's good practice to explicitely close your files.
I'd say using the first solution is the best practice, though open is generally preferred to file. Note that you can shorten it a little though if you prefer the brevity of the second example:
def file_get_contents(filename):
with open(filename) as f:
return f.read()
The __exit__ part of the context manager will execute when you leave the body for any reason, including exceptions and returning from the function - there's no need to use an intermediate variable.

import os
def file_get_contents(filename):
if os.path.exists(filename):
fp = open(filename, "r")
content = fp.read()
fp.close()
return content
This case it will return None if the file didn't exist, and the file descriptor will be closed before we exit the function.

Using the with statement is actually the nicest way to be sure that the file is really closed.
Depending on the garbage collector behavior for this task might work, but in this case, there is a nice way to be sure in all cases, so...

with will make sure that the file is closed when the block is left.
In your second example, the file handle might remain open (Python makes no guarantee that it's closed or when if you don't do it explicitly).

You can also use Python's v3 feature:
>>> ''.join(open('htdocs/config.php', 'r').readlines())
"This is the first line of the file.\nSecond line of the file"
Read more here http://docs.python.org/py3k/tutorial/inputoutput.html