FDs will be closed after exception automatically in python2.7?
as title,
I confused will fd will be close after exception automatically?
Like:
try:
fd=open("file","w+")
fd.get() //any useless function of fd
except Exception:
print 'hi'
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Re: FDs will be closed after exception automatically in python2.7?
On 2019-06-10 18:18:07 +0800, lampahome wrote:
> as title,
>
> I confused will fd will be close after exception automatically?
>
> Like:
> try:
> fd=open("file","w+")
> fd.get() //any useless function of fd
> except Exception:
> print 'hi'
Please try to preserve indentation when sending code to the list.
Indentation is part of the Python syntax and if you smash everything to
the left margin your program is not only hard to read but syntactically
wrong.
To answer your question:
No, an exception will not automatically close open file descriptors
(which file descriptors should it close and why?).
However, while handling an exception the variable holding the file
descriptor may go out of scope (for example if you leave a "with"
block[1] or return from a function). In that case the file descriptor
will be closed (in the case of a with block immediately, otherwise when
the garbage collector gets around to it).
hp
[1] Don't know if those exist in Python 2.x. You should upgrade to 3.x
anyway.
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|_|_) || because we have much more sophisticated
| | | h...@hjp.at | management tools.
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Re: FDs will be closed after exception automatically in python2.7?
On 10Jun2019 12:47, Peter J. Holzer wrote:
On 2019-06-10 18:18:07 +0800, lampahome wrote:
I confused will fd will be close after exception automatically?
Like:
try:
fd=open("file","w+")
fd.get() //any useless function of fd
except Exception:
print 'hi'
Please try to preserve indentation when sending code to the list.
Indentation is part of the Python syntax and if you smash everything to
the left margin your program is not only hard to read but syntactically
wrong.
To answer your question:
No, an exception will not automatically close open file descriptors
(which file descriptors should it close and why?).
However, while handling an exception the variable holding the file
descriptor may go out of scope (for example if you leave a "with"
block[1] or return from a function). In that case the file descriptor
will be closed (in the case of a with block immediately, otherwise when
the garbage collector gets around to it).
We want to be more precise here.
By using the term "file descriptor" above you've conflated 2 things: the
Python file object and the OS file descriptor. They're different things
with different behaviours.
What you get from "open()" is a python "file object". And what the OP is
getting in his example code is also a file object. And as you say, when
all references to that are removed (variable goes out of scope), the
Python interpreter will close the file.
So far so good.
However the term "file descriptor", at least in POSIX (UNIX and Linux),
is _not_ the same as a Python "file object". Instead, it is the integer
you get from a OS level file open such as returned by os.open(). It is
_not_ managed by the Python interpreter and does not get closed when
nobody references it, because it is not an object.
Now, for the OP's edification: when you use Python's open() function you
get a file object, which includes a reference to the _underlying_ OS
level file descriptor. When that object get recovered _it_ will close
the file descriptor as part of the close operation.
However, if you do an OS level file open which returns _just_ the file
descriptor, this does not happen because there's no object cleanup and
nothing to close the descriptor, because nothing is managing it.
Example code:
#!/usr/bin/python
import os
def open_and_fail():
fd = os.open('/dev/null',os.O_RDONLY)
raise RuntimeError("blam")
try:
open_and_fail()
except RuntimeError:
pass
os.system("lsof -p "+str(os.getpid()))
Here's some code which does an OS level open, getting a fie descriptor.
The opening function raises an exception. After we've caught it, we run
"lsof" on our own process. here's it on my Mac:
[~]fleet*1> /usr/bin/python fd.py
COMMAND PIDUSER FD TYPE DEVICE SIZE/OFF NODE NAME
Python 97417 cameron cwdDIR1,5 9214934731 /Users/cameron
Python 97417 cameron txtREG1,5 25152 199261280
/System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python
Python 97417 cameron txtREG1,5 2633024 199261275
/System/Library/Frameworks/Python.framework/Versions/2.7/Python
Python 97417 cameron txtREG1,5 52832 199261366
/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload/_locale.so
Python 97417 cameron txtREG1,5643792 199264919 /usr/lib/dyld
Python 97417 cameron txtREG1,5 560200432 200219840
/private/var/db/dyld/dyld_shared_cache_x86_64
Python 97417 cameron0u CHR 16,470t1529 1991 /dev/ttys047
Python 97417 cameron1u CHR 16,470t1529 1991 /dev/ttys047
Python 97417 cameron2u CHR 16,470t1529 1991 /dev/ttys047
Python 97417 cameron3r CHR3,2 0t0 305 /dev/null
See the last line. That is file descriptor 3, attached to /dev/null. It
is still open.
When your programme exits the OS will clean this up, but until then the
_descriptor_ remains open.
If you rewrote this with plain Python "open("/dev/null")" instead of the
os.open you would not see the file descriptor lying around because the
close of the _file object_ would have cleaned up the related OS file
descriptor.
Cheers,
Cameron Simpson
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Re: FDs will be closed after exception automatically in python2.7?
On 06/10/2019 04:18 AM, lampahome wrote:
> as title,
>
> I confused will fd will be close after exception automatically?
> Like:
> try:
> fd=open("file","w+")
> fd.get() //any useless function of fd
> except Exception:
> print 'hi'
No. What if you want to work with the fd object after dealing with the
exception?
Probably you should be using Python 3, which uses a print() function.
But even in Python 2.7, the recommended way to do this is like this:
fd = open("file","w+")
with fd:
fd.get()
After the "with" block exits, the fd object is automatically closed.
This happens regardless of whether an exception occurred within the with
block or not. You may use try and except within the with block.
Normally I wouldn't bother, though.
Be careful about catching exceptions. In most cases, it is not
appropriate in my opinion to "except Exception" which will catch *all*
exceptions including syntax errors in your code.
Only catch exceptions that your code is specifically equipped to deal
with (expecting). For example, if a user provides a file name, and you
open it, you may want to catch the FileNotFound exception, in order to
inform the user and prompt for a new file name.
Exceptions that your code is not expecting and cannot handle should be
left alone to bubble up and be caught somewhere else, or crash the
program with a traceback, which will aid in debugging.
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Re: FDs will be closed after exception automatically in python2.7?
On 2019-06-10 10:48, Michael Torrie wrote:
> Probably you should be using Python 3, which uses a print() function.
> But even in Python 2.7, the recommended way to do this is like this:
>
> fd = open("file","w+")
> with fd:
There is still a small window there if there are asynchronous events
happening. Use this instead:
with open("file","w+") as fd:
*rest of good advice elided.*
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can not use pycharm
Hello,this is my first time trying to learn coding and programming and I wanted to start with python.Though,when I download pycharm, I go to configure>settings>project interpreter and i can not put a project interpreter( I have download python version 3) .What should I do?thank you for your time. --- Αυτό το e-mail ελέγχθηκε για ιούς από το πρόγραμμα Avast antivirus. https://www.avast.com/antivirus -- https://mail.python.org/mailman/listinfo/python-list
Re: FDs will be closed after exception automatically in python2.7?
D'Arcy Cain writes:
> On 2019-06-10 10:48, Michael Torrie wrote:
> > fd = open("file","w+")
> > with fd:
>
> There is still a small window there if there are asynchronous events
> happening. Use this instead:
>
> with open("file","w+") as fd:
That makes the window smaller, but it doesn't actually eliminate it. Look
at the generated byte code. In both cases the call to open() is over and
the open file is created _before_ the SETUP_WITH instruction is executed.
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Alan Bawden
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Re: FDs will be closed after exception automatically in python2.7?
On 2019-06-10 15:46, Alan Bawden wrote:
> D'Arcy Cain writes:
>> with open("file","w+") as fd:
>
> That makes the window smaller, but it doesn't actually eliminate it. Look
> at the generated byte code. In both cases the call to open() is over and
> the open file is created _before_ the SETUP_WITH instruction is executed.
Am I correct in assuming that the window is there for process interrupts
but not threads?
--
D'Arcy J.M. Cain
Vybe Networks Inc.
http://www.VybeNetworks.com/
IM:da...@vex.net VoIP: sip:da...@vybenetworks.com
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