Re: [Tutor] try and file existence
In a message of Sat, 15 Aug 2015 15:20:19 -0700, Clayton Kirkwood writes: If you want to locate dangling symlinks, os.path.exists will return False, so the symlink is there, but the file it pointed to is long gone. Can't you do that with os.path.open() and get a value in os.path.status? (I think that is the thing to call) crk Laura There is no os.path.open I assume you are thinking of os.open? or open the builtin? or maybe os.stat? If what you really want to do is open the file, if it exists, then trying to open it and then if that fails handle whatever problem you get is most often the way to go. But often you never wanted to open it in the first place. Maybe it is a directory. Maybe it is a lockfile --- oops, I will come back later. There are a lot of times when the sense you really want is not 'check if this valued file exists' but rather 'check for cruft you don't want to find'. Open is also slow, which again isn't a problem if you need to open the file anyway, but will matter if all you want to do is check your entire large filesystem for files named 'core'. Laura ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On Sat, Aug 15, 2015 at 07:04:47PM -0500, boB Stepp wrote: On Sat, Aug 15, 2015 at 6:41 PM, Steven D'Aprano st...@pearwood.info wrote: On Sat, Aug 15, 2015 at 02:24:21PM -0500, boB Stepp wrote: I understand your points, but wonder then what is the intended use for os.path.exists()? That is, in what types of circumstances would it be both appropriate and safe to use? def print_file_names(possible_names): print(List of file names checked) print(-- for name in possible_names: if os.path.exists(name): print(name) else: print(missing:, name) Chuckle! Your example, giving about the most benign possible uses, is for emphasis? Well, not really. I was trying to think of a case where you want to check whether a file exists but not actually open the file (or at least, not open the file *yet*). -- Steve ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
In a message of Sat, 15 Aug 2015 14:24:21 -0500, boB Stepp writes: I understand your points, but wonder then what is the intended use for os.path.exists()? That is, in what types of circumstances would it be both appropriate and safe to use? boB If you want to locate dangling symlinks, os.path.exists will return False, so the symlink is there, but the file it pointed to is long gone. Laura ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On 15Aug2015 15:20, Clayton Kirkwood c...@godblessthe.us wrote: Behalf Of Laura Creighton [..] To: boB Stepp robertvst...@gmail.com In a message of Sat, 15 Aug 2015 14:24:21 -0500, boB Stepp writes: I understand your points, but wonder then what is the intended use for os.path.exists()? That is, in what types of circumstances would it be both appropriate and safe to use? If you want to locate dangling symlinks, os.path.exists will return False, so the symlink is there, but the file it pointed to is long gone. Can't you do that with os.path.open() and get a value in os.path.status? (I think that is the thing to call) Open does more that os.stat (which is what os.path.exists uses underneath). There are plenty of times you will want to know a file exists but not have permission to open it. Also, open can have side effects if the target file is a device or a socket/pipe. Always use the smallest thing you can to achieve an effect: stat is smaller than open. Cheers, Cameron Simpson c...@zip.com.au Q: How many user support people does it take to change a light bulb? A: We have an exact copy of the light bulb here and it seems to be working fine. Can you tell me what kind of system you have? ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
-Original Message- From: Tutor [mailto:tutor-bounces+crk=godblessthe...@python.org] On Behalf Of Laura Creighton Sent: Saturday, August 15, 2015 2:49 PM To: boB Stepp robertvst...@gmail.com Cc: l...@openend.se; tutor tutor@python.org Subject: Re: [Tutor] try and file existence In a message of Sat, 15 Aug 2015 14:24:21 -0500, boB Stepp writes: I understand your points, but wonder then what is the intended use for os.path.exists()? That is, in what types of circumstances would it be both appropriate and safe to use? boB If you want to locate dangling symlinks, os.path.exists will return False, so the symlink is there, but the file it pointed to is long gone. Can't you do that with os.path.open() and get a value in os.path.status? (I think that is the thing to call) crk Laura ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On Sat, Aug 15, 2015 at 02:24:21PM -0500, boB Stepp wrote: On Fri, Aug 14, 2015 at 10:39 PM, Steven D'Aprano st...@pearwood.info wrote: There is also os.path.exists(filename), but you should avoid using that if possible. The problem is this: if os.path.exists(filename): # file exists *right now* # but a millisecond later, some other program deletes it... # and now it doesn't exist any more with open(filename) as f: # gives an error ... I understand your points, but wonder then what is the intended use for os.path.exists()? That is, in what types of circumstances would it be both appropriate and safe to use? def print_file_names(possible_names): print(List of file names checked) print(-- for name in possible_names: if os.path.exists(name): print(name) else: print(missing:, name) -- Steve ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On Sat, Aug 15, 2015 at 6:41 PM, Steven D'Aprano st...@pearwood.info wrote: On Sat, Aug 15, 2015 at 02:24:21PM -0500, boB Stepp wrote: I understand your points, but wonder then what is the intended use for os.path.exists()? That is, in what types of circumstances would it be both appropriate and safe to use? def print_file_names(possible_names): print(List of file names checked) print(-- for name in possible_names: if os.path.exists(name): print(name) else: print(missing:, name) Chuckle! Your example, giving about the most benign possible uses, is for emphasis? boB ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On 15/08/2015 04:39, Steven D'Aprano wrote: On Fri, Aug 14, 2015 at 06:28:09PM -0700, Clayton Kirkwood wrote: try: fp = open( user_preferences ) except( PermissionError ): else: with open(user_preferences ) as f: try: fp = open(user_preferences) except (IOError, OSError) as e: handle_error() else: with fp as f: handle_file() I'll just point out that you can catch finer grained errors owing to https://www.python.org/dev/peps/pep-3151/. There is a handy little table here https://docs.python.org/3/library/exceptions.html#exception-hierarchy -- My fellow Pythonistas, ask not what our language can do for you, ask what you can do for our language. Mark Lawrence ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On Fri, Aug 14, 2015 at 10:39 PM, Steven D'Aprano st...@pearwood.info wrote: On Fri, Aug 14, 2015 at 06:28:09PM -0700, Clayton Kirkwood wrote: what is the best way to find out if a file exists? Try to open it and see what happens. If the open() succeeds, then the file exists and can be read. If it fails, then either the file doesn't exist, or it can't be read. Inspect the error to find out which. There is also os.path.exists(filename), but you should avoid using that if possible. The problem is this: if os.path.exists(filename): # file exists *right now* # but a millisecond later, some other program deletes it... # and now it doesn't exist any more with open(filename) as f: # gives an error ... I understand your points, but wonder then what is the intended use for os.path.exists()? That is, in what types of circumstances would it be both appropriate and safe to use? boB ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On 15/08/2015 02:28, Clayton Kirkwood wrote: try: fp = open( user_preferences ) except( PermissionError ): You need a pass statement here if you don't intend doing anything with the error, but see my comments at the bottom. else: with open(user_preferences ) as f: I originally only had the bottom open statement. Ran but file didn't exist, and my run failed with file doesn't exist. I figured I'd check to see if the file existed. This is one of those situations where a search of documentation for fd_exist (which I thought I'd seen once), or exist turns up either nothing or nothing relevant. I finally found that the try: clause with the open statement might help and I copied the snippet to my code. I am getting an indentation error: expected an indent block. What is wrong, and what is the best way to find out if a file exists? TIA, Clayton There's nothing to stop you using multiple except statements with one try. So something like this is how I'd go about it. try: with open(user_preferences) as f: do_something() except PermissionError: whatever() except FileNotFoundError: oh_heck() etc. Seee this for an explanation of exception handling https://docs.python.org/3/tutorial/errors.html#handling-exceptions. A full list of the exceptions you'd need to consider is here https://docs.python.org/3/library/exceptions.html#os-exceptions -- My fellow Pythonistas, ask not what our language can do for you, ask what you can do for our language. Mark Lawrence ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] try and file existence
On 14Aug2015 18:28, Clayton Kirkwood c...@godblessthe.us wrote:
try:
fp = open( user_preferences )
except( PermissionError ):
else:
with open(user_preferences ) as f:
I originally only had the bottom open statement. Ran but file didn't exist,
and my run failed with file doesn't exist. I figured I'd check to see if the
file existed. This is one of those situations where a search of
documentation for fd_exist (which I thought I'd seen once), or exist turns
up either nothing or nothing relevant. I finally found that the try: clause
with the open statement might help and I copied the snippet to my code. I am
getting an indentation error: expected an indent block. What is wrong, and
what is the best way to find out if a file exists?
In purely syntactic terms you need some code in the suite under the except
clause, and you don't want the brackets:
try:
fp = open( user_preferences )
except PermissionError as e:
print(open(%r) fails: %s % (user_preferences, e))
else:
with open(user_preferences ) as f:
In logical terms, the with is not wanted - you're opening the file again.
Leaving aside the logical issue there, this structure (test then operate) is
also racy: suppose the file has its attributes changed or is removed between
the first open and the second.
Next: you're catching PermissionError. That normally means that you have not
got rights for opening the file; you will get a different exception if the file
does not exist. You're being too precise if you want both.
But maybe you don't. You need to decide
Finally, the usual Python pattern is not to catch exceptions _at all_ unless
you have a deliberate polciy plan for what to do. Normally you would have some
function looking like this:
def read_prefs(filename):
prefs = {}
with open(filename) as fp:
... read preferences, save in prefs dict for example ...
return prefs
If the file is missing or you have no permissions, that will raise an
exception. Let it!
Then in an outer layer of your program you might catch the exception, where you
can make a policy decision because you have a wider view of what is going on:
try:
prefs = read_prefs(prefs_filename)
except FileNotFoundError as e:
print(warning: file not found: %r: %s % (prefs_filename, e))
# proceed with empty preferences, not an error
prefs = {}
This bit of code catches _only_ FileNotFoundError on the (presumed) policy that
a missing preferences file is ok - your program will proceed with default
behaviours - but any _other_ kind of exception is not expected - let your
program abort! Do not proceed!
Cheers,
Cameron Simpson c...@zip.com.au
Capitalism is the extraordinary belief that the nastiest of men, for the
nastiest of reasons, will somehow work for the benefit of us all.
- John Maynard Keynes
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Re: [Tutor] try and file existence
On Fri, Aug 14, 2015 at 06:28:09PM -0700, Clayton Kirkwood wrote: try: fp = open( user_preferences ) except( PermissionError ): else: with open(user_preferences ) as f: try: fp = open(user_preferences) except (IOError, OSError) as e: handle_error() else: with fp as f: handle_file() [...] what is the best way to find out if a file exists? Try to open it and see what happens. If the open() succeeds, then the file exists and can be read. If it fails, then either the file doesn't exist, or it can't be read. Inspect the error to find out which. There is also os.path.exists(filename), but you should avoid using that if possible. The problem is this: if os.path.exists(filename): # file exists *right now* # but a millisecond later, some other program deletes it... # and now it doesn't exist any more with open(filename) as f: # gives an error ... This is called a time of check to time of use bug, and it is a major cause of security bugs in software. Remember, even if you're the only *person* using your computer, there could be hundreds of other programs running in the background, and one of those might delete the file after you've checked its existence. Also, just because the file *exists* doesn't mean you can open it. Perhaps the file is locked, unreadable, you don't have permissions, or maybe even the disk is faulty and the file is corrupt and unreadable. -- Steve ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
