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Python File Error Checking


Python Library Reference summarizes all the built-in exceptions. AssertionError Raised in case of failure of the Assert statement. Notice what you're doing here: assigning the function default_getpass to the variable getpass. What is the best way to open a file for exclusive access in Python?. this contact form

Then if its type matches the exception named after the except keyword, the except clause is executed, and then execution continues after the try statement. elif IOError: print "Unable to open file: "+str(fName) This way you can attempt to open the file, but if it doesn't exist (if it raises an IOError), alert the user! You should instead go directly to opening the file instead of checking its permissions. (EAFP vs LBYP). share|improve this answer answered May 16 '10 at 14:59 Ned Batchelder 180k31346497 add a comment| up vote 2 down vote The following extends from ~unutbu's example. you can try this out

Python Exception Message

by another process) between checking that it exists and trying to open it. –Liquid_Fire Apr 11 '11 at 20:59 1 It's also possible that fName could be the name of It is that having multiple conditions, some of which are superfluous, is less clear and explicit. –wim Apr 9 '13 at 5:45 os.access() may encounter permission problem. –diabloneo Oct User-defined Exceptions¶ Programs may name their own exceptions by creating a new exception class (see Classes for more about Python classes). Assuming we want to ask the user to enter an integer number.

Based on the os.py documentation, typing in os.F_OK, will check the existence of the path. demanded by your business logic) you should throw own exceptions, maybe even derive from Exception. No. –Debosmit Ray Apr 14 at 23:33 | show 1 more comment up vote 6 down vote import os path = /path/to/dir root,dirs,files = os.walk(path).next() if myfile in files: print "yes Python Try Except Else Virtually every module in the standard Python library uses them, and Python itself will raise them in a lot of different circumstances.

If no exception occurs during the execution, the execution will reach the break statement and the while loop will be left. Python Raise Custom Exception NameError Raised when an identifier is not found in the local or global namespace. x, y = inst.args # unpack args ... http://stackoverflow.com/questions/5627425/what-is-a-good-way-to-handle-exceptions-when-trying-to-read-a-file-in-python Is there a way to get a job without recommendation letters Open a text file and remove any blank lines Flight between non-Schengen countries with Schengen connection after exhausting 90/180 limit

This follows symbolic links, so both islink() and isfile() can be true for the same path. Is Nested Try Block Possible In Python Steepest descent/gradient descent as dynamical system Next number in sequence, understand the 1st mistake to avoid the 2nd Hotel cancellation from booking.com How to flood the entire lunar surfaces? That hits on the confusing nature of this specific example, and why they added the with statement. Checking is a file exists is not the same as checking if you can open it.

  1. try: ...
  2. The contents of the argument vary by exception.
  3. share|improve this answer answered May 16 '10 at 12:31 jhwist 7,84922942 add a comment| Your Answer draft saved draft discarded Sign up or log in Sign up using Google Sign

Python Raise Custom Exception

more hot questions question feed lang-py about us tour help blog chat data legal privacy policy work here advertising info mobile contact us feedback Technology Life / Arts Culture / Recreation http://stackoverflow.com/questions/82831/how-do-i-check-whether-a-file-exists-using-python This must be either an exception instance or an exception class (a class that derives from Exception). Python Exception Message The assert Statement When it encounters an assert statement, Python evaluates the accompanying expression, which is hopefully true. Syntax For Generic Except Clause In Python If I don't need an except block, then why do I need the inner try/finally block at all?

else: ... http://vealcine.com/in-python/python-raw-input-error-checking.php Suggestion error with Coveo search box Can I use my client's GPL software? Join them; it only takes a minute: Sign up What is a good way to handle exceptions when trying to read a file in python? Therefore using opening a file as a proxy for checking if the file exists is not correct: will have false negatives. –Chris Johnson Feb 17 at 18:58 add a comment| up Python Print Exception

Without using the assert statement, we can formulate it like this in Python: if not : raise AssertionError() The following code, using the assert statement, is semantically equivalent, i.e. Interviewee offered code samples from current employer -- should I accept? os.path.exists returns true for things that aren't files, such as directories. navigate here open a file before actually doing so using open() creates a security hole, because the user might exploit the short time interval between checking and opening the file to manipulate it.

Thanks for pointing that out. –Riegz Aug 16 '15 at 20:23 add a comment| up vote 44 down vote It doesn't seem like there's a meaningful functional difference between try/except and Name Of Errors In Python Why don't browser DNS caches mitigate DDOS attacks on DNS providers? Was Sigmund Freud "deathly afraid" of the number 62?

In general, when a Python script encounters a situation that it cannot cope with, it raises an exception.

Ask for forgiveness instead" is the pythonic way of doing things. So depending on if you want only plain files or also directories, you'll use isfile() or exists(). Summary In this chapter, you will dive into exceptions, file objects, for loops, and the os and sys modules. An Exception Can Be In Python OK, you didn't have termios, so let's try msvcrt, which is a Windows-specific module that provides an API to many useful functions in the Microsoft Visual C++ runtime services.

This follows symbolic links, so both islink() and isfile() can be true for the same path. Re-re-reading, it looks like Tim's answer is what you want. SRC: http://www.pfinn.net/python-check-if-file-exists.html share|improve this answer edited Jul 18 '15 at 17:41 answered Apr 28 '15 at 2:45 Pedro Lobito 27.8k760111 The OP asked how to check if a file his comment is here The Python Software Foundation is a non-profit corporation.

There are at least two possible exceptions: an IOError ValueError Just in case we have an additional unnamed except clause for an unexpected error: import sys try: f = open('integers.txt') s SystemError Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit. more hot questions question feed lang-py about us tour help blog chat data legal privacy policy work here advertising info mobile contact us feedback Technology Life / Arts Culture / Recreation However, if you are on windows, it replicates find with an efficient filesystem walker.

share|improve this answer edited Sep 18 '14 at 4:39 community wiki 2 revs, 2 users 67%aitchnyu add a comment| up vote 6 down vote Testing for files and folders with os.path.isfile(), This gives false positives. try: ... You might want os.path.lexists() which will return True for files, directories, and even broken links.

You really should just use try if you can, but it is not possible in some cases. –travc Oct 22 '15 at 5:11 @Bulwersator does it actually yield or import os.path os.path.isfile(fname) if you need to be sure it's a file. Can Feudalism Endure Advanced Agricultural Techniques? As others have pointed out, asserts can be turned off.

This object will responds to open but raises an IOError or OSError on every read. The with statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.