I have a txt file called test.txt with 4 lines in it. I want to copy lines containing the word 'exception' into a new file from command line argument. I have managed this far. But I also want to exception handle this for IO error. That is if from the command line, somebody misspells the word test.txt, it will throw an exception. This is my current code. Please help! I'm a beginner. Presently, if I misspell it intentionally, it is not showing the error message I intend it to show.
import sys
def Cat(filename):
try:
f = open(filename, 'rU')
for line in f:
print (line),
return 3
except IOError:
print('\nIO error!!', filename)
def main():
f1 = open(sys.argv[1])
f2 = open(sys.argv[2], 'w')
for line in f1:
if 'exception' in line:
f2.write(line)
if __name__ == '__main__':
main()
First check if source file exists and readable -
if not (os.path.exists(file1) and os.access(file1, os.R_OK)):
print "file1 does not exists or not readable"
sys.exit(1)
else:
//good to go
and then handle writing of destination file with try except block.
You need to put the open() inside a try-except block, just as you did in Cat().
Currently, you are not calling Cat(), and the open() in main() is not inside a try-except block.
Since you said you are a beginner in Python I'll assume this a sort of "learning code". So, I won't tell you anything about the design. Also, you should do what #NPE says too.
You can try this in your main function in order to reach your goal:
def main():
filename = open(sys.argv[1])
if filename != "test.txt":
raise Exception("Here goes the message error you want to show")
f2 = open(sys.argv[2], 'w')
for line in f1:
if 'exception' in line:
f2.write(line)
You forgot to call Cat()
before
f2 = open(sys.argv[2], 'w')
Cat(f1)
for line in f1:
and in the Cat function you will need to raise exception to stop the execution
print('\nIO error!!', filename)
raise IOError('Invalid filename')
Related
I have written a program that takes a file name and returns numbers and it works as expected when the file name is inputted correctly but when I try to purposely write an incorrect file name, instead of giving me a "error: file not found" from the exception, it will say:
def getFile(fileName):
lines = []
try:
infile = open(fileName, 'r')
if infile != None:
for line in infile:
lines.append(line)
except IOError:
print('Error: file not found.')
finally:
infile.close()
return lines
If an exception is raised by open, the local variable infile is never declared let alone assigned, so the attempt to call infile.close() in the finally block will raise an UnboundLocalError as you see here. You can 'fix' this somewhat by declaring infile with some special uninitialized value (e.g. None) and checking explicitly like so:
def getFile(fileName):
lines = []
infile = None
try:
infile = open(fileName, 'r')
for line in infile:
lines.append(line)
except IOError:
print('Error: file not found.')
finally:
if infile is not None:
infile.close()
return lines
Alternatively, since file objects are context managers, you can write something like:
def getFile(fileName):
lines = []
try:
with open(fileName, 'r') as infile:
for line in infile:
lines.append(line)
except IOError:
print('Error: file not found.')
return lines
... which will ensure infile is closed in a more syntactically concise & structured manner.
Note that on failure open raises an OSError (e.g. a FileNotFoundError) rather than returning None, so your existing check is redundant.
Additionally, an IOError might be raised when iterating over the file rather than opening it initially, so the error message printed may be incorrect in those circumstances.
Lastly, since infile is an iterable, you can iteratively construct a list from it readily using the constructor that accepts an iterable directly like so:
return list(infile)
I have a function that opens a file and returns an opened file object.
def read_any():
try:
opened = gzip.open(fname, 'r')
except IOError:
opened = open(fname, 'r')
return opened
When I attempt to run this function on some non-zipped file except condition does not get triggered and the function crashes with the message: IOError: Not a gzipped file.
Ok, now I try and do the same with with statement:
def read_any2():
try:
with gzip.open(fname, 'r') as f:
return f.read()
except IOError:
with open(fname, 'r') as f:
return f.read()
Now, if I try to run the same file the function works as intended.
Can you explain why doesn't except condition get triggered?
To see what's going on, test it in a REPL:
>>> import gzip
>>> f = gzip.open('some_nongzipped_file', 'r')
You will see that this doesn't raise an error. Once you, however, read from the object:
>>> f.read()
... (snip)
OSError: Not a gzipped file
, it raises the error.
In short: Simply creating the file object doesn't read anything from the file yet, and thus doesn't know if it should fail or not.
Since in the first example you just return the file object, when you try to read from it later it will raise the exception there (outside your raise-except block). In your second example you return f.read() which reads and therefore raises the exception. It has nothing to do with the with block, as you can see if you remove it:
def read_any_mod():
try:
opened = gzip.open(fname, 'r')
return opened.read()
except IOError:
opened = open(fname, 'r')
return opened.read()
I'm trying to use readline on file in a for loop. The problem is that I start getting I/O errors. It seems that I get I/O error after 10 readlines.
Here is my function:
def getAll():
with open("nodes2.txt", "r+") as f:
for i in range(0, 200):
print "**%s**"%(i)
try:
file = f.readline()
file = file[:-1]
# print "*%s*" % (file)
entities = getAllPagesEntities(file)
# print entities
for en in entities:
try:
dict = getFirmAttributes(en)
printToFile(dict)
except Exception,e:
with open("log_getFirmAttributes.txt","a") as f:
f.write(str(e))
f.write("\n")
except Exception,e:
with open("log_readFile.txt","a") as f:
f.write(str(e))
f.write("\n")
Here is a printed catched exception:
I/O operation on closed file
I think that this problem can't be caused by another used functions so I don't attach them here. I thought that it is caused by the file used but when I try to readline 200 and print them, everything works perfect.
with open("nodes2.txt", "r+") as f:
for i in range(0, 200):
print f.readline()
Have you any idea what could be the problem? Thanks
Following lines in the except block overwrites f causing open file to be closed.
with open("log_readFile.txt","a") as f:
f.write(str(e))
f.write("\n")
Change the name f for the file for appending to another name will solve the problem:
with open("log_readFile.txt", "a") as logf:
logf.write(str(e))
logf.write("\n")
I'm trying to read all files from a folder that matches a certain criteria. My program crashes once I have an exception raised. I am trying to continue even if there's an exception but it still stops executing.
This is what I get after a couple of seconds.
error <type 'exceptions.IOError'>
Here's my code
import os
path = 'Y:\\Files\\'
listing = os.listdir(path)
try:
for infile in listing:
if infile.startswith("ABC"):
fo = open(infile,"r")
for line in fo:
if line.startswith("REVIEW"):
print infile
fo.close()
except:
print "error "+str(IOError)
pass
Put your try/except structure more in-wards. Otherwise when you get an error, it will break all the loops.
Perhaps after the first for-loop, add the try/except. Then if an error is raised, it will continue with the next file.
for infile in listing:
try:
if infile.startswith("ABC"):
fo = open(infile,"r")
for line in fo:
if line.startswith("REVIEW"):
print infile
fo.close()
except:
pass
This is a perfect example of why you should use a with statement here to open files. When you open the file using open(), but an error is catched, the file will remain open forever. Now is better than never.
for infile in listing:
try:
if infile.startswith("ABC"):
with open(infile,"r") as fo
for line in fo:
if line.startswith("REVIEW"):
print infile
except:
pass
Now if an error is caught, the file will be closed, as that is what the with statement does.
Move the try/except inside the for loop.
Like in:
import os
path = 'C:\\'
listing = os.listdir(path)
for infile in listing:
try:
if infile.startswith("ABC"):
fo = open(infile,"r")
for line in fo:
if line.startswith("REVIEW"):
print infile
fo.close()
except:
print "error "+str(IOError)
You're code is doing exactly what you're telling it to do. When you get an exception, it jumps down to this section:
except:
print "error "+str(IOError)
pass
Since there's nothing after that, the program ends.
Also, that pass is superfluous.
As per strictest interpretation of the question "continue even if there's an exception". Python gives us a keyword "finally" which executes a block of code no matter what precedes it. The only issue with this method will run a block of code regardless of the type of error, which might not be desirable for all cases.
try:
unreal = 3/0 # raises divide by zero exception
print(unreal)
# handles zerodivision exception
except :
print("Can't divide by zero, 0 has no multiplicative inverse")
finally:
# this block is always executed
print("Brahmagupta claimed that “zero divided by a zero is zero.”)
hi im slowly trying to learn the correct way to write python code. suppose i have a text file which i want to check if empty, what i want to happen is that the program immediately terminates and the console window displays an error message if indeed empty. so far what ive done is written below. please teach me the proper method on how one ought to handle this case:
import os
def main():
f1name = 'f1.txt'
f1Cont = open(f1name,'r')
if not f1Cont:
print '%s is an empty file' %f1name
os.system ('pause')
#other code
if __name__ == '__main__':
main()
There is no need to open() the file, just use os.stat().
>>> #create an empty file
>>> f=open('testfile','w')
>>> f.close()
>>> #open the empty file in read mode to prove that it doesn't raise IOError
>>> f=open('testfile','r')
>>> f.close()
>>> #get the size of the file
>>> import os
>>> import stat
>>> os.stat('testfile')[stat.ST_SIZE]
0L
>>>
The pythonic way to do this is:
try:
f = open(f1name, 'r')
except IOError as e:
# you can print the error here, e.g.
print(str(e))
Maybe a duplicate of this.
From the original answer:
import os
if (os.stat(f1name).st_size == 0)
print 'File is empty!'
If file open succeeds the value of 'f1Cont` will be a file object and will not be False (even if the file is empty).One way you can check if the file is empty (after a successful open) is :
if f1Cont.readlines():
print 'File is not empty'
else:
print 'File is empty'
Assuming you are going to read the file if it has data in it, I'd recommend opening it in append-update mode and seeing if the file position is zero. If so, there's no data in the file. Otherwise, we can read it.
with open("filename", "a+") as f:
if f.tell():
f.seek(0)
for line in f: # read the file
print line.rstrip()
else:
print "no data in file"
one can create a custom exception and handle that using a try and except block as below
class ContentNotFoundError(Exception):
pass
with open('your_filename','r') as f:
try:
content=f.read()
if not content:
raise ContentNotFoundError()
except ContentNotFoundError:
print("the file you are trying to open has no contents in it")
else:
print("content found")
print(content)
This code will print the content of the file given if found otherwise will print the message
the file you are trying to open has no contents in it