If I have a try except block in my python code, and the first line of my try statement raises an exception will it automatically move on to exception or will it finish the try block first?
try:
int(string)
string = "This was a mistake, can't int string"
except:
pass
Here is it checking if it can int(string), which it can't, and then it immediately moves onto except, or does it do the string assignment first?
When I run it, it seems like it stops right away, but I want to know if that's happening for sure or something else.
Thanks
Let's try it.
try:
1/0
print("what?")
except:
print("nope")
Output:
nope
>>>
When an exception is raised it moves on to the except.
From the python docs, I found this,
"The try statement works as follows.
First, the try clause (the statement(s) between the try and except keywords) is executed.
If no exception occurs, the except clause is skipped and execution of the try statement is finished.
If an exception occurs during execution of the try clause, the rest of the clause is skipped. 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.
If an exception occurs which does not match the exception named in the except clause, it is passed on to outer try statements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above."
Cheers!
Hope it helps!
#the try block is to run the process
try:
string = "This was a mistake, can't int string"
#you are parsing the string into an int
int(string)
#this block is to capture the exception when it is raised
except:
#here I am just printing the exception
print('Exception')
#the pass is to pass the execution and follow the remaining process.
pass
Output:
Exception
>>>
Well, that the whole point of an error handler. When an error occurs in out program, it crashes, it won't simply skip the error line.
try:
int(string)
string = "This was a mistake, can't int string"
except:
pass
So if the try block finishes even after an error occurred in the middle, then the error handler'd seem a bit pointless.
There seem to be some issue with your program and maybe your understanding of the try except block.
First of all, from the block of code you have provided, it seems to me like string isn't even defined yet when you try to pass int(string).
Secondly, the string variable which you seem to be defining under seems to me like is something you are trying to print to the console once the program spits out an error and passes to the except block. If this is what you are trying to do, then your code should look something like this:
try:
int(string)
except:
print("This was a mistake, can't int string")
If you were to run the code provided above, you would indeed confirm that the try except statement is working as it would print "This was a mistake, can't int string" into the console instead of giving an error. Hope this was somewhat helpful and understandable. You got this chief, keep grinding!
I'm in trouble about how to end a 'try' loop, which is occurred since I have the 'try', here is the code:
import time
class exe_loc:
mem = ''
lib = ''
main = ''
def wizard():
while True:
try:
temp_me = input('Please specify the full directory of the memory, usually it will be a folder called "mem"> ' )
if temp_me is True:
exe_loc.mem = temp_me
time.sleep(1)
else:
print('Error value! Please run this configurator again!')
sys.exit()
temp_lib = input('Please specify the full directory of the library, usually it will be a folder called "lib"> ')
if temp_lib is True:
exe_loc.lib = temp_lib
time.sleep(1)
else:
print('Invalid value! Please run this configurator again!')
sys.exit()
temp_main = input('Please specify the full main executable directory, usually it will be app main directory> ')
if temp_main is True:
exe_loc.main = temp_main
time.sleep(1)
I tried end it by using break, pass, and I even leaves it empty what I get is Unexpected EOF while parsing, I searched online and they said it is caused when the code blocks were not completed. Please show me if any of my code is wrong, thanks.
Btw, I'm using python 3 and I don't know how to be more specific for this question, kindly ask me if you did not understand. Sorry for my poor english.
EDIT: Solved by removing the try because I'm not using it, but I still wanna know how to end a try loop properly, thanks.
Your problem isn't the break, it's the overall, high-level shape of your try clause.
A try requires either an except or a finally block. You have neither, which means your try clause is never actually complete. So python keeps looking for the next bit until it reaches EOF (End Of File), at which point it complains.
The python docs explain in more detail, but basically you need either:
try:
do_stuff_here()
finally:
do_cleanup_here() # always runs, even if the above raises an exception
or
try:
do_stuff_here()
except SomeException:
handle_exception_here() # if do_stuff_here raised a SomeException
(You can also have both the except and finally.) If you don't need either the cleanup or the exception handling, that's even easier: just get rid of the try altogether, and have the block go directly under that while True.
Finally, as a terminology thing: try is not a loop. A loop is a bit of code that gets executed multiple times -- it loops. The try gets executed once. It's a "clause," not a "loop."
You have to also 'catch' the exception with the except statement, otherwise the try has no use.
So if you do something like:
try:
# some code here
except Exception:
# What to do if specified error is encountered
This way if anywhere in your try block an exception is raised it will not break your code, but it will be catched by your except.
I am not sure why we need finally in try...except...finally statements. In my opinion, this code block
try:
run_code1()
except TypeError:
run_code2()
other_code()
is the same with this one using finally:
try:
run_code1()
except TypeError:
run_code2()
finally:
other_code()
Am I missing something?
It makes a difference if you return early:
try:
run_code1()
except TypeError:
run_code2()
return None # The finally block is run before the method returns
finally:
other_code()
Compare to this:
try:
run_code1()
except TypeError:
run_code2()
return None
other_code() # This doesn't get run if there's an exception.
Other situations that can cause differences:
If an exception is thrown inside the except block.
If an exception is thrown in run_code1() but it's not a TypeError.
Other control flow statements such as continue and break statements.
You can use finally to make sure files or resources are closed or released regardless of whether an exception occurs, even if you don't catch the exception. (Or if you don't catch that specific exception.)
myfile = open("test.txt", "w")
try:
myfile.write("the Answer is: ")
myfile.write(42) # raises TypeError, which will be propagated to caller
finally:
myfile.close() # will be executed before TypeError is propagated
In this example you'd be better off using the with statement, but this kind of structure can be used for other kinds of resources.
A few years later, I wrote a blog post about an abuse of finally that readers may find amusing.
They are not equivalent. finally code is run no matter what else happens*.
It is useful for cleanup code that has to run.
*:
As Mark Byers commented, anything causes the process to terminate immediately also prevents the finally-code to run.
The latter could be an os._exit(). or powercut, but an infinite loop or other things also fall into that category.
To add to the other answers above, the finally clause executes no matter what whereas the else clause executes only if an exception was not raised.
For example, writing to a file with no exceptions will output the following:
file = open('test.txt', 'w')
try:
file.write("Testing.")
print("Writing to file.")
except IOError:
print("Could not write to file.")
else:
print("Write successful.")
finally:
file.close()
print("File closed.")
OUTPUT:
Writing to file.
Write successful.
File closed.
If there is an exception, the code will output the following, (note that a deliberate error is caused by keeping the file read-only.
file = open('test.txt', 'r')
try:
file.write("Testing.")
print("Writing to file.")
except IOError:
print("Could not write to file.")
else:
print("Write successful.")
finally:
file.close()
print("File closed.")
OUTPUT:
Could not write to file.
File closed.
We can see that the finally clause executes regardless of an exception. Hope this helps.
Here's a peice of code to clarify the difference:
...
try:
a/b
print('In try block')
except TypeError:
print('In except block')
finally:
print('In finally block')
print('Outside')
a, b = 0, 1
Output:
In try block
In finally block
Outside
(No errors, except block skipped.)
a, b = 1, 0
Output:
In finally block
Traceback (most recent call last):
a/b
ZeroDivisionError: division by zero
(No exception handling is specified for ZeroDivisionError and only the finally block is executed.)
a, b = 0, '1'
Output:
In except block
In finally block
Outside
(Exception is handled properly and the program is not interrupted.)
Note: If you have an except block to handle all types of errors, the finally block will be superfluous.
As explained in the documentation, the finally clause is intended to define clean-up actions that must be executed under all circumstances.
If finally is present, it specifies a ‘cleanup’ handler. The try
clause is executed, including any except and else clauses. If an
exception occurs in any of the clauses and is not handled, the
exception is temporarily saved. The finally clause is executed. If
there is a saved exception it is re-raised at the end of the finally
clause.
An example:
>>> def divide(x, y):
... try:
... result = x / y
... except ZeroDivisionError:
... print("division by zero!")
... else:
... print("result is", result)
... finally:
... print("executing finally clause")
...
>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'
As you can see, the finally clause is executed in any event. The TypeError raised by dividing two strings is not handled by the except clause and therefore re-raised after the finally clause has been executed.
In real world applications, the finally clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.
The code blocks are not equivalent. The finally clause will also be run if run_code1() throws an exception other than TypeError, or if run_code2() throws an exception, while other_code() in the first version wouldn't be run in these cases.
In your first example, what happens if run_code1() raises an exception that is not TypeError? ... other_code() will not be executed.
Compare that with the finally: version: other_code() is guaranteed to be executed regardless of any exception being raised.
Using delphi professionally for some years taught me to safeguard my cleanup routines using finally. Delphi pretty much enforces the use of finally to clean up any resources created before the try block, lest you cause a memory leak. This is also how Java, Python and Ruby works.
resource = create_resource
try:
use resource
finally:
resource.cleanup
and resource will be cleaned up regardless of what you do between try and finally. Also, it won't be cleaned up if execution never reaches the try block. (i.e. create_resource itself throws an exception) It makes your code "exception safe".
As to why you actually need a finally block, not all languages do. In C++ where you have automatically called destructors which enforce cleanup when an exception unrolls the stack. I think this is a step up in the direction of cleaner code compared to try...finally languages.
{
type object1;
smart_pointer<type> object1(new type());
} // destructors are automagically called here in LIFO order so no finally required.
Finally can also be used when you want to run "optional" code before running the code for your main work and that optional code may fail for various reasons.
In the following example, we don't know precisely what kind of exceptions store_some_debug_info might throw.
We could run:
try:
store_some_debug_info()
except Exception:
pass
do_something_really_important()
But, most linters will complain about catching too vague of an exception. Also, since we're choosing to just pass for errors, the except block doesn't really add value.
try:
store_some_debug_info()
finally:
do_something_really_important()
The above code has the same effect as the 1st block of code but is more concise.
finally is for defining "clean up actions". The finally clause is executed in any event before leaving the try statement, whether an exception (even if you do not handle it) has occurred or not.
I second #Byers's example.
Perfect example is as below:
try:
#x = Hello + 20
x = 10 + 20
except:
print 'I am in except block'
x = 20 + 30
else:
print 'I am in else block'
x += 1
finally:
print 'Finally x = %s' %(x)
A try block has just one mandatory clause: The try statement.
The except, else and finally clauses are optional and based on user preference.
finally:
Before Python leaves the try statement, it will run the code in the finally block under any conditions, even if it's ending the program. E.g., if Python ran into an error while running code in the except or else block, the finally block will still be executed before stopping the program.
Try running this code first without a finally block,
1 / 0 causes a divide by zero error.
try:
1 / 0
print(1)
except Exception as e:
1 / 0
print(e)
Then try running this code,
try:
1 / 0
print(1)
except Exception as e:
1 / 0
print(e)
finally:
print('finally')
For the first case you don't have a finally block,
So when an error occurs in the except block the program execution halts and you cannot execute anything after the except block.
But for the second case,
The error occurs but before the program halts python executes the finally block first and then causes the program to halt.
Thats is why you use finally and do stuff that is really important.
Run these Python3 codes to watch the need of finally:
CASE1:
count = 0
while True:
count += 1
if count > 3:
break
else:
try:
x = int(input("Enter your lock number here: "))
if x == 586:
print("Your lock has unlocked :)")
break
else:
print("Try again!!")
continue
except:
print("Invalid entry!!")
finally:
print("Your Attempts: {}".format(count))
CASE2:
count = 0
while True:
count += 1
if count > 3:
break
else:
try:
x = int(input("Enter your lock number here: "))
if x == 586:
print("Your lock has unlocked :)")
break
else:
print("Try again!!")
continue
except:
print("Invalid entry!!")
print("Your Attempts: {}".format(count))
Try the following inputs each time:
random integers
correct code which is 586(Try this and you will get your answer)
random strings
** At a very early stage of learning Python.
I was trying to run a code where i wanted to read excel sheets. Issue was, if there is a file which has no sheet named say : SheetSum I am not able to move it to error location!! Code i wrote was:
def read_file(data_file):
# data_file = '\rr\ex.xlsx'
sheets = {}
try:
print("Reading file: "+data_file)
sheets['df_1'] = pd.read_excel(open(data_file,'rb'), 'SheetSum')
except Exception as excpt:
print("Exception occurred", exc_info=True)
return sheets
read_file(file)
shutil.move( file, dirpath +'\\processed_files')
Giving Error :
[WinError 32] The process cannot access the file because it is being
used by another process
I had to add full try except with finally block and tell finally i need to close the file in any case like:
def read_file(data_file):
# data_file = '\rr\ex.xlsx'
sheets = {}
sheets_file = None
try:
print("Reading file: "+data_file)
sheets_file = open(data_file,'rb')
sheets['df_1'] = pd.read_excel(sheets_file, 'SheetSum')
except Exception as excpt:
print("Exception occurred", exc_info=True)
finally:
if sheets_file:
sheets_file.close()
return sheets
read_file(file)
shutil.move( file, dirpath +'\\processed_files')
Otherwise, file still remains open is the background.
If finally is present, it specifies a cleanup handler. The try
clause is executed, including any except and else clauses. If an
exception occurs in any of the clauses and is not handled, the
exception is temporarily saved. The finally clause is executed. If
there is a saved exception it is re-raised at the end of the finally
clause. If the finally clause raises another exception, the saved
exception is set as the context of the new exception.
..More Here
Just to enable Abhijit Sahu's comment on this answer to be seen much better and with syntax highlighting:
Like this, you can observe what happens with which code block when:
try:
x = Hello + 20
x = 10 + 20
except:
print 'I am in except block'
x = 20 + 30
else:
print 'I am in else block'
x += 1
finally:
print 'Finally x = %s' %(x)
Here you can see how try, except, else, and finally work together. Actually, since your code doesn't have 'else' then what you claimed is right. I.e., There is no difference between the two statements that you wrote. But if 'else' is used somewhere, then 'finally' makes a difference
import sys
def checkarg():
try:
filename=str(sys.argv[1])
if filename=="-mycommand":
print "SPECIFIC_TEXT"
sys.exit()
else:
return filename
except:
print "ERROR"
sys.exit()
Hello all...i have a problem with the code above. When i call the 'checkarg' function, if i did not pass any parameter on the command line i have the "ERROR" output and sys exit, just as expected.
But, if i provide a parameter on the command line (like "-mycommand") it prints the "SPECIFIC_TEXT" and then prints "ERROR" message from the EXCEPT block too.
The TRY block will only run when I provide a parameter, if I don't, then EXCEPT will get the turn. But, it is running the TRY and EXCEPT blocks together.
Does anybody knows the reason of this behavior?? Any mistake on my code? Tks for all !
I think I understand your question...
sys.exit() exits by raising a SystemExit exception, which your except statement is catching.
Answer found here: http://docs.python.org/library/sys.html
sys.exit([arg])
Exit from Python. This is implemented by raising the SystemExit exception, so cleanup actions specified by finally clauses of try statements are honored, and it is possible to intercept the exit attempt at an outer level.
sys.exit works by raising an exception. That's why your except block executes.
You really shouldn't be using try/except for situations where you can check the state using control flow logic.
Instead, in this case, check for if len(sys.argv) > 1.
Another reason never to use specifically a blank except: You will catch even system exceptions like SystemExit or KeyboardInterrupt, making it potentially impossible to terminate your program short of a messy kill.
I know you've already accepted an answer, but I think the root of the problem is that your try block contains code in which you do not necessarily wish to catch exceptions; rather, you merely wish these statements to be executed after the code in which you wish to catch exceptions if no exception occurs.
To address this, your try block should contain only filename=str(sys.argv[1]) and the rest of the code now in your try block should be moved to an else block, which will be executed only if no exception occurs. In other words:
try:
filename=str(sys.argv[1])
except:
print "ERROR"
sys.exit()
else:
if filename=="-mycommand":
print "SPECIFIC_TEXT"
sys.exit()
else:
return filename
Or in this case, since you exit the script entirely in the case of an exception, you don't actually need the else:
try:
filename=str(sys.argv[1])
except:
print "ERROR"
sys.exit()
if filename=="-mycommand":
print "SPECIFIC_TEXT"
sys.exit()
else:
return filename
The fact that you're catching every exception with your bare except is bad style and changing that would also avoid the problem, but to me, it's secondary. You do not wish to catch exceptions in your if/else code, so it should not be in the try block to begin with. IMHO, most admonitions against bare except would be moot if this guideline were followed more closely.
I can't understand this...
Cannot get this code to run and I've no idea why it is a syntax error.
try:
newT.read()
#existingArtist = newT['Exif.Image.Artist'].value
#existingKeywords = newT['Xmp.dc.subject'].value
except KeyError:
print "KeyError"
else:
#Program will NOT remove existing values
newT.read()
if existingArtist != "" :
newT['Exif.Image.Artist'] = artistString
print existingKeywords
keywords = os.path.normpath(relativePath).split(os.sep)
print keywords
newT['Xmp.dc.subject'] = existingKeywords + keywords
newT.write()
except:
print "Cannot write tags to ",filePath
Syntax error occurs on the last "except:". Again...I have no idea why python is throwing a syntax error (spent ~3hrs on this problem).
You can't have another except after the else. The try, except, and else blocks aren't like function calls or other code - you can't just mix and match them as you like. It's always a specific sequence:
try:
# execute some code
except:
# if that code raises an error, go here
# (this part is just regular code)
else:
# if the "try" code did not raise an error, go here
# (this part is also just regular code)
If you want to catch an error that occurs during the else block, you'll need another try statement. Like so:
try:
...
except:
...
else:
try:
...
except:
...
FYI, the same applies if you want to catch an error that occurs during the except block - in that case as well, you would need another try statement, like this:
try:
...
except:
try:
...
except:
...
else:
...
Reading the documentation would give you this phrase:
The try ... except statement has an optional else clause, which, when present, must follow all except clauses.
Move else to the end of your handler.
looking at the python documentation: http://docs.python.org/reference/compound_stmts.html#the-try-statement It doesn't look like you can have multiple elses with try. Maybe you meant finally at the end?