To handle exceptions, and to call code when an exception
occurs, you can use try/except statement. The try block contains code that
might throw an exceptions. If that exception occurs, the code in the try block
stops being executed, and the code in the except block is run. If no error
occurs, the code in the except doesn’t run.
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try:
a = 7
b = 0
print(a/b)
print('Done calculation')
except ZeroDivisionError:
print('An
error occured due to the zero division')
|
An error occured due to the zero division
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In
the code above, the except statement defines the type of exception to handle
(in our case, the ZeroDivisonError). We can expand this example with the
raw_input function in order to input numbers we want.
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userinput1 = raw_input('Enter first value: ')
userinput2 = raw_input('Enter second value: ')
try:
a =
int(userinput1)
b =
int(userinput2)
print(a/b)
print('Done calculation')
except ZeroDivisionError:
print('An
error occured due to the zero division')
except ValueError:
print("You've entered a non-integer value.")
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First
run:
Enter
first value: kill
Enter
second value: bill
You've
entered a non-integer value.
Second
run:
Enter
first value: 5
Enter
second value: 6
0
Done
calculation
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Example: What is the output of this code ?
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try:
variable =
10
print(10/2)
except ZeroDivisionError:
print('Error')
print('Finished')
|
5
Finished
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A
try statement can have multiple different except blocks to handle different
exceptions. Multiple exceptions can also be put into a single except block
using parenthesis, to have the except block handle all of them.
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try:
a = 10
print a + 'Hello'
print a/2
except
ZeroDivisionError:
print 'Divide by zero'
except
(ValueError, TypeError):
print 'Error occured'
|
Error
occured
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Example:
What is the output of this code?
|
try:
a = 30
print a / 0
print 'the meaning of life'
except(ValueError,
TypeError):
print 'ValueError or TypeError occured'
except
ZeroDivisionError:
print 'Divide by zero'
|
Divide
by zero
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An
except statement without any exception specified will catch all errors. These
should be used sparingly, as they can catch up unexpected errors and hid
programming mistakes.
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try:
word = 'SPAM'
print word/0
except:
print 'An error occurred'
|
An
error occurred
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Exception handling is particularly useful when dealing
with user input.
Example: fill in the blanks to handle all possible
exceptions.
|
___:
a =
_____(':')
b = input(':')
print
float(a)/_____(b)
_____:
_____
'Invalid input'
|
:5
:6
0.833333333333
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To
ensure that code runs no matter what errors occur, you can use a finally
statement. The finally statement is placed is placed at the bottom of a
try/except statement. Code within a finally statement always runs after
execution of the code in the try, and possibility in the except blocks.
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try:
print 'Hello'
print 1/0
except
ZeroDivisionError:
print 'Divided by zero'
finally:
print 'This code will run no matter what'
|
Hello
Divided
by zero
This
code will run no matter what
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Example:
what is the output of this code?
|
try:
print 1
except:
print 2
finally:
print 3
|
1
3
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Code
in a finally statement even runs if an uncaught exception occurs in one of the
preceding blocks.
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try:
print 1
print 10/0
except
ZeroDivisionError:
print unknown_var
finally:
print 'This is executed last'
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1
This is
executed last
NameError:
name 'unknown_var' is not defined
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Example:
Fill in the blanks to create a try/except/finally block.
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__:
print 1
______:
print 2
________:
print 42
|
1
42
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