Error_Handling

Error Handling ❗ 💥

Homework:

• URL Error
  • errorlog.txt
  • errorHTTPlog.txt
  • successfulHTTP.txt

Reading images using the pillow module:

• Image_Reader

Exceptions

• Errors that occur during execution of a program. When that error occurs, Python generates an exception that can be handled, which avoids the program to crash

Raising an exception

Raising an exception using the raise exception statement

Common exception errors

Exception Errors	Explanation
IOError	If the file cannot be opened
ImportError	If python cannot find the module
ValueError	Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value e.g. printing int instead of str
KeyboardInterrupt	Raised when user hits the interrupt key
EOFError	Raised when one of the built-in functions input() or raw_input() hits an end-of-file condition (EOF) without reading any data
NameError	Raised when Py interpreter does not recognise a local or global object name

NameError Example:

>>> print 10 * ten
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'ten' is not defined

and this output show it's a TypeError
>>> print 1 + 'ten'
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Try and Except

# code that may raise an error
try:
    fh = open("non_existing_file")

# code that should run if the code inside the try block fails
# Input/Output Error
except IOError:
    print("The file does not exist")

Handling ValueErrors

while True:
try:
    x = int(raw_input("Please enter a number:))
    break
except ValueError:
    print("That was not a valid number")

Or:

import sys
print "Lets fix the previous code with exception handling"

try:
number = int(raw_input("Enter a number between 1 - 10"))

except ValueError:
print "Err.. numbers only"
sys.exit()

print "you entered number", number