Exceptions

Even if a statement of expression is syntactically correct, errors may still occure when a program is running.

Exceptions:

• are errors deteccted during Execution.
• are thrown or raised from anywhere in a program.
• print a descriptive error message and/or debugging information (name of th file and the line where the eror occured).
• terminate a program.

ValueError:

• source: int(), float(). and many more
• Trigger: operand or argument has the right type but an inappropriate value.

print(int('one handred'))
# int will throw a Value exception
# invalid literal for int()

TypeError:

• Source: multiple operators and fucntions
• Trigger: operand or argument has the wrong type

ZeroDivisionError:

• Source: division and modulo operators
• Trigger: dividing by zero

NameError:

• Source: expression with variable
• Trigger: usign a variable that has not been declared

IndexError:

• Source: sequence index operator
• Trigger: accessing an index that is out of range

mylist = [1,2,3]
print(mylist[10])
#this will cause an indexerror
print('finished')

KeyError:

• Source: dictionary index operator

• Trigger: accessing a key that does not exist

• friendlier error message
• fixing problem without exiting
• offer alternatives

Catching Exceptions

try:
    try_block
Except exception_type:
    exception_block

# multiple
try:
    try_block
Except exception_type1:
    exception_block1
Except exception_type2:
    exception_block2

Example

mylist = [1,2,3]
try:
    print(mylist[10])
    print('got item at index 10')
except IndexError:
    print('mylist index is out of bounds')
print('finished')

Open function Exceptions

FileExistsError:

• Source:
• Trigger: attempting to overwrite to a file that exists

FileNotFoundError:

• Source:
• Trigger: filename does not exist

PermissionError:

• Source:
• Trigger: operation not allwoed based on permissions

IsADirectoryError:

• Source:
• Trigger: accessing a directory as if it were a file

try:
    with open('does_not_exist.txt') as f:
        print(f.read())
except PermissionError:
    print('file permission error')
except FileNotFoundError:
    print('does_not_exist.txt' not found)

Exceptions handling questions could be

• what are Exceptions
• Exception Types
• Why Catch Exceptions
• Catchign Exceptions
• Open function Exceptions

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