Errors & Exception

 

# Exceptions

try:
    result = 1 / 0
except Exception as e:
    print(f"'{e}' error occurred!")

lst = [1, 2, 3]
try:
    print(lst[3])
except Exception as e:
    print(f"'{e}' error occurred!")

# 결과
'division by zero' error occurred!
'list index out of range' error occurred!

 

# Try, except, else
# - The else block in a try statement is executed only if
# no exceptions are raised in the try block.

# Try, except, else, finally
# - The finally block is particularly useful for cleanup actions
# that must be executed under all circumstances, such as closing
# a file or releasing resources.

 

# Defining a Custom Exception
class NegativeValueError(Exception):
    def __init__(self, msg):
        self.msg = msg
        super().__init__(self.msg)

def check_positive(number):
    if number < 0:
        raise NegativeValueError("The number is negative.")

try:
    check_positive(10)
    check_positive(-1)
except NegativeValueError as e:
    print(f"Caught an exception: {e.msg}")

# 결과
Caught an exception: The number is negative.

 

# 간단한 함수를 하나 만든다.

 

def exceptionHandling():

a = 10

b = 2

 

c = (a / b)

print(c)

 

exceptionHandling()

 

# 결과

5.0

 

# 0으로 나누기 에러를 만들어 본다.

 

def exceptionHandling():

a = 10

b = 0

 

c = (a / b)

print(c)

 

exceptionHandling()

 

# 결과

ZeroDivisionError: division by zero

 

# 타입 에러를 만들어 본다.

 

def exceptionHandling():

a = 10

b = "string"

 

c = (a / b)

print(c)

 

exceptionHandling()

 

# 결과

TypeError: unsupported operand type(s) for /: 'int' and 'str'

 

# 타입 에러에 대한 예외 처리를 만든다.

 

def exceptionHandling():

try:

a = 10

b = "string"

 

c = (a / b)

print(c)

except TypeError:

print("Can't divide by string ")

 

exceptionHandling()

 

# 결과

Can't divide by string

 

# 0으로 나누기 에러에 대한 예외 처리를 만든다.

 

def exceptionHandling():

try:

a = 10

b = 0

 

c = (a / b)

print(c)

except TypeError:

print("Can't divide by string ")

except ZeroDivisionError:

print("Zero Division")

 

exceptionHandling()

 

# 결과

Zero Division

 

# 에러에 대한 공통 예외 처리를 만든다.

 

def exceptionHandling():

try:

a = 10

b = 0

 

c = (a / b)

print(c)

except:

print("In the except block")

 

exceptionHandling()

 

# 결과

In the except block

 

# 또 다른 에러에 대해 공통 예외 처리가 적용되는지 확인한다.

 

def exceptionHandling():

try:

a = 10

b = "string"

 

c = (a / b)

print(c)

except:

print("In the except block")

 

exceptionHandling()

 

# 결과

In the except block

 

# 에러가 없을 경우 실행하는 로직을 정의할때

 

def exceptionHandling():

try:

a = 10

b = 2

 

c = (a / b)

print(c)

except:

print("In the except block")

 

else:

print("Because there was no exception, else is executed")

 

exceptionHandling()

 

# 결과

5.0

Because there was no exception, else is executed

 

# 에러가 있던, 없든 간에 무조건 실행하는 로직을 정의할때

## 에러 없을 경우

 

def exceptionHandling():

try:

a = 10

b = 2

 

c = (a / b)

print(c)

except:

print("In the except block")

 

else:

print("Because there was no exception, else is executed")

 

finally:

print("Finally, always executed")

 

exceptionHandling()

 

# 결과

5.0

Because there was no exception, else is executed

Finally, always executed

 

## 에러 있을 경우

 

def exceptionHandling():

try:

a = 10

b = 0

 

c = (a / b)

print(c)

except:

print("In the except block")

 

else:

print("Because there was no exception, else is executed")

 

finally:

print("Finally, always executed")

 

exceptionHandling()

 

# 결과

In the except block

Finally, always executed

 

 

# 예외 문구를 담는 변수 이용

t = (1, 2, 3)

 

try:

t.append(4)

except AttributeError as e:

print("Error formed: ", e)

 

# 결과

Error formed:  'tuple' object has no attribute 'append'

 

# 기본적인 에러 예외 처리
result= None

x = int(input('number 1: '))
y = int(input('number 2: '))

try:
    result = x / y
except Exception as e:
    print(e)
# this will be executed only when we dont have any exception
else:
    print('inside else')
# this will be executed for both the cases when we executed only
finally:
    print('inside finally')

print('result = ', result)

# 결과
number 1: 10
number 2: 0
division by zero
inside finally
result =  None

 

class Coffee:
    def __init__(self, temperature):
        self.__temperature = temperature
    
    def drink_coffee(self):
        if self.__temperature > 50:
            print('Hot to drink.')
            # Or
            raise Exception('Coffee too hot.')
        elif self.__temperature < 20:
            print('Cold to drink.')
        else:
            print('Coffee OK to drink.')

cup = Coffee(100)
cup.drink_coffee()

# 결과
...
    raise Exception('Coffee too hot.')
Exception: Coffee too hot.

 

class CoffeeException(Exception):
    def __init__(self, arg):
        self.msg = arg

class CoffeeWithMyException:
    def __init__(self, temperature):
        self.__temperature = temperature
    
    def drink_coffee(self):
        if self.__temperature > 50:
            raise CoffeeException('Coffee too hot.')
        elif self.__temperature < 20:
            CoffeeException('Cold to drink.')
        else:
            print('Coffee OK to drink.')

cup = CoffeeWithMyException(50)
cup.drink_coffee()

cup = CoffeeWithMyException(80)
cup.drink_coffee()

# 결과
Coffee OK to drink.
...
    raise CoffeeException('Coffee too hot.')
CoffeeException: Coffee too hot.

 

# equation solver
#
# a * x + b = c는
# 1) a * x + b - b = c - b
# 2) a * x = c - b
# 3) a * x / a = (c - b) / a
# 4) x = (c - b) / a

def solve_equation(a: float, b: float, c: float) -> float:
    try:
        return (float(c) - float(b)) / float(a)
    except (ZeroDivisionError, ValueError):
        print('Error! Enter a valid value.')
        raise

print('ax + b = c linear equation solver')
a = input('Enter a: ')
b = input('Enter b: ')
c = input('Enter c: ')

try:
    x = solve_equation(a, b, c)
except Exception:
    print('Something bad happened.')
else:
    print(f'x is {x}')

# 결과
ax + b = c linear equation solver
Enter a: 10
Enter b: 2
Enter c: 5
x is 0.3

Enter a: 0
Enter b: 3
Enter c: 6
Error! Enter a valid value.
Something bad happened.

Enter a: a
Enter b: 5
Enter c: 6
Error! Enter a valid value.
Something bad happened.

 

# when to use try-except

# if somekey in d:
    # extract_data(d)
# else:
    # skip_this_one(d)


# versus

# try:
    # extract_data(d)
# except:
    # skip_this_one(d)

# in general, when using a try-except or deciding whether to use if/else,
# it's better to be as specific as possible.
# So, if it's one particular key that you know is either going to be
# there or not there, I'd use an if/else.
# If I don't know what might go wrong but
# I know that something might go wrong,
# so a dictionary might be in a different format than I'm expecting,
# then I might use a try-except.

 

# try - except의 예
import sys

total: float = 0

while True:
    user_input: str = input("Enter a number: ")

    if user_input == '0':
        print(f"Total: {total}")
        sys.exit()
    
    try:
        total += float(user_input)
    except ValueError:
        print("Invalid... try again.")

# 결과
Enter a number: 10
Enter a number: -5
Enter a number: a
Invalid... try again.
Enter a number: 0
Total: 5.0

 

# The difference netween a beginner and a professional
# is that a professional has run into practically
# every error that has ever existed, meaning that
# if something ever goes wrong in a program, they're
# going to be able to identify that error and
# fix it immediately because they know exactly what it was
# due to their experience with all these errors.

def check_age(age: int) -> bool:
    if age < 0:
        raise ValueError("Not a valid age...")
    elif age >= 19:
        print("You are old enough!")
        return True
    else:
        print("You are not old enough...")
        return False

check_age(10)
check_age(20)
check_age(-1)

# 결과
You are not old enough...
You are old enough!
...
ValueError: Not a valid age...

 

import string
import sys

def is_letters_only(text: str) -> None:
    alphabet: str = string.ascii_letters + " "
    
    for char in text:
        if char not in alphabet:
            raise ValueError("Text can only contain letters from the alphabet!")

    print(f"'{text}' is letters-only, good job!")

def main() -> None:
    while True:
        try:
            user_input: str = input("Check text: ('exit' to quit)")
            if user_input == 'exit':
                sys.exit()
            is_letters_only(user_input)
        except ValueError:
            print("Please only enter English letters...")
        except Exception as e:
            print(f"Encounted and unknown exception: {type(e)}, {e}")

main()

# 결과
Check text: ('exit' to quit)hello
'hello' is letters-only, good job!
Check text: ('exit' to quit)hello!
Please only enter English letters...
Check text: ('exit' to quit)exit

 

# Take advantage of each block
#
# try:
#   # Do something
# except MyException as e:
#   # Handle exception
# else:
#   # Runs when there are no exceptions
# finally:
#   # Always runs after try:

import json

def load_json_key(data, key):
    try:
        result_dict = json.loads(data)
    except ValueError as e:
        raise KeyError from e
    else:
        return result_dict[key]

def main():
    try:
        # load_json_key('{"foo": bad payload}', 'foo')
        print(load_json_key('{"foo": "bar"}', 'foo'))
        # assert False
    except KeyError:
        print("Saw KeyError")

main()

# 결과
bar

 

def ask_for_int():
    while True:
        try:
            result = int(input("Please provide number: "))
        except:
            print("Whoops! That is not a number")
            continue
        else:
            print("Yes thank you")
            break
        finally:
            print("End of try/except/finally")

ask_for_int()

# Output:
Please provide number: asas
Whoops! That is not a number
End of try/except/finally
Please provide number: 11
Yes thank you
End of try/except/finally

 

def ask():
    waiting = True
    while waiting:
        try:
            n = int(input("Enter a number"))
        except:
            print("Please try again! \n")
            continue
        else:
            waiting = False

ask()

# Output:
Enter a number11

 

try:
    age = int(input("Enter your age: "))
    print(f"Next year, you will be {age+1} years old.")

except ValueError:
    print(f"That's not a valid age!")

# Output:
Enter your age: 10
Next year, you will be 11 years old.

 

try:
    with open('nonexistent.txt', 'r') as f:
        content = f.read()

except FileNotFoundError:
    print("The file does not exist.")

except IOError:
    print("An error occurred while reading the file.")

# Output:
The file does not exist.