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When something goes wrong an exception is raised. For example, if you try to divide by zero, ZeroDivisionError is raised or if you try to access a nonexistent key in a dictionary, KeyError is raised.
empty_dict = {}
# empty_dict['key'] # Uncomment to see the traceback
try-except structure¶If you know that a block of code can fail in some manner, you can use try-except structure to handle potential exceptions in a desired way.
# Let's try to open a file that does not exist
file_name = "not_existing.txt"
try:
with open(file_name) as my_file:
print("File is successfully open")
except FileNotFoundError as e:
print(f"Uups, file: {file_name} not found")
print(f"Exception: {e} was raised")
Uups, file: not_existing.txt not found Exception: [Errno 2] No such file or directory: 'not_existing.txt' was raised
If you don't know the type of exceptions that a code block can possibly raise, you can use Exception which catches all exceptions. In addition, you can have multiple except statements.
def calculate_division(var1, var2):
result = 0
try:
result = var1 / var2
except ZeroDivisionError as ex1:
print("Can't divide by zero")
except Exception as ex2:
print(f"Exception: {ex2}")
return result
result1 = calculate_division(3, 3)
print(f"result1: {result1}")
result2 = calculate_division(3, "3")
print(f"result2: {result2}")
result3 = calculate_division(3, 0)
print(f"result3: {result3}")
result1: 1.0 Exception: unsupported operand type(s) for /: 'int' and 'str' result2: 0 Can't divide by zero result3: 0
try-except can be also in outer scope:
def calculate_division(var1, var2):
return var1 / var2
try:
result = calculate_division(3, "3")
except Exception as e:
print(e)
unsupported operand type(s) for /: 'int' and 'str'
In your own applications, you can use custom exceptions for signaling users about errors which occur during your application run time.
import math
# Define your own exception
class NegativeNumbersNotSupported(Exception):
pass
# Dummy example how to use your custom exception
def secret_calculation(number1, number2):
if number1 < 0 or number2 < 0:
msg = f"Negative number in at least one of the parameters: {number1}, {number2}"
raise NegativeNumbersNotSupported(msg)
return math.sqrt(number1) + math.sqrt(number2)
# Uncomment to see the traceback
# result = secret_calculation(-1, 1)