Python uses try and except to handle errors gracefully. A graceful exit (or graceful handling) of errors is a simple programming idiom - a program detects a serious error condition and "exits gracefully", in a controlled manner as a result. Often the program prints a descriptive error message to a terminal or log as part of the graceful exit, this makes our application more robust. The cause of an exception is often external to the program itself. An example of exceptions could be an incorrect input, wrong file name, unable to find a file, a malfunctioning IO device. Graceful handling of errors prevents our applications from crashing.
We have covered the different Python error types in the previous section. If we use try and except in our program, then it will not raise errors in those blocks.
try:
code in this block if things go well
except:
code in this block run if things go wrongExample:
try:
print(10 + '5') # TypeError: unsupported operand type(s) for +: 'int' and 'str'
except:
print('Something went wrong') In the example above the second operand is a string. We could change it to float or int to add it with the number to make it work. But without any changes, the second block, except, will be executed.
Example:
try:
name = input('Enter your name:')
year_born = input('Year you were born:')
age = 2022 - year_born
print(f'You are {name}. And your age is {age}.')
except:
print('Something went wrong')Something went wrongIn the above example, the exception block will run and we do not know exactly the problem. To analyze the problem, we can use the different error types with except.
In the following example, it will handle the error and will also tell us the kind of error raised.
try:
name = input('Enter your name: ')
year_born = input('Year you were born: ')
age = 2022 - year_born
print(f'You are {name}. And your age is {age}.')
except TypeError:
print('Type error occurred.')
except ValueError:
print('Value error occurred.')
except ZeroDivisionError:
print('zero division error occurred.')Enter your name: Case
Year you born: 1940
Type error occurred.In the code above the output is going to be TypeError. (TypeError: unsupported operand type(s) for +: 'int' and 'str') Now, let's add an additional block:
try:
name = input('Enter your name: ')
year_born = input('Year you born: ')
age = 2022 - int(year_born)
print('You are {name}. And your age is {age}.')
except TypeError:
print('Type error occur.')
except ValueError:
print('Value error occur.')
except ZeroDivisionError:
print('zero division error occur.')
else:
print('I usually run with the try block.')
finally:
print('I always run.')Enter your name: Case
Year you born: 1940
You are Case. And your age is 82.
I usually run with the try block
I always run.It is also shorten the above code as follows:
try:
name = input('Enter your name: ')
year_born = input('Year you born: ')
age = 2022 - int(year_born)
print('You are {name}. And your age is {age}.')
except Exception as e:
print(e)We use two operators:
- * for tuples
- ** for dictionaries
Let us take as an example below. It takes only arguments but we have list. We can unpack the list and changes to argument.
def sum_of_five_nums(a, b, c, d, e):
return a + b + c + d + e
lst = [1, 2, 3, 4, 5]
print(sum_of_five_nums(lst)) # TypeError: sum_of_five_nums() missing 4 required positional arguments: 'b', 'c', 'd', and 'e'When we run the this code, it raises an error, because this function takes numbers (not a list) as arguments. Let us unpack/destructure the list.
def sum_of_five_nums(a, b, c, d, e):
return a + b + c + d + e
lst = [1, 2, 3, 4, 5]
print(sum_of_five_nums(*lst)) # 15We can also use unpacking in the range built-in function that expects a start and an end.
numbers = range(2, 7) # normal call with separate arguments
print(list(numbers)) # [2, 3, 4, 5, 6]
args = [2, 7]
numbers = range(*args) # call with arguments unpacked from a list
print(numbers) # [2, 3, 4, 5, 6]A list or a tuple can also be unpacked like this:
countries = ['Finland', 'Sweden', 'Norway', 'Denmark', 'Iceland']
fin, sw, nor, *rest = countries
print(fin, sw, nor, rest) # Finland Sweden Norway ['Denmark', 'Iceland']
numbers = [1, 2, 3, 4, 5, 6, 7]
one, *middle, last = numbers
print(one, middle, last) # 1 [2, 3, 4, 5, 6] 7def unpacking_person_info(name, country, city, age):
return f'{name} lives in {country}, {city}. He is {age} year old.'
dct = {'name':'Case', 'country':'Chiba City Blue', 'city':'Turing', 'age':240}
print(unpacking_person_info(**dct)) # Case lives in Chiba City Blue, Turing. He is 240 years old.Sometimes we never know how many arguments need to be passed to a python function. We can use the packing method to allow our function to take unlimited number or arbitrary number of arguments.
def sum_all(*args):
s = 0
for i in args:
s += i
return s
print(sum_all(1, 2, 3)) # 6
print(sum_all(1, 2, 3, 4, 5, 6, 7)) # 28def packing_person_info(**kwargs):
# check the type of kwargs and it is a dict type
# print(type(kwargs))
# Printing dictionary items
for key in kwargs:
print("{key} = {kwargs[key]}")
return kwargs
print(packing_person_info(name="Case",
country="Chiba City Blue", city="Turing", age=250))name = Case
country = Chiba City Blue
city = Turing
age = 250
{'name': 'Case', 'country': 'Chiba City Blue', 'city': 'Turing', 'age': 250}Like in JavaScript, spreading is possible in Python. Let us check it in an example below:
lst_one = [1, 2, 3]
lst_two = [4, 5, 6, 7]
lst = [0, *list_one, *list_two]
print(lst) # [0, 1, 2, 3, 4, 5, 6, 7]
country_lst_one = ['Finland', 'Sweden', 'Norway']
country_lst_two = ['Denmark', 'Iceland']
nordic_countries = [*country_lst_one, *country_lst_two]
print(nordic_countries) # ['Finland', 'Sweden', 'Norway', 'Denmark', 'Iceland']If we are interested in an index of a list, we use enumerate built-in function to get the index of each item in the list.
for index, item in enumerate([20, 30, 40]):
print(index, item)countries = ['Finland', 'Sweden', 'Norway', 'Denmark', 'Iceland']
for index, i in enumerate(countries):
if i == 'Finland':
print(f'The country {i} has been found at index {index}.')
else:
print(f'hi, {i} at index {index}.')The country Finland has been found at index 0.
hi, Sweden at index 1.
hi, Norway at index 2.
hi, Denmark at index 3.
hi, Iceland at index 4.Sometimes we would like to combine lists when looping through them. See the example below:
fruits = ['banana', 'orange', 'mango', 'lemon', 'lime']
vegetables = ['Tomato', 'Potato', 'Cabbage','Onion', 'Carrot']
fruits_and_veges = []
for f, v in zip(fruits, vegetables):
fruits_and_veges.append({'fruit':f, 'veg':v})
print(fruits_and_veges)[{'fruit': 'banana', 'veg': 'Tomato'}, {'fruit': 'orange', 'veg': 'Potato'}, {'fruit': 'mango', 'veg': 'Cabbage'}, {'fruit': 'lemon', 'veg': 'Onion'}, {'fruit': 'lime', 'veg': 'Carrot'}]