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# Exception Handling

## Exception handling:

* We don't want to scare the user away
* We don't want the user to see log like stack traces when errors are shown
* That's where exception handling comes in

#### Raw code before exception handling:

```python
def sum(num1, num2):
	print(num1+num2)
	
number1 = input("Enter a number: ")

sum(number1, 12)
```

* Will generate a type error as the `input` function converts anything you give it to string
* Any code other than 0 generates an error
* Error:

```python
Traceback (most recent call last):
  File "c:\Users\$USERNAME\Desktop\test.py", line 6, in <module>
    sum(number1, 12)
  File "c:\Users\$USERNAME\Desktop\test.py", line 2, in sum     
    print(num1+num2)
TypeError: can only concatenate str (not "int") to str
```

* By handling exceptions, you don't display the above stack to the user

#### Code after exception handling:

* To handle the errors there is a special syntax called `try` and `except`

```python
def sum(num1, num2):
	try:
		print(num1+num2)
	except:
		print("There was an error")
		
print('HELLO')
	
number1 = input("Enter a number: ")

sum(number1, 12)
```

* By handling the Exception, the rest of the code in the grand scope will run
  * Such as `print('HELLO')`
  * This is due to scope
* The exception will be handled gracefully and will not crash the rest of the program

#### **Exception handling should be used only if you don't have any control over how it is used**

* Example:
  * The software goes to the internet and downloads files, and the user runs your program with no internet access, that could cause a crash
  * Another example is if the program relies on a file to be on the file system. If that is not there it will crash

#### The above code does not need any exception handling really, as the data type is in our control

```python
def sum(num1, num2):
	if isinstance(num1, int) and isinstance(num2, int):
		print(num1+num2)
	else:
		print('Please enter numbers only')
		
number1 = input("Enter a number: ")
sum(number1, 12)
```

* `isinstance` checks for data types specified