A Guide To TryExcept.md

TryExcept

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Additional Materials found at:
Python Try Except - w3schools

What is TryExcept?

In python, exception or error handling is done via the use of try ... except blocks. Typically, you may have observed python stop running due to encountering an error. However, through the use of exception handling we can prevent a total shutdown of the program by adding in extra security and management in areas where we know functions may occur.

try:
    # do something that could cause an issue
except:
    # uh oh, an error occured! This code will now execute...

What are the components of TryExcept?

TryExcept is composed of up to 4 blocks of code which can execute. Those blocks are the try, except, else and finally blocks. Below, each block's purpose will be covered in more detail.

Try and Except

The try block will allow you to execute code which may have errors in it safely. It signifies that there is a potential for an error to occur and that it is being sufficiently handled by the program.

except is the code which executes when the try block code triggers an error. Except is capable of catching specific errors from the try block, and executing code dependant upon that type of error. Both try and except (or alternatively finally) are required components for making proper try ... except routine.

list = [2, 15]

for item in list:
    try:
        if item == 2:
            print(x)

        else:
            print("the value of x is " + item)

    except NameError:
        print("x was undefined!")

    except TypeError:
        print("x is not a string!")

    except:
        print("somehow a different error occurred!")

The loop above shows an example of catching two different errors. This is done by defining the error type being caught by the except block. Alternatively, a plain except can be used to catch any and all errors that may be thrown by the try block.

Finally and Else

The finally block will allow for you to execute code regardless of the results of what occurs in the try and except blocks. This can be useful for cleaning up resources such as opened files, or just ensuring that code will execute, even if an early return occurs during a try or except block.

def Foo():
    try:
        print(x)
    except:
        print("no x defined")
        return
    finally:
        print("this was executed after returning")

Foo()

else blocks can be used to run code when specifically no issues arose. You may want to implement this if you have two operations that could raise an error, but you explicitly do not want to catch the second operations potential error. Or alternatively use it for logging purposes.

try:
    x = 15
    print(x)
except NameError:
    print("x was undefined")
else:
    print("operation succeeded")

Throw

Whilst not an actual component of try ... except, the throw keyword is still worth mentioning. This keyword allows for you to throw specific exceptions should an error occur. This can be useful when making functions which require specific types of values. For example, this function which requires a list of numbers (integer or decimal) only

def SumIntegers(nums):
    result = 0
    for num in nums:
        if type(num) not in [int, float]:
            raise TypeError("Only numbers are allowed!")
        else:
            result += num

    return result

Some Helpful Exceptions

Function	Exception	When
int() or other type casts	ValueError	When the value cannot be converted
open()	OSError	When the file failed to open
- + / * maths functions	TypeError	When attempting to use with a number and non-number

Typically, if you search for a function's documentation then it will be able to inform you about the errors that it raises.