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Friends, here we are going to see how to create custom exceptions.

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First of all, what is a custom exception?

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Simple, wantedly if you want to raise an exception, then you

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can raise. So that type of exceptions are called custom exceptions.

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And you can create this custom exceptions in two ways -

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by using 'raise' and 'assert'.

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So 'raise' we are going to use to raise an exception

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which are existing with your Python.

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Right. See let me explain this with an example.

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So I am going to save my script name as something like

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'custom_exceptions', right. See simply I am using a 'raise'

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'Exception("This is a exception"). That's it.

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Now see the output. If I run this

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it is giving like an exception, right. See what is the exception

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name? "Exception" itself.

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And what is the error?

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"This is a Exception". Not only that, suppose if any variable

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is not defined you know, right. "NameError" is the exception.

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So, "Variable is not defined". Wantedly

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I'm raising this.

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That's it.

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But this is not the way to use in your real-time. Suppose,

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just assume that you are trying to read a variable.

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It may be any variable. Let's say simply 'age=' some 23.

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So what I want to do is, 'if age > 30:' then

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I don't have any problem.

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'print("Valid age")'.

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But if 'age' is not less than, I mean not greater

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than 30 then I want to raise an exception.

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Instead of printing simply a message wantedly now, actually

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previously we have studied to avoid exceptions,

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but now we are trying to learn how to create exceptions if

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it is required.

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Right. Suppose if age is less than 30 then I don't want to

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run my script. I want to terminate my script.

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I want to stop. I need to raise an exception.

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Yes, you can raise, simply raise.

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So I am taking suppose, 'ValueError' exception.

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Right. So see I am using existing keywords, 'ValueError' that

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is already there with your exceptions, right?

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So that's why 'raise' keyword is used to raise an existing exception wantedly.

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"Age is less than 30".

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That's it.

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This is the way how we can raise a custom exception using

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'raise' keyword.

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Now we have one more thing called 'assert'.

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So 'assert' is going to be used to create your own exceptions with

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a name called 'AssertError'. First

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let me do that.

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See I am writing simply 'assert'.

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See, first of all how your 'assert' is going to work, 'assert'

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will raise assertion error, if the condition becomes false.

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I mean suppose I am writing 4 < 3, that is false.

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But if I write 4 > 3, that is correct, right. Now

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see that I am running,

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it is not raising any exception because condition is true,

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right. If I write condition as false, now see the result what

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you are getting. You are getting error called, exception

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called "AssertionError". That's it.

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So 'assert' is used to create "AssertionError", and whenever

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if you get any "AssertionError" you have to assume that

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wantedly you created

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an exception, that's it.

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Right. So here also you can take something like, suppose

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if instead of 'if' condition, now directly you can give it. So

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suppose I am trying to write 'try:', actually assume that I am

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writing here suppose 'age=30'.

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Right. So now I no need to use 'if' condition. Directly if I want

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to raise an exception based on 'age' then I no need to use 'if'

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condition, 'assert' itself is going to work as 'assert',

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if 'age < 30', then only create,

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create an exception. If it is true then see,

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"Valid age".

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Let me run it and see the result.

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Yeah, this is 'try' and 'except', right?

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You need to write your 'except' block as well.

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Let me write 'except:'.

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'print("Exception

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occurred")', right.

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So what type of assertion, sorry what type of exception

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you're creating with the help of 'assert', 'AssertionError'.

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Let me run it and see the result.

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See both are same. Let me take 33, then condition will become true, right?

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I mean, now see what you are getting, if age is, if it is less than 30

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Yeah, if it is greater than, right, as per your requirement.

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If 'age' is greater than 30, no problem.

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Right. Let me take my age as something like 20,

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now if 'age' is not greater than 30, right, then

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I want to create an 'AssertionError'.

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Yes, exception is occurring means 'AssertionError' is running,

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raising. So if you want to handle your raised 'AssertionError',

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yes, you can handle it like, 'except: if error is 'AssertionError'

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then 'print("Raised with assert because age is lessthan 30")'.

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That's it. Now see the result.

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So 'assert' is going to give an exception called

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'AssertionError'. That's it.

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Right. So guys for time being just have a look about this

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'assert' and 'raise', right.

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So while writing real-time scripts based on requirement,

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I will use 'raise' sometime,

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I will use 'assert' sometime, and simply I will use 'try:' and

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'except:'. It's based on situation.

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So up to that you have to remember the syntaxes, right and

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then we will use there while writing our scripts.

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Okay guys, thank you for watching this video.

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