CODEWITHSUNDEEP
pythonexceptiontypeerror
elegent
elegent

Reputation: 4007

Confusing TypeError exception error message caused by wrong number of method arguments in python

Let’s code for example a class called Rocket:

class Rocket():
    def __init__(self, name):
        print("Created a Rocket called '" +  str(name) + "'!")

    def liftOff(self):
        print("10, 9 ,8 ,7 ... Lift Off!!")

and create an instance SaturnV of this class, but pass no arguments to the __init__ method:

SaturnV = Rocket()

We produce a TypeError:

Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    SaturnV = Rocket()
TypeError: __init__() takes exactly 2 arguments (1 given)

Why does the compiler print, that __init__() takes exactly 2 arguments instead of 1 required name argument?

Why do they not change the error message to something like...

Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    SaturnV = Rocket()
TypeError: __init__() takes exactly 1 argument in addition to the instance reference (0
           arguments given)

Upvotes: 1

Views: 2578

Answers (1)

ljetibo
ljetibo

Reputation: 3094

Python counts the self keyword as an argument as well. That just means that it expects to be called on an class instance via the famous . (as in satrurnV.name)or you could optionally do a getattr(saturnV, "name") to retrieve the same thing.

The __init__() method is special because it gets called implicitly when you do Rocket() which is basically Rocket.__init__(). Now you see that the other argument, the one that you're missing is the name.

you also have an error in your code:

print("Created a Rocket called" +  "'str(name)'" + "!")

should be

print("Created a Rocket called" +  str(name) + "!")

str() function takes the object in the () and returns a string so there's no need to ut that under " "

Upvotes: 1

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