Interesting takes exactly 1 argument 2 given Python error

Question

For the error   TypeError  takes exactly 1 argument  2 given    With the following class method   def extractAll tag            and calling it   e extractAll  th     The error seems very odd when I m giving it 1 argument  the method should take only 1 argument  but it s saying I m not giving it 1 argument    I know the problem can be fixed by adding self into the method prototype but I wanted to know the reasoning behind the error   Am I getting it because the act of calling it via e extractAll  th   also passes in self as an argument  And if so  by removing the self in the call  would I be making it some kind of class method that can be called like Extractor extractAll  th

User · Answer

Am I getting it because the act of calling it via e.extractAll("th") also passes in self as an argument?

Yes, that's precisely it. If you like, the first parameter is the object name, e that you are calling it with.

And if so, by removing the self in the call, would I be making it some kind of class method that can be called like Extractor.extractAll("th")?

Not quite. A classmethod needs the @classmethod decorator, and that accepts the class as the first paramater (usually referenced as cls). The only sort of method that is given no automatic parameter at all is known as a staticmethod, and that again needs a decorator (unsurprisingly, it's @staticmethod). A classmethod is used when it's an operation that needs to refer to the class itself: perhaps instantiating objects of the class; a staticmethod is used when the code belongs in the class logically, but requires no access to class or instance.

But yes, both staticmethods and classmethods can be called by referencing the classname as you describe: Extractor.extractAll("th").

User · Answer

Summary  Some examples of how to define methods in classes in python      usr bin env python      if running from bash   class Class1 object        def A self  arg1           print arg1           this method requires an instance of Class1              can access self variable name  and other methods in Class1       classmethod     def B cls  arg1           cls C arg1            can access methods B and C in Class1        staticmethod     def C arg1           print arg1           can access methods B and C in Class1             i e  via Class1 B      and Class1 C         Example   my obj Class1    my obj A  1     Class1 A  2     TypeError  method A   must be called with Class1 instance  my obj B  3   Class1 B  4   my obj C  5   Class1 C  6

User · Answer

Yes  when you invoke e extractAll foo   Python munges that into extractAll e  foo    From http   docs python org tutorial classes html     the special thing about methods is   that the object is passed as the first   argument of the function  In our   example  the call x f   is exactly   equivalent to MyClass f x   In   general  calling a method with a list   of n arguments is equivalent to   calling the corresponding function   with an argument list that is created   by inserting the method   s object   before the first argument    Emphasis added

User · Answer

If a non-static method is member of a class  you have to define it like that   def Method self  atributes      So  I suppose your  e  is instance of some class with implemented method that tries to execute and has too much arguments

User · Answer

try using   def extractAll self tag     attention to self

User · Answer

The call  e extractAll  th     for a regular method extractAll   is indeed equivalent to  Extractor extractAll e   th     These two calls are treated the same in all regards  including the error messages you get   If you don t need to pass the instance to a method  you can use a staticmethod    staticmethod def extractAll tag             which can be called as e extractAll  th     But I wonder why this is a method on a class at all if you don t need to access any instance

[python] Interesting 'takes exactly 1 argument (2 given)' Python error

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