Question

class makeCode:
    def __init__(self,code):
            self.codeSegment = code.upper()
            if checkSegment(self.codeSegment):
                    quit()
            self.info=checkXNA(self.codeSegment)
    def echo(self,start=0,stop=len(self.codeSegment),search=None): #--> self not defined
            pass

Not working…

• it says that the variable self is not defined when it actually was;
• the function checkSegment returns 1 if the input was not a string made of nucleotids letter, or if that contains nucleotids that can’t be together;
• it quits if that happens, that’s ok it works perfectly;
• then it assigns the information (if it’s RNA or DNA) checking with the function checkXNA that returns a string with the information “dnaSegment” or “rnaSegment”; works perfectly.

But then the function echo which will be designed for printing more specific information tells me that self is not defined, but why?

Answer 1

self is not defined at function definition time, you cannot use it to create a default argument.

Expressions in a function definition are evaluated when the function is created, not when it is being called, see "Least Astonishment" and the Mutable Default Argument.

Use the following technique instead:

def echo(self, start=0, stop=None, search=None):
    if stop is None:
        stop = len(self.codeSegment)

If you need to support None as a possible value for stop (e.g. None is a valid value for stop if specified explicitly), you’ll need to pick a different unique sentinel to use:

_sentinel = object()

class makeCode:
    def echo(self, start=0, stop=_sentinel, search=None):
        if stop is _sentinel:
            stop = len(self.codeSegment)