Set keyword argument's default value as length of string

Question

I've taken up a challenge to recreate some popular Python functions, one of them being string.count() . Other than the substring argument and the start index (optional), it also takes the end index (optional as well). This is my code:

def count(self,substring,start=0):
    self.substring = substring
    self.start = start

    self.queue = 0
    self.counter = 0

    for a in self.string[start:]:
        if a == self.substring[0]:
            self.queue += 1
        if a == self.substring[len(self.substring) - 1] and self.queue != 0:
            self.queue -= 1
            self.counter += 1

    return self.counter

Some variables I have defined outside of this function: self.string , which is the original string itself. You can see that I have not included the end argument, and that is why I'm here today. I would like to set the argument as: end=len(self.string) - 1 , but it just throws and error and says: NameError: name 'self' is not defined . Is there a solution to this? So far I can say that my code is not the problem, since it works perfectly without the argument, but I may be wrong.

Answer 1

I think one simple solution is the following

def count(self,substring,start=0, end=None):
    if end is None:
        end = len(self.string) - 1
    self.substring = substring
    self.start = start
    self.end = end # i suppose this needs to be set

    self.queue = 0
    self.counter = 0

    for a in self.string[start:]:
        if a == self.substring[0]:
            self.queue += 1
        if a == self.substring[len(self.substring) - 1] and self.queue != 0:
            self.queue -= 1
            self.counter += 1
    
    return self.counter

Answer 2

Try adding that end is a null value, then end = None in parentheses of def count() :

def count(substring, start = 0, end = None):
    # implementation omitted

Later, thanks to the null value of end , you can create a condition inside the function, that is, if end is a null value, then end is -1 .

You can write if end is None: end = len (self.string) - 1

Answer 3

Let's categorize parameters into 3 types:

• required (positional) parameter (like substring )
• optional parameter (like start , end ), often with None as type-less placeholder
• parameter with default argument values (like start , end )

See also:

• "Least Astonishment" and the Mutable Default Argument
• Effbot/Fredrik Lundh (July 17, 2008): Default Parameter Values in Python , via Wayback archive, using None as default value

Use slicing and default for end parameter

See also the runnable demo on IDEone :

class StringUtil:

    def __init__(self, s):
        self.string = s
    
    
    def count(self, substring, start=0, end=-1):  # end index/slice: -1 for the last, instead len(self.substring) - 1
        self.substring = substring
        self.start = start
        self.end = end
        self.queue = 0
        self.counter = 0

        for a in self.string[start:]:
            if a == self.substring[0]:
                self.queue += 1
            if a == self.substring[end] and self.queue != 0:  # -1 for the last, instead len(self.substring) - 1
                self.queue -= 1
                self.counter += 1
        
        return self.counter


sub = "o*o"
s =  "Hell" + sub + " W" + sub + "rld"
cnt = StringUtil(s).count(sub)
print(f" counted '{sub}' in '{s}' times: {cnt}")