Private helper functions in Python classes

Question

I am creating a class which holds a dict, and has some methods to extract information based on the data in the dict. Each key points to a list of strings. Each string in the list is of the form name(data).

I am very new to programming in Python and have mainly used Java previously. I have created a couple of private helper functions that are to be used for in my public methods. Here is an example to illustrate what I'm trying to do:

def _getItemName(item):
    str = ""
    for c in item:
        if c!= '(':
            str += c
        else:
            break
    return str

This method is then used in several public methods, like in this example:

def getCertainItemsByName(self, key, name):
    foundItems = []
    for item in self.itemMap[key]:
        if _getItemName(item) == name:
            foundItems.append(item)
    return foundItems

This is giving me an error equivalent to "global name "_getItemName" is not defined. I realize I could just declare the method outside of the class, but the method is class-specific so that's not something I want to do. What is the best to do this?

Solution

In your question you are missing self;

def _getItemName(self, item):
    str = ""
    for c in item:
        if c!= '(':
            str += c
        else:
            break
    return str

def getCertainItemsByName(self, key, name):
    foundItems = []
    for item in self.itemMap[key]:
        if self._getItemName(item) == name:
            foundItems.append(item)

You should look at static method, class method and instance method in Python.

Static methods are a special case of methods. Sometimes, you'll write code that belongs to a class, but that doesn't use the object itself at all. For example:

class Pizza(object):
     @staticmethod
     def mix_ingredients(x, y):
         return x + y

     def cook(self):
         return self.mix_ingredients(self.cheese, self.vegetables)

For more, you can visit here

Comments

[myildirim]

you can use @staticmethod

[jorgenfar]

I don't want to pass it self as an argument because it isn't an instance method. I guess I will make it static, but that isn't very appealing to me either because it is a method that is only to be used inside the class, so it seems unneccesarily clunky to need to prefix the class name when calling it inside the class definition.

[jorgenfar]

Thanks for the link! I was already aware of the possibility of making this static but I guess I was just looking for a more elegant way of doing this, similar to how it is done in Java.