Passing constructor parameter to a function that generates default value fails

Question

I am very new to Python so I assume I am doing something terribly wrong, but I don't see what and Google has not helped this far too. What is wrong with this ?

def lookup_permille(name):
    # TODO: implement a permille lookup table
    return 0

def lookup_known_product(name):
    # TODO: implement a known product lookup table
    return 0

class ProductEntry:
    def __init__(self, source, name, price, volume, permille = lookup_permille(name), known_product_id = lookup_known_product(name), category = 0):
        self.source = source
        self.name = name
        self.price = price
        self.volume = volume
        self.permille = permille
        self.price_per_permille = self.permille / self.price;
        self.category = category
        self.known_product_id = known_product_id

Calling the constructor of ProductEntry fails with:

def __init__(self, source, name, price, volume, permille = lookup_permille(name), known_product_id = lookup_known_product(name), category = 0):
NameError: name 'name' is not defined

khelwood · Accepted Answer · 2016-02-06 17:44:00Z

1

The expressions defining default arguments are evaluated when the function is defined, not when it is called. At the point when __init__ is being defined, name does not exist, so it cannot be used in an expression to calculate a default argument.

The usual way to do something like this is to have a stand-in value as your default argument, and replace it with whatever value you actually want inside the body of your function.

def __init__(self, source, name, price, volume,
             permille=None, known_product_id=None, category=0):
     if permille is None:
         permille = lookup_permille(name)
     if known_product_id is None:
         known_product_id = lookup_known_product(name)
     ...

answered Feb 6, 2016 at 17:44

khelwood

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1 Comment

Rudolfs Bundulis Over a year ago

Thanks, I guess I was expecting too much from Python :D I mean this would not work in C++ and for some reason I expected Python to be a magical "do-it-all" thingy:D

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