Passing constructor parameter to a function that generates default value fails - python-2.7

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

Answer 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)
...