[Tutor] How to reuse code in Python

[Kent Johnson]

Negroup - wrote:
> Hi.
> Suppose that I need to use in my project a module that belongs to
> another project, because - with some tuning - it can fit my needings.
> 
> module.py
> limit = 30
> 
> class A:
>   def __init__(self):
>     self.num = 20
>   def info(self):
>     return limit
>   def inc_num(self):
>     self.num += 1
>   def check(self):
>     return self.num > limit
> 
> Now suppose that I'd like to have the same class A in my module, but
> also I want that the initial value of limit is set to 25.
> 
> 
>>>>limit = 25
>>>>from module import A
>>>>a = A()
>>>>a.info()
> 
> 30

Obviously this doesn't do what you want. The problem is that class A is seeing the limit defined in it's module. 'global' variables in Python actually have module scope, there is no truly global scope in Python (OK, there's the __builtin__ scope, but this is a beginner's list!)

IMO the best solution is to pass limit as a parameter to A's constructor and save it as an instance attribute:

class A:
  def __init__(self, limit):
    self.limit = limit
    self.num = 20
  def info(self):
    return self.limit
  def inc_num(self):
    self.num += 1
  def check(self):
    return self.num > self.limit

Now you can create whatever kind of A's you want:
from module import A
a = A(30)
etc.

Another way to do this is to make limit a class attribute. Then you can change it in subclasses:

class A:
  limit = 25
  def __init__(self):
    self.num = 20
  def info(self):
    return self.limit
  def inc_num(self):
    self.num += 1
  def check(self):
    return self.num > self.limit

class B(A):
  limit = 30

a = A()
a.info() --> 25
b = B()
b.info() --> 30

Kent
Note you still refer to self.limit so the value of limit in the current class will be found.

> where is the value 30 coming from?

from module.limit.

Kent
-- 
http://www.kentsjohnson.com