any ways to judge whether an object is initilized or not in a class

Duncan Booth duncan.booth at invalid.invalid
Mon Mar 19 09:50:15 CET 2007


"momobear" <wgwigw at gmail.com> wrote:

> in C++ language we must initilized a variable first, so there is no
> such problem, but in python if we don't invoke a.boil(), we will not
> get self.temp to be initilized, any way to determine if it's initilzed
> before self.temp be used.
> 
The simplest thing is simply never to attempt to use a variable or an 
attribute until you know that is has been initialized, so initialize all 
variables before using them, and initialize all attributes in the class's 
__init__ method.

If you don't have a suitable value for the attribute until later then just 
start off with None and then you can check for 'a.boil is not None': if you 
forget to check then Python's strong type checking will stop you from using 
the None value in an expression expecting a number (or a string). 

For cases where you aren't sure whether an object has a specific attribute 
you can use getattr with 3 arguments:

   if getattr(a, 'boil', 80):
       ...

If that isn't convenient (or it's a variable rather than an attribute) you 
should fall back on the principle that 'is it better to ask forgiveness  
than permission': i.e. just try to use the value and handle the exception 
which is thrown if it doesn't exist. (If the fallback is to substitute a 
simple value use getattr, if the fallback is complicated or takes a long 
time to calculate use exception handling).

There is also a function 'hasattr' which will tell you whether or not the 
object has the specified attribute, but internally it just calls 'getattr' 
and handles the exception so (IMHO) it is generally best just not to bother 
with 'hasattr'.



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