is this bad coding in python?

Question

Is there is some python rule that says declare/define all global variable in the "init" method? Why would python allow this?

for example, see the following code:

class Myclass():
  def __init__(self,input):
      self.input = input
  def a_method(self):
      self.sneaky = {}
      local_var = 'a var'
      return local_var
  def another_method(self):
      print self.input
      print self.sneaky, ', O sneaky you bad var'
      return

If my class had many methods the variable 'self.sneaky' would have been hard to spot. And "a_method" has to be called first other wise AttributeError is raised when "another_method" is called. I just spotted it in somebody's code and want to know if it is common practice.

possible duplicate of Must all Python instance variables be declared in def __init__? — user764357
– user764357, Commented Mar 27, 2014 at 0:36
@Lego I object to your editing of my first line that you entirely removed. — user1318806
– user1318806, Commented Mar 27, 2014 at 0:40

zmo · Accepted Answer · 2014-03-27 01:34:31Z

First of all, you're not talking about global variables, but about instance variables. Then whether or not declaring variables in a method or in the constructor depends on the scenario you're using.

Basically, the only rule that prevail is to always declare a variable before using it. (or you will get the AttributeError exception).

Usually, it's a good idea to declare all members of your class in init, so you can work on the assumption they exist with a default value. Though, sometimes you may prefer to defer the creation of the class type, and the attribution of the elements within.

A good example would be if within a loop you create a million instances, but then you only select a few of those to work with at a given time. Then you may want to have the constructor be as simple as possible with what's needed at instanciation, and create a populate() method that you call just before using it to add the default members you're about to use. That pattern is often seen in building graphical UIs.

HTH

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