This is a sample code of what I want to do...
class A():
number = 0
List = []
for i in range(5):
state = A()
state.number = i
List.append(state)
print List
Numbers = [1,2,3]
Now I want to delete class objects in List whose number is in Numbers. What is the fastest way to do it ? Also if someone can suggest a more pythonic way of doing this than by simply creating two for loops one for that of Numbers and other of List and then removing from the List.
First of, instead of having a class variable like A.number as in:
class A():
number = 0
you should generally write a __init__ method that sets the number only on the instance.
class A(object):
def __init__(self, number):
self.number = number
Now the rest of your code can be written simply as:
List = [A(i) for i in range(5)]
print List
To efficiently find items (without two loops) you need sets:
Numbers = set([1,2,3])
x in Numbers now takes constant time. Otherwise Python would have to loop through the List to find a number.
The result you want looks like:
filtered = [a for a in List if a.number in Numbers]
class level is shared between all instances. For example, all the methods are shared, which is exactly what you'd expect from a class. But when you want to store something on a instance of a class, you need to write a method. In this case, the initialisation method __init__ takes the instance that is passed into this method as self and stores the number on it.
number is not shared between all instances: codepad.org/r9itOCcK. (instance).number will just set the number property. From the docs: By definition, all attributes of a class that are function objects define corresponding methods of its instances. If an attribute is not a function it will not be defined on the instance.number is a shared class variable on A. But when you do A().number=i, a new variable (by the same name) is added to the instance. So there is nothing shared in the end. I still think you should write a __init__ method though, if only to avoid having to think about this after all.
Numbersis a list of ints. You are assigning an individual int to the.numberof each element in List. A list of numbers is never going to be the same as an individual number. Did you mean "...whosenumberis inNumbers"?