断言如何影响Python中的变量分配？

Question

I am having some sort of issue with list aliasing or variable assignment in Python 2.7. I will show you a minimal example. There are two different results with and without the assertion , and I don't know why/how the assertion affects this.

Somehow it is overwriting an attribute when I append something to the object_list shown below:

class Object1(object):                                                                                 

    def __init__(self):                                                                                    
        self.object_list = []  

    def add_thing(self, thing):                                                                                          
        # this next line makes all the difference
         assert thing.name not in [thing.name for thing in self.object_list], 'agent id already exists, use another one'
        self.object_list.append(thing)

class Thing(object):

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return self.name

Here is a minimal example:

With the assertion

mfd = Object1()
myAgent = Thing('blah')
myAgent_2 = Thing('blew')

mfd.add_thing(myAgent)
mfd.add_thing(myAgent_2)

print mfd.object_list

>> [blah, blah]

Without the assertion

mfd = Object1()
myAgent = Thing('blah')
myAgent_2 = Thing('blew')

mfd.add_thing(myAgent)
mfd.add_thing(myAgent_2)

print mfd.object_list

>> [blah, blew]

Can someone please explain to me how the assertion above is affecting this? Thank you.

Answer 1

In python 2, implicit assignments made in the scope of a list comprehension leak outside of the comprehension to the caller's scope.

So in your example, by executing the assert, thing 's value changes and the final value of thing is the final thing from object_list . Therefore, without assert , you are appending a different thing than after the assert has been executed.