Why do these .append() statements overwrite each other?

Question

I have a function as follows:

def control(qstat):
    gatnum = int(input("What number of control gates is this control qubit a part of?"))
    global qstatnum
    qstatnum = {}
    qstatnum[gatnum] = []
    qstatnum[gatnum].append(qstat) #seems to be a problem
    return qstat

However, there is a problem. Let's say I run it once. There will be one item in the list. Then, I run it a second time, with an item distinguishable from the second supposed to be added to the list. When I print qstatnum[gatnum], the list contains only the second item, leading me to believe that the .append() statement is somehow incorrectly written and overwriting any previous additions to the list.

Is this a correct diagnosis? Why would this be? Any help would be appreciated. Thanks!

Also `qstatnum[gatnum] = []` is questionable. I would have expected to see `qstatnum.setdefault(gatnum, []).append(qstat)`. — Dan D., Mar 12 '17 at 22:52
@DanD., out of curiosity, why is that better, and what exactly does it do? I've never seen `.setdefault` before — Auden Young, Mar 12 '17 at 22:54
It says if you have a list there, then get it. Otherwise, make an empty one — OneCricketeer, Mar 12 '17 at 22:55
Refer: http://stackoverflow.com/questions/3483520/use-cases-for-the-setdefault-dict-method#3483652 — OneCricketeer, Mar 12 '17 at 22:56
Could you provide an example of the expected content for qstatnum? — slackmart, Mar 12 '17 at 22:57
@slackmart, sure - `qstatnum = {1:[np.array([0,1]), np.array([1,0])}` in this particular case, but there might be more than one key value pair, obviously, and more or less items in the list. — Auden Young, Mar 12 '17 at 22:59
@cricket_007, this *seems* to be working, but now an unrelated error is popping up. — Auden Young, Mar 12 '17 at 23:02

slackmart · Accepted Answer · 2017-03-12T23:11:28.463

2

Each time you call the function, you are creating a new qstatnum dict, so the solution is to create the dictionary outside the function:

qstatnum = {}

def control(qstat):
    gatnum = int(input("What number of control gates is this control qubit a part of?"))
    try:
        qstatnum[qstat].append(gatnum)
    except:
        qstatnum[qstat] = [gatnum]
    return qstat

You need a try: except: block to verify if the key already exists in the dictionary, if it doesn't exists, just add the first value, else use append.

@DanD. approach seems to be shorter, please take a look:

qstatnum = {}

def control(qstat):
    gatnum = int(input("What number of control gates is this control qubit a part of?"))

    qstatnum.setdefault(qstat, []).append(gatnum)

    return qstat

edited Mar 12 '17 at 23:11

answered Mar 12 '17 at 22:52

slackmart

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ah, I need to keep the `global qstatnum` line - let me try that. – Auden Young Mar 12 '17 at 22:52
Okay, I tried that, and it still gives the exact same error. – Auden Young Mar 12 '17 at 22:53
You're still reseting `qstatnum[gatnum] = []` – OneCricketeer Mar 12 '17 at 22:54
@cricket_007, okay - how do I set that but without resetting each time? – Auden Young Mar 12 '17 at 22:55
Could you provide an example of the expected content for qstatnum? – slackmart Mar 12 '17 at 22:57
@slackmart, one example could be `qstatnum = {1:[np.array([1,0]),np.array([0,1])]}`. – Auden Young Mar 12 '17 at 23:01
@slackmart, thank you, I think yours and cricket_007's help fixed the problem. +1 and an accept =) – Auden Young Mar 12 '17 at 23:08
With DanD.'s approach, wouldn't you still need to include the `global qstatnum` line or no? – Auden Young Mar 12 '17 at 23:12

score 0 · Answer 2 · answered Mar 12 '17 at 22:53

0

Every time the method is called, qstatnum is set to empty. So basically you are appending to nothing every time.

answered Mar 12 '17 at 22:53

QB_

109
7

Why do these .append() statements overwrite each other?

2 Answers2