Looping for loop in Python

Question

I have the following code.

    for idx in range(len(networks)):
        net_ = networks[idx]
        lastId=0
        for layerUptID in range(len(net_[1])):
            retNet,lastId=cn_.UpdateTwoConvLayers(deepcopy(net_),lastId)
            networks.append(retNet)
            if(lastId==-1):
               break

networks has only one net at the beginning.

After running the line retNet,lastId=cn_.UpdateTwoConvLayers(deepcopy(net_),lastId), I have additional six nets and appended to networks.

So after this lastId ==-1, go back to first for loop with len(networks) is 7.

For the next idx, idx=1 and continue.

Then, len(networks) is 13. Then go back to first for loop. After this, the first for loop breaks. I am expecting to continue for idx is 2, but it breaks.

What could be the issue?

can you provide a Minimal, Complete, and Verifiable example? as it is written now it is hard to tell what's wrong, my guess that you are mutating list while iterating and expecting that its length in line for idx in range(len(networks)): will change as well? — Azat Ibrakov
– Azat Ibrakov, Commented Aug 2, 2018 at 8:21

Ohad the Lad · Accepted Answer · 2018-08-02 08:46:16Z

1

If you try using a WHILE loop instead of FOR loop, the break statement would be check if the loop is on the last item in 'networks' collection. This way the network length would be calculated in each loop iteration

edited Aug 2, 2018 at 8:46

answered Aug 2, 2018 at 8:23

Ohad the Lad

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Comments

Reckless Engineer · Accepted Answer · 2018-08-02 08:31:12Z

For starters: Iterating, or looping, over the list (or data) you're editing is bad practice. Keep that in mind while coding.

This means if you plan to edit what you're looping on, in your case networks, then you're going to have a bad time looping over it. I would advise to break it up into two code parts:

The first part creates a new list of whatever it is you want WHILE looping. The second part replaces the list you've used to generate what you wanted.

Another thing which could go wrong is net_[i] may not be set up for some i, and you're trying to access it here:

    for layerUptID in range(len(net_[1])):

What if there is nothing in net_[1]?

To avoid these errors, usually verifying your data is a great way to start. If it is not null, then proceed, otherwise, print an error.

This is what I can think of. Hope it helps.

Azat Ibrakov · Accepted Answer · 2018-08-02 08:46:08Z

If I understood correctly your problem is that you've added new elements to networks, i.e. have increased length of networks and expect that for-loop will pick up this changes, well it's not, let's look at following snippet

elements = [1]

indices = range(len(elements))
for index in indices:
    print('index is', index)
    elements.append(2)
    print('elements count is', len(elements))
    print('indices count is', len(indices))

outputs are

index is 0
elements count is 2
indices count is 1

so as we can see despite the fact that length of elements list has changed, range object which is used in for-loop has not. This happens because len returns int object which are immutable, so when you change list length its length becomes different object and range function has no idea about this changes.

Finally, we can use while loop here like

while networks:
    net_ = networks.pop()
    lastId = 0
    for layerUptID in range(len(net_[1])):
        retNet, lastId = cn_.UpdateTwoConvLayers(deepcopy(net_), lastId)
        networks.append(retNet)
        if lastId == -1:
            break

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