Question

I have a nested list something like this:

PLACES = (
    ('CA', 'Canada', (
        ('AB', 'Alberta'),
        ('BC', 'British Columbia' (
            ('van', 'Vancouver'),
        ),
        ...
    )),
    ('US', 'United States', (
        ('AL', 'Alabama'),
        ('AK', 'Alaska'),
        ...

I need to retrieve some data out of it. If depth is 0 I need to retrieve all the countries (and their codes), if depth == 1, I need to retrieve all the states/provinces, if depth == 2 I need to retrieve all the cities… and so forth. Is there some set library for doing stuff like this? Or can someone point me in the right direction? I started coding up a solution only to realize it wouldn’t work for levels deeper than 1 because you have to go in and out of each list…

Also notice that not all items have a 3rd part (i.e., we’re pretending Alberta doesn’t have any cities, so retrieving items at depth 2 would just return ('van','Vancouver') in this limited scenario).

---

I didn’t realize this before, but I also needed the parent value. So, I modified interjay’s solution:

def depth_gen(seq, depth, par=None):
    if depth==0:
        for x in seq:
            yield par, x[0], x[1] 
        return

    for x in seq:
        if len(x)==3:
            par = x[0]
            for y in depth_gen(x[2], depth-1, par):
                yield y

Which I’m using to generate some HTML:

<label for="id-pickup_address-province">Province</label>
<select id="id-pickup_address-province" rel="pickup_address-country" name="pickup_address-province">
    <option rel="CA" value="AB">Alberta</option>
    <option rel="CA" value="BC">British Columbia</option>
    <option rel="CA" value="MB">Manitoba</option>
    ...
    <option rel="US" value="WV">West Virginia</option>
    <option rel="US" value="WI">Wisconsin</option>
    <option rel="US" value="WY">Wyoming</option>
</select>
<label for="id-pickup_address-country">Country</label>
<select id="id-pickup_address-country" name="pickup_address-country">
    <option value="CA">Canada</option>
    <option value="US">United States</option>
</select>

And then I should be able to easily filter the list with jQuery based on which country is selected…

Answer 1

Here is a solution which will work for any depth:

def depthGenerator(seq, depth):
    if depth==0:
        for x in seq:
            yield x[:2] #strip subsequences
        return

    for x in seq:
        if len(x)==3:   #has subsequence?
            for y in depthGenerator(x[2], depth-1):
                yield y

Example:

>>> list(depthGenerator(PLACES, 1))
[('AB', 'Alberta'), ('BC', 'British Columbia'), ('AL', 'Alabama'), ('AK', 'Alaska')]