Python中列表列表的列表

Question

I'm working with the Flask framework in Python, and need to hand off a list of lists to a renderer.

I step through a loop and create a list, sort it, append it to another list, then call the render function with the masterlist, like so:

for itemID in itemsArray:
    avgQuantity = getJitaQuantity(itemID)
    lowestJitaSell = getJitaLowest(itemID)
    candidateArray = findLowestPrices(itemID, lowestJitaSell, candidateArray, avgQuantity)
    candidateArray.sort()
    multiCandidateArray.append(candidateArray)
renderPage(multiCandidateArray)

My problem is that I need to clear the candidateArray and create a new one each time through the loop, but it looks like the candidateArray that I append to the multiCandidateArray is actually a pointer, not the values themselves.

When I do this:

for itemID in itemsArray:
    avgQuantity = getJitaQuantity(itemID)
    lowestJitaSell = getJitaLowest(itemID)
    candidateArray = findLowestPrices(itemID, lowestJitaSell, candidateArray, avgQuantity)
    candidateArray.sort()
    multiCandidateArray.append(candidateArray)
    **del candidateArray[:]**
renderPage(multiCandidateArray)

I end up with no values.

Is there a way to handle this situation that I'm missing?

Answer 1

I would probably go with something like:

for itemID in itemsArray:
    avgQuantity = getJitaQuantity(itemID)
    lowestJitaSell = getJitaLowest(itemID)
    candidateArray = findLowestPrices(itemID, lowestJitaSell, candidateArray, avgQuantity)
    multiCandidateArray.append(sorted(candidateArray))

No need to del anything here, and sorted returns a new list , so even if FindLowestPrices is for some reason returning references to the same list (which is unlikely), then you'll still have unique lists in the multiCandidateArray (although your unique lists could hold references to the same objects).

Answer 2

Your code already creates a new one each time through the loop.

candidateArray = findLowestPrices(...)

This assigns a new list to the variable, candidateArray . It should work fine.

When you do this:

del candidateArray[:]

...you're deleting the contents of the same list you just appended to the master list.

Don't think about pointers or variables; just think about objects, and remember nothing in Python is ever implicitly copied. A list is an object. At the end of the loop, candidateArray names the same list object as multiCandidateArray[-1] . They're different names for the same thing. On the next run through the loop, candidateArray becomes a name for a new list as produced by findLowestPrices , and the list at the end of the master list is unaffected.

I've written about this before ; the C way of thinking about variables as being predetermined blocks of memory just doesn't apply to Python at all. Names are moved onto values, rather than values being copied into some fixed number of buckets.

(Also, nitpicking, but Python code generally uses under_scores and doesn't bother with types in names unless it's really ambiguous. So you might have candidates and multi_candidates . Definitely don't call anything an "array", since there's an array module in the standard library that does something different and generally not too useful. :))