I only find a fraction of the existing duplicates in two Lists of Strings

user_15444992 0
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This method receives lists of String as parameters and is supposed to return all Strings in uncheckeds that do not have a match in valids. (The Strings are e-mail addresses)

If I input two Lists that are exactly the same, some of the duplicates are found, but not all of them. This behaviour is consistent if i input two unique Lists. Some duplicates are found and removed, some are not.

I have found similar questions answered, but i cant find information regarding this specific problem.

My method:

def getUnmatchedAddresses(valids, uncheckeds):
    
    unmatcheds = uncheckeds

    for unchecked in uncheckeds:
        for valid in valids:
            if(unchecked == valid):
                unmatcheds.remove(valid)
                
    return unmatcheds
list python comparison string
3 Answers

When you assigned unmatcheds = uncheckeds, you are not creating a copy of the list, just another name for it, but it is the same list under the hood. You need to create a copy of the list.

def getUnmatchedAddresses(valids, uncheckeds):
    
    unmatcheds = uncheckeds.copy()

    for unchecked in uncheckeds:
        for valid in valids:
            if(unchecked == valid):
                unmatcheds.remove(valid)
                
    return unmatcheds
user_50037560

The issue comes from unmatcheds = uncheckeds which do not perform a deep copy: it only copies the reference. As a result, the loop iterate on a mutating list resulting in your issue. While you can copy the list before iterating over it, this would be clearly inefficient. You can just build directly the new one efficiently using the following code:

def getUnmatchedAddresses(valids, uncheckeds):
    return [unchecked for unchecked in uncheckeds 
               if not any(unchecked == valid for valid in valids)]
user_129395570

You can also avoid the deep copy issue by iterating on the valids list instead. This leads to the same result, but avoids removing an item of the list being iterated. Nb: this alters the uncheckeds list.

Here is an example/rework tested and working:

def getUnmatchedAddresses(valids, uncheckeds):
    
    for va in valids:
        while va in uncheckeds:
            uncheckeds.remove(va)

    return uncheckeds

Example:

v = [1, 2, 3, 4]
u = [1, 2, 3, 4]

print(getUnmatchedAddresses(v, u))

Output is:

[]
user_146621790
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