for … in … loop (Python) skips some values in list [duplicate]

Answer 1

The operator % returns an integer value not a boolean, so when the return value is 0 it skips the statement. Also you are modifiying the list you are iterating over, not good...

Try this:

def purify (lst):
    retLst = []
    for key in lst:
        if key % 2 == 0:
            lst.append(key)
    return retLst

Should work.