Question

How can I update the upper limit of a loop in each iteration? In the following code, List is shortened in each loop. However, the lenList in the for, in loop is not, even though I defined lenList as global. Any ideas how to solve this? (I’m using Python 2.sthg)
Thanks!

def similarity(List):
import difflib
lenList = len(List)
for i in range(1,lenList):
    import numpy as np
    global lenList
    a = List[i]
    idx = [difflib.SequenceMatcher(None, a, x).ratio() for x in List]
    z = idx > .9
    del List[z]
    lenList = len(List)


X = ['jim','jimmy','luke','john','jake','matt','steve','tj','pat','chad','don']
similarity(X)

Answer 1

Looping over indices is bad practice in python. You may be able to accomplish what you want like this though (edited for comments):

def similarity(alist):
  position = 0
  while position < len(alist):
    item = alist[position]
    position += 1
    # code here that modifies alist

A list will evaluate True if it has any entries, or False when it is empty. In this way you can consume a list that may grow during the manipulation of its items.

Additionally, if you absolutely have to have indices, you can get those as well:

for idx, item in enumerate(alist):
  # code here, where items are actual list entries, and 
  # idx is the 0-based index of the item in the list.

In … 3.x (I believe) you can even pass an optional parameter to enumerate to control the starting value of idx.