remove last element in a dictionary of lists in python

Question

What's a more pythonic or efficent way of removing the last element of each list, in a dictionary of lists?

For example, take this:

listDict = {'tom': [-2,10,2,-8], 'sam': [-9,-10,-10,-7]}

And transform it into this:

listDict = {'tom': [-2,10,2], 'sam': [-9,-10,-10]}

This is what I'm currently doing:

new = {}
for t,item in listDict.iteritems():
    new[t] = [item[0], item[1], item[2]]
listDict= new

Answer 1

Blender's answer grows very inefficient as the lengths of the lists increases, compared to this solution:

for k, v in listDict.items():
    v.pop()

For your example listDict, the difference is not big, just 27%. But using a dict with 100 keys and lists of length from 50 to 100, and popping 50 of them, the dict comprehension method takes more than 12 times longer. This is because this solution modifies the existsing lists instead of creating copies of each list every time.

I'm afraid that there is no one-liner version of this, unless you're cheating. The reason I mention this at all is to prevent that some dofus feels compelled to point it out in comments. Please don't use dict/list comprehensions for side effects.

Of course you can do it on one line as follows, but PEP8 says "rather not":

for k, v in listDict.items(): v.pop()

Answer 2

I would use a dictionary comprehension:

new_dict = {key: value[:-1] for key, value in listDict.items()}

For older Python versions you'll have to use the dict() constructor:

new_dict = dict((key, value[:-1]) for key, value in listDict.items())