Python: determine if all items of a list are the same item

Question

In some of my code I put a series of objects in a list and I build an additional list out of their attributes, which is a string. I need to determine if all the items in this second list have the exact same value, without knowing beforehand which value it is, and return a bool so that I can do different things in my code depending on the result.

I can't know the names of the properties beforehand, that is why I'm trying to make something as generic as possible.

To make the example clear, an ideal function, called "all_same" would work like this:

>>> property_list = ["one", "one", "one"]
>>> all_same(property_list)
True
>>> property_list = ["one", "one", "two"]
>>> all_same(property_list)
False

I was thinking of making a list of unique elements and then check if its length is 1, but I'm not sure if it's the most elegant solution out there.

Answer 1

You could cheat and use set:

def all_same( items ):
    return len( set( items ) ) == 1 #== len( items )

or you could use:

def all_same( items ):
    return all( map(lambda x: x == items[0], items ) )

or if you're dealing with an iterable instead of a list:

def all_same( iterable ):
    it_copy = tee( iterable, 1 )
    return len( set( it_copy) ) == 1

Answer 2

def all_same(items):
    return all(x == items[0] for x in items)

Example:

>>> def all_same(items):
...     return all(x == items[0] for x in items)
...
>>> property_list = ["one", "one", "one"]
>>> all_same(property_list)
True
>>> property_list = ["one", "one", "two"]
>>> all_same(property_list)
False
>>> all_same([])
True

Answer 3

I originally interpreted you to be testing identity ("the same item"), but you're really testing equality ("same value"). (If you were testing identity, use is instead of ==.)

def all_same(items):
  it = iter(items)
  for first in it:
    break
  else:
    return True  # empty case, note all([]) == True
  return all(x == first for x in it)

The above works on any iterable, not just lists, otherwise you could use:

def all_same(L):
  return all(x == L[0] for x in L)

(But, IMHO, you might as well use the general version—it works perfectly fine on lists.)

Answer 4

wrong: reduce(lambda x,y:x==y,iterable)

Answer 5

This works on sequences and iterables (python >= 2.6 because of next built-in):

def all_same(items):
  it = iter(items)
  first = next(it, None)
  return all(x == first for x in it)

assert all_same([])
assert all_same([1])
assert all_same([1, 1])
assert all_same([None, None])
assert not all_same([1, 2])
assert not all_same([None, 42])