The nltk.stem.wordnet.WordNetLemmatizer object can be used to lemmatize and find a simple form for a word.
Note, if the part-of-speech is not indicated, the word is treated as a noun. We make this explicit, here, so we may use * to indicate the parameter should be treated as individual values and not as a tuple, in which case we have an error.
from __future__ import print_function
from nltk.stem.wordnet import WordNetLemmatizer
lem = WordNetLemmatizer()
words = [('order','v'),'order',('orders','v')]
for word in words:
if type(word) == str:
word = (word,'n')
lemma = lem.lemmatize(*word)
# For: ('order', 'v') Lemma = order
# For: ('order', 'n') Lemma = order
# For: ('orders', 'v') Lemma = order