How do I enumerate() over a list of tuples in Python?

Question

I've got some code like this:

letters = [('a', 'A'), ('b', 'B')]
i = 0
for (lowercase, uppercase) in letters:
    print "Letter #%d is %s/%s" % (i, lowercase, uppercase)
    i += 1

I've been told that there's an enumerate() function that can take care of the "i" variable for me:

for i, l in enumerate(['a', 'b', 'c']):
    print "%d: %s" % (i, l)

However, I can't figure out how to combine the two: How do I use enumerate when the list in question is made of tuples? Do i have to do this?

letters = [('a', 'A'), ('b', 'B')]
for i, tuple in enumerate(letters):
    (lowercase, uppercase) = tuple
    print "Letter #%d is %s/%s" % (i, lowercase, uppercase)

Or is there a more elegant way?

Sputter

enumerate vs itertools: what should we take into account when choosing between them? – Aerometeorograph

Answer 1

This is a neat way to do it:

letters = [('a', 'A'), ('b', 'B')]
for i, (lowercase, uppercase) in enumerate(letters):
    print "Letter #%d is %s/%s" % (i, lowercase, uppercase)

Cassirer

What is this called? I want to look up more about this syntax. – Risteau

@Zak: If you mean assigning to (lowercase, uppercase), it's called "unpacking". – Cassirer

Neat. I didn't know that the unpacking syntax can be nested. – Shaikh

Awesome! I always wrote a helper generator to achieve this. I didn't know that nesting is possible. Came across this question by lucky accident. Thanks! – Inure

Answer 2

This is how I'd do it:

import itertools

letters = [('a', 'A'), ('b', 'B')]
for i, lower, upper in zip(itertools.count(),*zip(*letters)):
    print "Letter #%d is %s/%s" % (i, lower, upper)

EDIT: unpacking becomes redundant. This is a more compact way, which might work or not depending on your use case:

import itertools

letters = [('a', 'A'), ('b', 'B')]
for i in zip(itertools.count(),*zip(*letters)):
    print "Letter #%d is %s/%s" % i

Rabiah

Answer 3

You can do this way too:

letters = [('a', 'A'), ('b', 'B')]
for i, letter in enumerate(letters):
    print "Letter #%d is %s/%s" % (i, letter[0], letter[1])

Barbour

Answer 4

You could also write a generator:

def enumerate_nested(nested_collection, start=0):

    for index, row in enumerate(nested_collection, start):
        yield index, *row

Which then allows you to iterate over the collection of tuple (or list) and to unpack the values:

names = [["Heinz", "Steiner"], ["Fred", "Glauser"], ["Nicole", "Hauser"]]

for index, first_name, last_name in enumerate_nested(names, 1):
    print(index, first_name, last_name)

---

However, there is a simpler built-in solution to achieve this. Have a look at Richie Hindle's answer:

names = [["Heinz", "Steiner"], ["Fred", "Glauser"], ["Nicole", "Hauser"]]

for index, (first_name, last_name) in enumerate(names, 1):
    print(index, first_name, last_name)

Inure