# Python Programming/Tuples

A tuple in Python is much like a list except that it is immutable (unchangeable) once created. A tuple of hashable objects is hashable and thus suitable as a key in a dictionary and as a member of a set.

## Overview

Tuples in Python at a glance:

```tup1  = (1, 'a')
tup2  = 1, 'a'                # Brackets not needed
tup3  = (1,)                  # Singleton
tup4  = 1,                    # Singleton without brackets
tup5 = ()                     # Empty tuple
list1 = [1, 'a']
it1, it2 = tup1               # Assign items
print tup1 == tup2            # True
print tup1 == list1           # False
print tup1 == tuple(list1)    # True
print list(tup1) == list1     # True
print tup1                 # First member
for item in tup1: print item  # Iteration
print (1, 2) + (3, 4)         # (1, 2, 3, 4)
tup1 += (3,)
print tup1                    # (1, 'a', 3), despite immutability
print len(tup1)               # Length AKA size AKA item count
print 3 in tup1               # Membership - true
return r1, r2                 # Return multiple values
r1, r2 = myfun()              # Receive multiple values
tup6 = ([1,2],)
tup6=3
print tup6                    # The list within is mutable
set1 = set( (1,2) )           # Can be placed into a set
#set1 = set( ([1,2], 2) )     # Error: The list within makes it unhashable
```

## Tuple notation

Tuples may be created directly or converted from lists. Generally, tuples are enclosed in parentheses.

```>>> l = [1, 'a', [6, 3.14]]
>>> t = (1, 'a', [6, 3.14])
>>> t
(1, 'a', [6, 3.14])
>>> tuple(l)
(1, 'a', [6, 3.14])
>>> t == tuple(l)
True
>>> t == l
False
```

A one item tuple is created by an item in parentheses followed by a comma:

```>>> t = ('A single item tuple',)
>>> t
('A single item tuple',)
```

Also, tuples will be created from items separated by commas.

```>>> t = 'A', 'tuple', 'needs', 'no', 'parens'
>>> t
('A', 'tuple', 'needs', 'no', 'parens')
```

## Packing and Unpacking

You can also perform multiple assignment using tuples.

```>>> article, noun, verb, adjective, direct_object = t   #t is defined above
>>> noun
'tuple'
```

Note that either, or both sides of an assignment operator can consist of tuples.

```>>> a, b = 1, 2
>>> b
2
```

The example above: article, noun, verb, adjective, direct_object = t is called "tuple unpacking" because the tuple t was unpacked and its values assigned to each of the variables on the left. "Tuple packing" is the reverse: t=article, noun, verb, adjective, direct_object. When unpacking a tuple, or performing multiple assignment, you must have the same number of variables being assigned to as values being assigned.

## Operations on tuples

These are the same as for lists except that we may not assign to indices or slices, and there is no "append" operator.

```>>> a = (1, 2)
>>> b = (3, 4)
>>> a + b
(1, 2, 3, 4)
>>> a
(1, 2)
>>> b
(3, 4)
>>> a.append(3)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'append'
>>> a
(1, 2)
>>> a = 0
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: object does not support item assignment
>>> a
(1, 2)
```

For lists we would have had:

```>>> a = [1, 2]
>>> b = [3, 4]
>>> a + b
[1, 2, 3, 4]
>>> a
[1, 2]
>>> b
[3, 4]
>>> a.append(3)
>>> a
[1, 2, 3]
>>> a = 0
>>> a
[0, 2, 3]
```

### Tuple Attributes

Length: Finding the length of a tuple is the same as with lists; use the built in len() method.

```>>> len( ( 1, 2, 3) )
3
>>> a = ( 1, 2, 3, 4 )
>>> len( a )
4
```

### Conversions

Convert list to tuples using the built in tuple() method.

```>>> l = [4, 5, 6]
>>> tuple(l)
(4, 5, 6)
```

Converting a tuple into a list using the built in list() method to cast as a list:

```>>> t = (4, 5, 6)
>>> list(t)
[4, 5, 6]
```

Dictionaries can also be converted to tuples of tuples using the items method of dictionaries:

```>>> d = {'a': 1, 'b': 2}
>>> tuple(d.items())
(('a', 1), ('b', 2))
```

## Uses of Tuples

Tuples can be used in place of lists where the number of items is known and small, for example when returning multiple values from a function. Many other languages require creating an object or container to return, but with Python's tuple assignment, multiple-value returns are easy:

```def func(x, y):
# code to compute x and y
return x, y
```

This resulting tuple can be easily unpacked with the tuple assignment technique explained above:

```x, y = func(1, 2)
```

## Using List Comprehension to process Tuple elements

Occasionally, there is a need to manipulate the values contained within a tuple in order to create a new tuple. For example, if we wanted a way to double all of the values within a tuple, we can combine some of the above information in addition to list comprehension like this:

```def double(T):
'double() - return a tuple with each tuple element (e) doubled.'
return tuple( [ e * 2 for e in T ] )
```

## Exercises

1. Create the list ['a', 'b', 'c'], then create a tuple from that list.
2. Create the tuple ('a', 'b', 'c'), then create a list from that tuple. (Hint: the material needed to do this has been covered, but it's not entirely obvious)
3. Make the following instantiations simultaneously: a = 'a', b=2, c='gamma'. (That is, in one line of code).
4. Create a tuple containing just a single element which in turn contains the three elements 'a', 'b', and 'c'. Verify that the length is actually 1 by using the len() function.