Python Programming

4.1 Dictionaries

• A dictionary is like a list, but more general. In a list, the index positions have to be integers; in a dictionary, the indices can be (almost) any type.

>>> x = {} #This is False
>>> print(x)
{}

• You can think of a dictionary as a mapping between a set of indices (which are called keys) and a set of values.

>>> x['one'] = 'apple'
>>> print(x)
{'one': 'apple'}

• In general, the order of items in a dictionary is unpredictable. (the order you add items)

>>> x = {'one':'apple','two':'banana','three':'orange'}
>>> print(x)
{'one':'apple','three':'orange','two':'banana'}

>>> print(x['two'])
'banana'

>>> len(x)
3

• Adding New Key-Value Pairs

>>> alien = {}
>>> alien['color'] = 'green'
>>> alien['point'] = 5
>>> print(alien)
{'color': 'green', 'points': 5}

• To create a dictionary, start with an empty one, and add the key-value pairs. The keys should be immutable and there is no repeated key. Lists cannot be keys in the dictionary.

• Modifying Values in a Dictionary

>>> alien['color'] = 'yellow'
>>> print(alien)
{'color': 'yellow', 'points': 5}

• Removing Key-Value Pairs

>>> del alien['points']
>>> print(alien)
{'color': 'yellow'}

• Merge two dictionaries

>>> dict1 = { "name":"owen", "age": 18 }
>>> dict2 = { "birthday": "1999-11-22"}
>>> x = dict1|dict2
>>> print(x)
{'name': 'owen', 'age': 18, 'birthday': '1999-11-22'}

>>> a = { 'x' : 1 , 'y' : 2 }
>>> b = { 'y' : 3 , 'z' : 4 }
>>> c = a|b
>>> print(c)
{'x': 1, 'y': 3, 'z': 4}

• The in operator works on dictionaries. (keys) To see whether something appears as a value in a dictionary, you can use the method .values().

>>> x = {'one': 'apple', 'two': 'banana', 'three': 'orange'}
>>> 'one' in x
True
>>> 'uno' in x
False
>>>'orange' in x.values()
True

Methods of Dictionary

>>> stuff = {}
>>> dir(stuff)

['__class__', '__class_getitem__', '__contains__', '__delattr__', '__delitem__',
 '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__',
 '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__ior__',
 '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__or__',
 '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__ror__', '__setattr__',
 '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 
 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update','values']

>>> help(dict.get)
Help on method_descriptor:

get(self, key, default=None, /)
    Return the value for key if key is in the dictionary, else default.

• Dictionaries have a method called .get() that takes a key and a default value.

>>> counts = { 'chuck' : 1 , 'annie' : 42, 'jan': 100}
>>> print(counts.get('jan', 0)) #0 is the default value
100
>>> print(counts.get('anni', 1550))
1550

• Suppose you are given a string and you want to count how many times each letter appears.

word = 'banana'
d = {}
word_list = list('banana')
for c in word_list:
    d[c] = d.get(c,0) + 1
print(d)

{'b': 1, 'a': 3, 'n': 2}

Looping Through Dictionary

.items(), .keys(), .values()

# Looping Through All Key-Value Pairs
favorite = {
    'jen': 'python',
    'sarah': 'c',
    'edward': 'ruby',
    }
for x, y in favorite.items():
    print(x, y)

jen python
sarah c
edward ruby

# Looping Through All Values

for y in favorite.values():
    print(y)

python
c
ruby

# Looping Through All the Keys in a Dictionary

for x in favorite.keys():
    print(x)

jen
sarah
edward

Nesting

• You can nest a set of dictionaries inside a list, a list of items inside a dictionary, or even a dictionary inside another dictionary. A list or a dictionary cannot be the key (The key should be immutable).

• Dictionaries in a list

alien_0 = {'color': 'green', 'points': 5}
alien_1 = {'color': 'yellow', 'points': 10}
alien_2 = {'color': 'red', 'points': 15}

aliens = [alien_0, alien_1, alien_2]

• A List in a Dictionary

pizza = {
    'crust': 'thick',
    'toppings': ['mushrooms', 'extra cheese'],
    }
print(pizza['crust'], pizza['toppings'][1])

thick extra cheese

• A Dictionary in a Dictionary

users = {
    'aeinstein': {
        'first': 'albert',
        'last': 'einstein',
        'location': 'princeton',
        },

    'mcurie': {
        'first': 'marie',
        'last': 'curie',
        'location': 'paris',
        },
    }

full_name = users['aeinstein']['first']+" "+users['aeinstein']['last']
location = users['aeinstein']['location']

print("Full name: " + full_name)
print("Location: " + location)

Full name: Albert Einstein
Location: Princeton

Dictionaries: Summary

• Elements are key-value pairs. Empty dictionary {}

• Features：unpredictable order, key-value pairs are mutable (keys are immutable, but you can modify values). Do not make repeated keys.

• .get() (value or default value)

  Looping: .items(), .keys(), .values()

• in operator works on keys (but you can use .values())

• Nesting: A list of dictionaries, a list in a dictionary, a dictionary in a dictionary

4.2 Tuples

• A tuple is a sequence of values much like a list. The important difference is that tuples are immutable

• The empty tuple can be written as tuple().

>>> t = () #Empty is False
>>> print(t)
() 

>>> t = ('a', 'b', 'c', 'd', 'e')

• To create a tuple with a single element, you have to include the final comma:

>>> t1 = ('a',)
>>> type(t1)
tuple

>>> t2 = ('a')
>>> type(t2)
str

• Most list operators also work on tuples.

>>> t = ('a', 'b', 'c', 'd', 'e')
>>> print(t[0])
'a'

>>> print(t[1:3])
('b', 'c')

• Tuple is immutable. You can't modify the elements of a tuple, but you can replace one tuple with another:

>>> t[0] = 'A'
TypeError: object doesn't support item assignment

>>> t = ('A',) + t[1:]
>>> print(t)
('A', 'b', 'c', 'd', 'e')

• If you want to modify a tuple, you can convert it to a list, and then convert it back.

>>> t = ('a', 'b', 'c', 'd', 'e')
>>> s = list(t) 
>>> s[0] = 'A'
>>> t = tuple(s) 

Lists and Tuples: Common Features

• Comparing and sorting (the same for lists)

>>> (0, 1, 2) < (0, 3, 4)
True
>>> (0, 1, 2000000) < (0, 3, 4)
True

>>> t = [(2, 4), (0, 1, 2), (0, 3, 4)]
>>> t.sort()
>>> print(t)
[(0, 1, 2), (0, 3, 4), (2, 4)]

• in operator (the same for lists)

>>> t = ('a', 'b', 'c', 'd', 'e')
>>> print('a' in t) 
True

• Multiple Assignment (the same for lists)

>>> m = ('have', 'fun')  
>>> x, y = m 
>>> x
'have'
>>> y
'fun'
>>> x, y = y, x #swap

• Iteration with multiple values (the same for lists in lists, tuples in lists, lists in tuples)

t = [('a',1), ('b',2), ('c',3)]

# t = (('a',1), ('b',2), ('c',3))
# t = [['a',1], ['b',2], ['c',3]]

for x, y in t:
    print(x)
    print(y)

Dictionaries and Tuples

>>> d = {'a':10, 'b':1, 'c':22}
>>> t = list(d.items())
>>> t
[('b', 1), ('a', 10), ('c', 22)]

• Using tuples as keys in dictionaries

>>> directory = dict()
>>> directory[('Taylor', 'Swift')] = 100
>>> print(directory)

{('Taylor', 'Swift'): 100}

Tuples: Summary

• The element can be any type. The empty is ().

• Features: Ordered, Immutable, Repeatable

• Index and slice are the same with that of lists.

• You cannot modify the elements of a tuple, but you can replace one tuple with another.

• Comparison, multiple assignment, in operator

• Tuples can be keys of a dictionary.

4.3 List, dictionary, tuple

Comparison

Mutable and Immutable (可变和不可变)

• Values are immutable. Thus, numbers, strings, tuples are immutable. Lists, dictionaries are mutable.

• What does it mean for immutable or mutable? Assignment, Modification, Reference

• 1. what is assignment? variable $\rightarrow$ computer memory $\rightarrow$ object

2. Modification

• When you want to modify the immutable object, you can only assign a new object. It changes the address in the memory.

>>> x = "abc"
>>> x = "def"

• There are two ways for you to modify the mutable object. a. make a new assignment; b. make a modification by methods for lists or some ways for dictionaries.

a. make a new assignment

>>> x = [1,2,3]
>>> x = [4,5,6]

b. make a modification by methods for lists. You do not change the address.

>>> x = [1,2,3]
>>> del x[2]
>>> del x[1]
>>> del x[0]
>>> x.append(4)
>>> x.append(5)
>>> x.append(6)

3. Reference

>>> x = "abc"
>>> y = x
>>> x = "def"
>>> print(y)
"abc"

Summary

• Dictionaries, Tuples
• Reading: Python for Everybody
• Chapter 9, 10.1-10.5, 10.7-10.8