10 - Python Dictionary - PowerPoint Presentation

Slide1

10 - Python Dictionary

John R. WoodwardSlide2

Dictionary 1

A dictionary is

mutable

and is another container type that can store any number of Python objects, including other container types.

Dictionaries consist of

pairs

(called items) of

keys

and their corresponding

values

.

Think of the

key to a value

Python dictionaries are also known as associative arrays or hash tables.

dict

=

{

'Alice'

:

'2341'

,

'Beth'

:

'9102'

,

'Cecil'

:

'3258'

}Slide3

Keys

Each

key

is separated from its value by a colon (

:

), the items are separated by commas, and the whole thing is enclosed in

curly braces

.

An

empty dictionary

without any items is written with just two curly braces, like this:

{}

.

Keys

are

unique

within a dictionary while values may not be. The

values

of a dictionary can be of

any type

, but the

keys

must be of an

immutable

data type such as

strings, numbers, or tuples

.Slide4

Example

digitsStrings

= {

1

:

"one

"

,

2

:

"two"

,

3

:

"three"

}

print

digitsStrings

digitsStrings

[

4

] =

"four"

print

digitsStrings

del

digitsStrings

[

1

]

print

digitsStrings

digitsStrings

[

"five"

] =

5

print

digitsStrings

digitsStrings

[

1.2

]=

"

one.two

"

print

digitsStringsSlide5

Example

digitsStrings

= {

1

:

"one

"

,

2: "two", 3:"three"}print digitsStringsdigitsStrings[4] = "four"print digitsStringsdel digitsStrings[1]print digitsStringsdigitsStrings["five"] = 5print digitsStringsdigitsStrings[1.2]="one.two"print digitsStrings

{1: 'one', 2: 'two', 3: 'three

'}

{1: 'one', 2: 'two',

3

: 'three', 4: 'four

'}

{2: 'two', 3: 'three', 4: 'four

'}

{'five': 5, 2: 'two',

3

: 'three', 4: 'four

'}

{'five': 5, 2: 'two', 3: 'three',

4

: 'four', 1.2: '

one.two

'}Slide6

Keys – must be unique

Keys must be

unique

One key accesses

one value

.

I would keep

the same datatype

.(

string,int) I would not use floats. Which are possible keys Student ID number, date of birth, email address, car registration, full name, Slide7

Accessing Values in Dictionary

To access dictionary elements, you can use the familiar square brackets.

dict

=

{

'Name'

:

'Zara', 'Age': 7, 'Class': 'First'}; print "dict['Name']: ", dict['Name']; print "dict['Age']: ", dict['Age'];it produces the following result:dict['Name']: Zara dict['Age']: 7Slide8

Updating Dictionary

You can

update

a dictionary by

adding

a new entry or item (i.e., a key-value pair),

modifying

an existing entry, or

deleting

an existing entry as shown below in the simple example:dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}; dict['Age'] = 8; # update existing entry dict['School'] = "DPS School"; # Add new entry print "dict['Age']: ",

dict

[

'Age'

];

print

"

dict

['School']: "

,

dict

[

'School'

];

it produces

the following result

:

dict

['Age']: 8

dict

['School']: DPS SchoolSlide9

Delete Dictionary Elements:

dict

=

{

'Name'

:

'Zara', 'Age': 7, 'Class': 'First'};del dict['Name']; # remove entry with key 'Name' dict.clear(); # remove all entries in dict del dict ; # delete entire dictionary Slide10

Clear or delete a dictionary

dictNumbers

= {

1

:

"one"

,

2

:

"two", 3:"three"}print dictNumbersdictNumbers.clear()print dictNumbersdel dictNumbersprint dictNumbersSlide11

output

{1: 'one', 2: 'two', 3: 'three'}

{}

print

dictNumbers

NameError

: name '

dictNumbers

' is not definedSlide12

Properties of Dictionary Values:

Dictionary

values

have no restrictions.

They can be any

arbitrary Python object

, either standard objects or user-defined objects.

However, same is not true for the keys.Slide13

Properties of Dictionary Keys:

There are two important points to remember about dictionary keys:

More

than one entry per key not allowed. Which means

no duplicate key is allowed

.

When

duplicate keys encountered during assignment,

the last assignment wins

. Following is a simple example:Slide14

Example – do not do

dict

=

{

'Name'

:

'Zara', 'Age': 7, 'Name': 'Manni'}; print "dict['Name']: ", dict['Name'];it produces the following result:dict['Name']: ManniNote that we lost “'Name': 'Zara',”Slide15

Keys must be immutable

you

can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed.

dict

=

{[

'Name'

]: 'Zara', 'Age': 7}; print "dict['Name']: ", dict['Name']; it produces the following result:Traceback (most recent call last): File "test.py", line 3, in <module> dict = {['Name']: 'Zara', 'Age': 7}; TypeError: list objects are unhashableSlide16

Built-in Dictionary Functions 

cmp

(dict1,

dict2)

Compares

elements of both dict.

len

(

dict) Gives the total length of the dictionary. This would be equal to the number of items in the dictionary.str(dict) Produces a printable string representation of a dictionarytype(variable) Returns the type of the passed variable. If passed variable is dictionary, then it would return a dictionary type.Slide17

Built-in Dictionary Methods

dict.clear

()

Removes

all elements of dictionary 

dict

dict.copy

()

Returns a shallow copy of dictionary dictdict.fromkeys() Create a new dictionary with keys from seq and values set to value.dict.get(key, default=None) For key key, returns value or default if key not in dictionary

dict.has_key

(key)

Returns

true

 if key in dictionary 

dict

, 

false

 otherwise

dict.items

()

Returns

a list of 

dict

's

(key, value) tuple pairs

dict.keys

()

Returns

list of dictionary

dict's

keys

dict.setdefault

(key,

default=None)

Similar

to get(), but will set

dict

[key]=default if 

key

 is not already in

dict

dict.update

(dict2)

Adds

dictionary 

dict2

's key-values pairs to 

dict

dict.values

()

Returns

list of dictionary 

dict

's

valuesSlide18

fromkeys

seq

= (

'name'

,

'age'

,

'sex'

)

dict = dict.fromkeys(seq)print "New Dictionary : %s" % str(dict)dict = dict.fromkeys(seq, 10)print "New Dictionary : %s" % str(dict)OUTPUTNew Dictionary : {'age': None, 'name': None, 'sex': None}New Dictionary : {'age': 10, 'name': 10, 'sex': 10}Slide19

What does the following print?

dict1

= {"key1": "value1", "key2": "value2"}

dict2

= dict1

p

rint dict2

dict2

["key2"] = "WHY?!"

print dict1Slide20

What does the following print?

dict1

= {"key1": "value1", "key2": "value2"}

dict2

= dict1

p

rint dict2

dict2

["key2"] = "WHY?!"

print dict1 output {'key2': 'value2', 'key1': 'value1'}dict2["key2"] = "WHY?!"print dict1{'key2': 'WHY?!', 'key1': 'value1'}Slide21

Copy a dictionary

If you want to copy the

dict

(

which is rare

), you have to do so explicitly

with

dict2 =

dict

(dict1) ordict2 = dict1.copy()Slide22

What is the output?

def

inc

(x):

print

"

inc" ,x x = x +1 print "inc" ,xy = 99 print "outside function ", yinc(y)print "outside function ", ySlide23

def

incList

(x):

print

"

inc

" ,x x[0] = x[0] + 1 print "inc" ,xy = [99] print "outside function ", yincList(y)print "outside function ", yWhat is the output?Slide24

def

inc

(x):

print

"

inc

" ,x x = x +1 print "inc" ,x return x #addedy = 99 print "outside function ", yy = inc(y) #assignmentprint "outside function ", yWhat is the output?Slide25

What is the output?

def

inc

(x):

print

"

inc" ,x x = x +1 print "inc" ,xy = 99 print "outside function ", yinc(y)print "outside function ", ySlide26

What is the output?

def

inc

(x):

print

"

inc" ,x x = x +1 print "inc" ,xy = 99 print "outside function ", yinc(y)print "outside function ", youtputoutside function 99inc 99inc 100outside function 99Slide27

def

incList

(x):

print

"

inc

" ,x x[0] = x[0] + 1 print "inc" ,xy = [99] print "outside function ", yincList(y)print "outside function ", yWhat is the output?Slide28

def

incList

(x):

print

"

inc

" ,x x[0] = x[0] + 1 print "inc" ,xy = [99] print "outside function ", yincList(y)print "outside function ", youtputoutside function [99]inc [99]inc [100]outside function [100]What is the output?Slide29

def

inc

(x):

print

"

inc

" ,x x = x +1 print "inc" ,x return x #addedy = 99 print "outside function ", yy = inc(y) #assignmentprint "outside function ", yWhat is the output?Slide30

What is the output?

def

inc

(x):

print

"

inc" ,x x = x +1 print "inc" ,xy = 99 print "outside function ", yinc(y)print "outside function ", youtputoutside function 99inc 99inc 100outside function 100