Python collections

#Python Collections
P.D.ManushaDilan
manushadilan@gmail.com
2015-August-15

Page | 1
Contents
I. #importing…………………………………………………………………………………………………..2
II. #str……………………………………………………………………………………………………….3
III. #bytes……………………………………………………………………………………………………7
IV. #for-loop………………………………………………………………………………………………8
V. #list………………………………………………………………………………………………………9
VI. #tuple…………………………………………………………………………………………………..15
VII. #dict…………………………………………………………………………………………………….18
VIII. #range………………………………………………………………………………………………..23
IX. #set…………………………………………………………………………………………………….25
X. #Collection protocols………………………………………………………………………..28
XI. #Comprehensions……………………………………………………………………………..29
XII. #Generators……………………………………………………………………………………….31

Page | 2
#importing
Import modules
import math
s=math.sqrt(8)
print(s)
2.8284271247461903
from math import factorial
n=5
k=3
s=factorial(n)/(factorial(k)*factorial(n-k))
print(s)
10.0
from math import factorial as fac
n=5
k=3
s=fac(n)/(fac(k)*fac(n-k))
print(s)
10.0
# // use for integer division

Page | 3
#str
Immutable sequence of Unicode code points
#can use both ‘_’ and “_”
'This is a string'
"This is also string"
s="first""Second"
print(s)
firstSecond
#’’’_’’’ for multiline
‘’’Multi line
string
:D ‘’’
#universal newline n
print("This is nnew line")
This is
new line
#escape sequence
print("This is " in a string")
This is " in a string
#row string
path=r"C:UsersExamDocdir"
print(path)
C:UsersExamDocdir

Page | 4
#call by index
s="Parrot"
print(s[4])
o
#length of string
s="Beautiful is better than ugly.Explicit is better than implicit."
print(len(s))
63
#concatenates
w="new"+"found"+"land"
print(w)
newfoundland
r="new"
r+="found"
r+="land"
print(r)
newfoundland
#join
color=';'.join(["#ff2","#a34","#3542"])
print(color)
#ff2;#a34;#3542

Page | 5
#splitting and joining
print(color.split(';'))
['#ff2', '#a34', '#3542']
print(''.join(["high","way","man"]))
highwayman
#partition
x="unforgetable"
print(x.partition("forget"))
('un', 'forget', 'able')
#dummy variable '_'
origin,_,destination="Seattle-Boston".partition("-")
print(origin,destination)
Seattle Boston
#format
q="My name is {0}.I like {1}".format("Sumudu","Apple")
print(q)
My name is Sumudu.I like Apple
p="This {} can {} do this way.".format('method','also')
print(p)
This method can also do this way.

Page | 6
y="My name is {Name}.I am {Age} years
old.".format(Name="Sumudu",Age="22")
print(y)
My name is Sumudu.I am 22 years old.
pos=(34,23.6,67)
q="galactic position x={pos[0]},y={pos[2]},z={pos[1]}".format(pos=pos)
print(q)
galactic position x=34,y=67,z=23.6
import math
s="Math constants: pi={m.pi} e={m.e}".format(m=math)
print(s)
Math constants: pi=3.141592653589793 e=2.718281828459045
#get up to 3 floating point values
s="Math constants: pi={m.pi:.3f} e={m.e:.3f}".format(m=math)
print(s)
Math constants: pi=3.142 e=2.718

Page | 7
#bytes
Immutable sequence of bytes
#declare bytes
d=b'Some bytes'
print(d.split())
[b'Some', b'bytes']
#encoding and decoding
#using in data streaming through the networks
data="I love my girlfriend".encode('utf-8')
dc=data.decode('utf-8')
print(data)
print(dc)
b'I love my girlfriend'
I love my girlfriend

Page | 8
#for loop
for ITEM in SEQUENCE
cities=["Matara","Ambalangoda","Colombo","Gampaha","Halawatha"]
for city in cities:
print(city)
Matara
Ambalangoda
Colombo
Gampaha
Halawatha
color={"Red":43,"Yellow":23,"Blue":45,"Green":22}
for clr in color:
print(clr,color[clr])
Green 22
Yellow 23
Red 43
Blue 45

Page | 9
#list
Mutable sequence of objects
a=["apple","orange","pears"]
print(a[2])
pears
a[2]=7
print(a)
['apple', 'orange', 7]
#add items
b=[]
b.append(1.345)
b.append(1.534)
#adding element to the end of the list
s=list("This is a list")
print(s)
['T', 'h', 'i', 's', ' ', 'i', 's', ' ', 'a', ' ', 'l', 'i', 's', 't']
#use last comma

Page | 10
#negative indexing
#last element at -1
l="show how to index into sequence".split()
print(l)
print(l[-3],l[-1])
['show', 'how', 'to', 'index', 'into', 'sequence']
index sequence
#slice
#slice=seq[start:stop]
print(l[1:3])
print(l[3:])
print(l[:3])
print(l[:])
['how', 'to']
['index', 'into', 'sequence']
['show', 'how', 'to']
#copying
#copies are shallow
print(l[:])
print(l.copy())
print(list(l))

Page | 11
#repeat
q=l*3
print(q)
['show', 'how', 'to', 'index', 'into', 'sequence', 'show', 'how', 'to', 'index', 'into', 'sequence',
'show', 'how', 'to', 'index', 'into', 'sequence']
#index()
w="Sumudu is a good girl.Sumudu is beautiful".split()
i=w.index('good')
print(i)
3
#count()
print(w.count('is'))
2
#membership
print('Sumudu' in w)
print('love' not in w)
True
True
#delete
u="Jack is a woodcutter".split()
del u[3]
print(u)
['Jack', 'is', 'a']

Page | 12
#raise value error if value is not in list
u.remove('Jack')
#insert
u.insert(0,'Sumudu')
u.insert(3,'girl')
print(u)
['Sumudu', 'is', 'a', 'girl']
#concatenate
m=[1,3,5,7,9]
n=[2,4,6,8,10]
k=m+n
print(k)
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
k+=[20,30,40]
print(k)
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10, 20, 30, 40]
k.extend([90,100])
print(k)
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10, 20, 30, 40, 90, 100]

Page | 13
#reverse
g=[1,2,3,4,5]
g.reverse()
print(g)
[5, 4, 3, 2, 1]
#sort
h=[4,7,2,1,5,9,8]
h.sort()
print(h)
[1, 2, 4, 5, 7, 8, 9]
h.sort(reverse=True)
print(h)
[9, 8, 7, 5, 4, 2, 1]
h="Moon is so beautiful when you are with me".split()
h.sort(key=len)
print(h)
['is', 'so', 'me', 'you', 'are', 'Moon', 'when', 'with', 'beautiful']
#join
print(' '.join(h))
is so me you are Moon when with beautiful
#empty
e=[]

Page | 14
#sorted
#not change the original list
x=[10,2,3,5,8,7]
y=sorted(x)
print(x,y)
[10, 2, 3, 5, 8, 7] [2, 3, 5, 7, 8, 10]
#reversed
p=[8,7,6,5,4,3]
q=reversed(p)
print(q)
print(list(q))
<list_reverseiterator object at 0x03CEFFB0>
[3, 4, 5, 6, 7, 8]

Page | 15
#tuple
Heterogeneous immutable sequence
t=("This","Is",23,43.5)
print(t[3])
43.5
print(len(t))
4
print(t)
('This', 'Is', 23, 43.5)
for item in t:
print(item)
This
Is
23
43.5
#concatenate
print(t+("new",66))
('This', 'Is', 23, 43.5, 'new', 66)
#repeat
print(t*3)
print(type(t))
('This', 'Is', 23, 43.5, 'This', 'Is', 23, 43.5, 'This', 'Is', 23, 43.5)
<class 'tuple'>

Page | 16
#nested tuple
a=((1,2),(1,3))
print(a[1])
print(a[1][0])
(1, 3)
1
#single tuple by adding trailing comma
k=(99,)
#empty tuple
e=()
#sequence take as tuple
p=1,2,3,4,5,4
print(p)
(1, 2, 3, 4, 5, 4)
#tuple are useful for multiple return values
def minmax(item):
return min(item),max(item)
print(minmax([1,3,45,3,4,2,22]))
(1, 45)

Page | 17
#tuple unpacking
lower,upper=minmax([3,2,11,4,5,7,8,44,0])
print(lower)
print(upper)
0
44
#tuple unpacking works with nested tuples
(a,(b,(c,d)))=(4,(6,(3,5)))
print(b)
6
#idiomitic python swap
#a,b=b,a
#converting to tuple
print(tuple("this is string"))
print(tuple([1,2,3,4,5,6]))
('t', 'h', 'i', 's', ' ', 'i', 's', ' ', 's', 't', 'r', 'i', 'n', 'g')
(1, 2, 3, 4, 5, 6)
#membership
print(5 in (1,2,3,4,5,6))
print(10 not in (1,2,3,4,5))
True
True

Page | 18
#dict
Mutable mapping of keys to values
#{k1:v1,k2:v2,k3:v3}
#keys must be immutable
#values can be mutable
#order is arbitrary
d={"SriLanka":"Colombo","India":"Mumbai","France":"Paris","US":"NewYo
rk"}
print(d)
print(d['SriLanka'])
e={}
print(e)
{'US': 'NewYork', 'India': 'Mumbai', 'SriLanka': 'Colombo', 'France': 'Paris'}
Colombo
{}
names_ages=[('Sumudu',22),('Manusha',25),('Bob',34)]
d=dict(names_ages)
print(d)
{'Manusha': 25, 'Sumudu': 22, 'Bob': 34}
phonetic=dict(a='alpha',b='bravo',c='charly',d='delta')
print(phonetic)
{'b': 'bravo', 'a': 'alpha', 'd': 'delta', 'c': 'charly'}

Page | 19
#copy
x=d.copy()
print(x)
f=dict(d)
print(f)
#update
g=dict(Thilina=24,Nimal=21)
d.update(g)
print(d)
{'Manusha': 25, 'Sumudu': 22, 'Nimal': 21, 'Thilina': 24, 'Bob': 34}
#if keys already exists values will be replaced
for key in d:
print("{key} => {val}".format(key=key,val=d[key]))
Manusha => 25
Sumudu => 22
Nimal => 21
Thilina => 24
Bob => 34

Page | 20
#values
for value in d.values():
print(value)
25
22
21
24
34
#no efficient way to get key from value
for key in d.keys():
print(key)
Manusha
Sumudu
Nimal
Thilina
Bob
#items give both key and values
for key,val in d.items():
print("{key} => {val}".format(key=key,val=val))
Manusha => 25
Sumudu => 22
Nimal => 21
Thilina => 24
Bob => 34

Page | 21
#membership only work with keys
print("Sumudu" in d)
print("Manusha" not in d)
True
False
#delete
del d['Thilina']
print(d)
{'Manusha': 25, 'Sumudu': 22, 'Nimal': 21, 'Bob': 34}
#add
m={'H':[1,2,3],'He':[1,2],'Li':[3,4,5],'Be':[1]}
m['Be'] += [2,3,4,6]
print(m)
{'Li': [3, 4, 5], 'Be': [1, 2, 3, 4, 6], 'He': [1, 2], 'H': [1, 2, 3]}
m['N']=[7,6,8]
print(m)
{'Li': [3, 4, 5], 'Be': [1, 2, 3, 4, 6], 'He': [1, 2], 'N': [7, 6, 8], 'H': [1, 2, 3]}

Page | 22
#pretty printing
from pprint import pprint as pp
pp(m)
{'Be': [1, 2, 3, 4, 6],
'H': [1, 2, 3],
'He': [1, 2],
'Li': [3, 4, 5],
'N': [7, 6, 8]}

Page | 23
#range
Arithmetic progression of integers
print(range(5))
range(0, 5)
for i in range(5):
print(i)
0
1
2
3
4
print(list(range(5,10)))
print(list(range(2,10,2)))
[5, 6, 7, 8, 9]
[2, 4, 6, 8]

Page | 24
#enumerate for counters
t=[1,2,3,4,5,6]
for p in enumerate(t):
print(p)
(0, 1)
(1, 2)
(2, 3)
(3, 4)
(4, 5)
(5, 6)
for i,v in enumerate(t):
print("i={},v={}".format(i,v))
i=0,v=1
i=1,v=2
i=2,v=3
i=3,v=4
i=4,v=5
i=5,v=6

Page | 25
#set
Unordered collection of unique immutable objects
p={1,2,3,4,5,6}
print(p)
{1, 2, 3, 4, 5, 6}
s=set([1,2,3,5])
print(s)
{1, 2, 3, 5}
#empty
e=set()
#duplicates removed
s={1,2,3,1,2,3,5,6,7,7,7}
print(s)
{1, 2, 3, 5, 6, 7}
#order is arbitrary
for x in {1,2,3,4,5,1,2}:
print(x)
1
2
3
4
5

Page | 26
#membership
print(1 in s)
print(1 not in s)
True
False
#add
s.add(9)
print(s)
{1, 2, 3, 5, 6, 7, 9}
s.update([23,21])
print(s)
{1, 2, 3, 5, 6, 7, 9, 21, 23}
#remove
#value Error will rise if element is not in set
s.remove(23)
#no value error in discard
s.discard(21)
print(s)
{1, 2, 3, 5, 6, 7, 9}

Page | 27
#copy
p=s.copy()
q=set(s)
print(p,q)
{1, 2, 3, 5, 6, 7, 9} {1, 2, 3, 5, 6, 7, 9}
#set functions
#Boolean values will return

Page | 28
#Collection protocols
Protocol Implementing Collections
Container str, list, range, tuple, set, bytes, dict
Sized str, list, range, tuple, set, bytes, dict
Iterable str, list, range, tuple, set, bytes, dict
Sequence str, list, range, tuple, set, bytes
Mutable Sequence list
Mutable Set set
Mutable Mapping dict

Page | 29
#Comprehensions
[expr(item) for item in iterable]
#list comprehensions
words="This is a sample word list to experiment".split()
x=[len(word) for word in words]
print(x)
[4, 2, 1, 6, 4, 4, 2, 10]
from math import factorial
f=[len(str(factorial(x))) for x in range(20)]
print(f)
[1, 1, 1, 1, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 18]
#set comprehensions
r={len(str(factorial(x))) for x in range(20)}
print(r)
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 18}
#dict comprehensions
from pprint import pprint as pp
c_c={'Sumudu':22,"Manusha":25,"Bob":23}
p={age:name for name,age in c_c.items()}
pp(p)
{22: 'Sumudu', 23: 'Bob', 25: 'Manusha'}

Page | 30
#filtering predicates
from math import sqrt
def is_prime(x):
if x <2:
return False
for i in range(2,int(sqrt(x))+1):
if x%i==0:
return False
return True
print([x for x in range(101) if is_prime(x)])
[5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49,
51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93,
95, 97, 99]
#iteration protocols
#end of list StopIteration error
iterable=['Spring','Summer','Autumn','Winter']
iterator=iter(iterable)
print(next(iterator))
print(next(iterator))
Spring
Summer

Page | 31
#Generators
#create independent generators
def gen123():
yield 1
yield 2
yield 3
g=gen123()
print(next(g))
1
for v in g:
print(v)
1
2
3
#generator comprehensions
million_squares=(x*x for x in range(1,100001))
print(million_squares)
<generator object <genexpr> at 0x041911E8>

Page | 32
#print(list(million_squares))
#generators are single used objects
print(sum(x*x for x in range(1,100001)))
print(sum(x for x in range(10001)if is_prime(x)))
333338333350000
24999996
#itertools
from itertools import islice,count
tp=islice((x for x in count() if is_prime(x)),1000)
print(sum(tp))
1004000
#any / all
print(any([False,False,True]))
print(all([False,True,True]))
print(any(is_prime(x) for x in range(1328,2001)))
True
False
True

Page | 33
#zip
sun=[12,14,11,15,11]
mon=[1,4,5,6,3]
for item in zip(sun,mon):
print(item)
(12, 1)
(14, 4)
(11, 5)
(15, 6)
(11, 3)
for s,m in zip(sun,mon):
print("Average= ",(s+m)/2)
Average= 6.5
Average= 9.0
Average= 8.0
Average= 10.5
Average= 7.0
#chain
from itertools import chain
temp=chain(sun,mon)
print(all(t>0 for t in temp))
True

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