Python String.

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Strings:

• strings are a sequence or a char, enclosed in qoutes.

creating a string

• a string can be created by encloing a char or sequence of chars in single qoutes or double qoutes or triple single qoutes or triple double qoutes

• we use multi-line stings for documentation purpose; doc_strings

# creating a string

string_1 = 'this a string enclosed in single qoutes'

string_2 = "this a string enclosed in double qoutes"

string_3 = '''This is a multiple line

              string, which has

              3 lines in it and is enclosed with triple single qoutes'''

string_4 = """This is a multiple line

              string, which has

              3 lines in it and is enclosed with triple double qoutes"""

strings can be indexed.

• since strings are a sequence of chars, and this seq has an order. We can access individgual chars by their index or position in that seq enclose in [].

  x = 'python'

  string x has a len of 6, (len of string is equals to the number of chars in that string) the first index is 0 (indexing in python starts with 0)

Ex ;

x = 'python'

print('len of sting x is ',len(x))

print(x[0]) # indexing

Output ; 

len of sting x is  6
p

reverse indexing

• we can also access the chars in a string using reverse indexing.
• reverse indexing starts at the end of the string, reverse indexing starts with -1

Ex ; 

x = 'Python'

print(x[-1])

Output ;

n

• positive index starts with 0 and ends at len(string)-1
• negative index starts with -1 and ends at -len(string
• giving a number which doesn't fall in the given range will result in IndexError

slicing

• as the word suggest, we can get a sub-string of the given string by performing slicing

• to slice a string, we need three values: start_index, end_index, step_size

  • start_index: from where the slicing has to start, or the starting index of sub-string.
  • end_index: in slicing, end index is end index - 1. end_index tells till which index the slicing has to happen, (we are not going to include char at end_index)
  • step_size: it talks about how many steps it has to move while performing the slicing. by default, step_size is +1

• syntax for slicing:

  • var[start_index: end_index: step_size]

Ex ;

x = 'Python is a great language to learn, it spoken by a large population of coders'

print(x[0:8])

print(x[0:8:2])

Output ;

Python i

Pto 

print(x[::-1]) # reversing a string using -1 as step size

sredoc fo noitalupop egral a yb nekops ti ,nrael ot egaugnal taerg a si nohtyP

• while using a negative step size, we travel from end to start.
• the start and end index values also follow this.
• while using -ve step size, the default start and end indicies are (-1 or len(string)-1) and (-len(string) or 0) respectively
• 
  

Ex ; 

x[5:15:-2] # you get a empty string as the direction and the indecies don't match

Output ; ''

string concatenation

Ex ;

a, b = 'hi', 'everyone'

c = a+' '+b

c

Output ;

'hi everyone'

membership operations on strings

Ex ; 

y = 'Python'

print( y in x)

print(y[::-1] not in x)

Output ;

True
True

Methods in strings

• when we assign a string to a variable, we are basically performing an object initialization. By doing this, we can invoke or use the methods that are there in str class.
• string methods are inbuilt prewritten functions that acts on the given string to get a desired output.
• results we get from string methods has to be saved in another variable to access them later as strings are immutable and these results have a different memory location than the given string.

print(dir(y))      =       ['__add__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'capitalize', 'casefold', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'format_map', 'index', 'isalnum', 'isalpha', 'isascii', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

Ex ; 

print(type(x))

output ; 

<class 'str'>

upper()

• upper() is goint to convert lowercase alphabets in given string to uppercase.

Ex ;

x = 'abc'

print(x.upper())

print(x)

# note: in other to use the new string or output of the string methods, we need assign or reassign to variable

print()

x = x.upper()

print(x)

Output ; 

ABC
abc

ABC

lower()

• lower() converts upper_case alphabets to lower_case

Ex ; 

print(x)

x = x.lower()

print(x)

Output ; 

ABC
abc

capitalize()

• if an alphabet is at 0th index, it is going convert it to capital case

Ex ;

x = 'abc def'

print(x)

x = x.capitalize()

print(x)

print()

x = '*abc def'

print(x)

x = x.capitalize()

print(x)

print()

x = '''a

       b

       c'''

print(x)

x = x.capitalize()

print(x)

print()

Output ;

abc def
Abc def

*abc def
*abc def

a
       b
       c
A
       b
       c

title()

• title() going to convert every first alphabet of every consecutive word separeted with a space to capital case.

Ex ;

x = 'momma, i killed a man!'

print(x)

x = x.title()

print(x)

print()

x = '''a

b

c'''

print(x)

x = x.title()

print(x)

Output ; 

momma, i killed a man!
Momma, I Killed A Man!

a
b
c
A
B
C

swapcase()

• swapcase() is going swap the cases of alphabets

Ex ;

x = 'AbCd EfGh'

print(x)

x=x.swapcase()

print(x)

Output ;

AbCd EfGh
aBcD eFgH

count()

• it will return the value of occurences of given sub-string in a string.

Ex ;

x = """Betty Botter bought some butter

But she said the butter’s bitter

If I put it in my batter, it will make my batter bitter

But a bit of better butter will make my batter better

So ‘twas better Betty Botter bought a bit of better butter"""

x = x.lower()

print(x.count('b'))

print(x.count('tt'))

Output ;

23
17

startswith() and endswith()

• these methods will return a boolean output.
• True, when the char or sub-string is passed is at the start and end of the string respectively
• False, otherwise

Ex ;

x = 'these'

print(x)

print()

# startswith()

print(x.startswith('t'), x.startswith('the'), x.startswith('e'))

#endswith()

print(x.endswith('e'), x.endswith('ese'), x.endswith('t'))

Output ;

these

True True False
True True False

find and index

• both methods will return the index position of first occurence of given char or sub-string.

• find and index performs same task, however they behave differently when a char or sub-string which is not present in the parent string is given as an input/arguement.

  • find will return -1
  • index will return an error

Ex ; 

x = """Betty Botter bought some butter

But she said the butter’s bitter

If I put it in my batter, it will make my batter bitter

But a bit of better butter will make my batter better

So ‘twas better Betty Botter bought a bit of better butter"""

x = x.lower()

print(x.find('m'), x.find('was'), x.find('zz'))

print()

print(x.index('m'), x.index('was'),)

print(x.index('zz'))

Output ;

22 180 -1

22 180

rfind and rindex

• both methods will return the index position of first occurence of given char or sub-string from the right side of the string.

• rfind and rindex performs same task, however they behave differently when a char or sub-string which is not present in the parent string is given as an input/arguement.

  • rfind will return -1
  • rindex will return an error

Ex ;

x = """Betty Botter bought some butter

But she said the butter’s bitter

If I put it in my batter, it will make my batter bitter

But a bit of better butter will make my batter better

So ‘twas better Betty Botter bought a bit of better butter"""

x = x.lower()

print(x.rfind('m'), x.rfind('was'), x.rfind('zz'))

print()

print(x.rindex('m'), x.rindex('was'),)

print(x.rindex('zz'))

Output ;

158 180 -1

158 180

replace()

• replaces every occurence of given char or sub-string with new char or sub-string

Ex ;

y = 'abc abc abc'

print(y)

print(y.replace('a', 'A'))

print(y.replace('bc', 'BC'))

print(y)

Output ;

abc abc abc
Abc Abc Abc
aBC aBC aBC
abc abc abc

split()

• split() returns a list of sub-strings which were broken/splitted on given char or sub-string, if no arg/input is provide, it'll split on space.

Ex ;

x = 'abc def ghi'

print(x.split())

print(x.split('e'))

Output ;

['abc', 'def', 'ghi']
['abc d', 'f ghi']

partition()

• partition() splits on the given char or sub-string.
• it'll break the string on the given char or sub-string into three parts and returns it as a tuple.

Ex ;

x = 'abc def ghi'

print(x.partition('e'))

#print(x.partition())

print(x.partition(' '))

Output ;

('abc d', 'e', 'f ghi')
('abc', ' ', 'def ghi')

rpartition() abd rsplit()

Ex ;

print(x.rpartition('e'))

print(x.rpartition(' '))

print('\n \n')

print(x.rsplit('e'))

print(x.rsplit())

Output ;

('abc d', 'e', 'f ghi')
('abc def', ' ', 'ghi')

 

['abc d', 'f ghi']
['abc', 'def', 'ghi']

center

• it is going to bring or readjust the given string to be at center of the width provided with spaces on either side (if no char input/arg is given) or with the given char

• the width given as an input/arg should be greated than len(string), if not, the orginal string will be given as input

Ex ;

x = 'python'

print(x)

xCentered = x.center(20) 

print(xCentered)

len(xCentered)

print()

xCent_ast = x.center(20,'*')

print(xCent_ast)

Output ;

python
       python       

*******python*******

ljust() & rjust()

• the width given as an input/arg should be greated than len(string), if not, the orginal string will be given as input

Ex ;

print(x)

print(x.ljust(20)) 

print(x.ljust(20,'*'))

print()

print(x.rjust(20))

print(x.rjust(20,'*'))

Output ;

python
python              
python**************

              python
**************python

zfill()

• zfill() is used to pad or fill the remaining of given width with zeros on left side

Ex ;

print(x)

print(x.zfill(20))

Output ;

python
00000000000000python

strip()

• it removes the whitespaces at the start and end of the string

• if a char or a sub-string is given as input/arg, it'll removes the given char/sub-string

Ex ;

xCentered

Output ;

'       python       '

lstrip() and rstrip()

• lstrip() is used to remove the prefix, it removes the given char/s if they are at start of the string. (space, if no char/s are provided)

• rstrip() is used to remove the suffix, it removes the given char/s if they are at end of the string. (space, if no char/s are provided)

Ex ;

print(xCent_ast)

print(xCent_ast.lstrip('*'))

print(xCent_ast.rstrip('*'))

Output ;

*******python*******
python*******
*******python

removeprefix() and removesuffix()

• removeprefix() and removesuffix() will remove only given char, but not the sequence of given char like in lstrip() and rstrip()

#print(xCent_ast)

#print(xCent_ast.removeprefix('*'))

#print(xCent_ast.removesuffix('*'))

join()

• join() is going to join given sequence of chars using the string on which it is called.

Ex ;

x = 'python is great lang :/'

print(x)

print()

x1 = x.split()

print(x1)

y = ''

z = '**'

print()

print(y.join(x1))

print()

print(z.join(x1))

Output ;

python is great lang :/

['python', 'is', 'great', 'lang', ':/']

pythonisgreatlang:/

python**is**great**lang**:/

methods with boolean output

Ex ;

x = 'Python is a great lang'

y = '123'

# isalpha() --> check wether given string is a string of alphabets

print(x.isalpha())

print(x[:5].isalpha())

print(y.isalpha())

print(y.isnumeric())

z = y[:5]+x

print(z.isalnum())

print('xyf'.isidentifier())

print('^ahg'.isidentifier())

# Explore the remaining methods starting with 'is'

#:)

Output ;

False
True
False
True
False
True
False

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