Strings

source: Python Tutorial/Strings.md

A string is a sequence of characters. You must have used strings in other languages as well. Python strings play the same role as character arrays in languages like C, but they are somewhat higher-level tools than arrays.

Unlike languages such as C, in Python, strings come with a powerful set of processing tools.

Create a String

A python string is zero or more characters written inside single quotes ‘ ‘ or double quotes “ “

S = 'Hello, World!'   # single quotes
S = "Hello, World!"   # double quotes

Multiline Strings

You can create a multiline string using triple-quotes: “”” “”” or ‘'' ‘''.

S = """String literals can
span multiple lines."""
print(S)
# String literals can
# span multiple lines.
The str() Constructor
You can convert almost any object in Python to a string using a type constructor called str()

# an integer to a string
S = str(42)
print(S)
# Prints '42'

# a complex number to a string
S = str(3+4j)
print(S)
# Prints '(3+4j)

# a list to a string
S = str([1,1])
print(S)
# Prints '[1, 1]'

Access Characters by Index

You can access individual characters in a string using an index in square brackets. The string indexing starts from 0.

You can also access a string by negative indexing. A negative string index counts from the end of the string.

The indices for the elements in a string are illustrated as below:

String Indexing
# Indexing
S = 'ABCDEFGHI'
print(S[0])    # Prints A
print(S[4])    # Prints E

# Negative Indexing
S = 'ABCDEFGHI'
print(S[-1])    # Prints I
print(S[-6])    # Prints D

Slicing a String

A segment of a string is called a slice and you can extract one by using a slice operator. A slice of a string is also a string.

The slice operator [n:m] returns the part of the string from the “n-th” item to the “m-th” item, including the first but excluding the last.

S = 'ABCDEFGHI'
print(S[2:5])      # Prints CDE
print(S[5:-1])     # Prints FGH
print(S[1:6:2])    # Prints BDF

The string slicing capability provided by python is extensive and covered in full detail here.

Modify a String

It is tempting to use the [] operator on the left side of an assignment, in order to convert a character into a string. for example:

S = 'Hello, World!'
S[0] = 'J'
# Triggers TypeError: 'str' object does not support item assignment
The reason for the error is that the strings are unchangeable (immutable) and because of which you cannot change the existing string. The best you can do is create a new string that is a variation of the original:

S = 'Hello, world!'
new_S = 'J' + S[1:]
print(new_S)
# Prints Jello, world!

String Concatenation

You can concatenate strings using the concatenation operator + or the augmented assignment operator +=

# concatenation operator
S = 'Hello,' + ' World!'
print(S)
# Hello, World!

# augmented assignment operator
S = 'Hello,'
S += ' World!'
print(S)
# Prints Hello, World!

In Python, two or more strings next to each other are automatically concatenated, known as Implicit concatenation.

S = 'Hello,' " World!"
print(S)
# Prints Hello, World!
Implicit concatenation only works with two literals though, not with variables or expressions.

You can also put several strings within parentheses to join them together. This feature is useful when you want to break long strings.

S = ('Put strings within parentheses '
    'to join them together.')
print(S)
# Put strings within parentheses to join them together.

You can replicate substrings in a string using the replication operator *

# the hard way
S = '--------------------'

# the easy way
S = '-' * 20

Find String Length

To find the number of characters in a string, use len() built-in function.

S = 'Supercalifragilisticexpialidocious'
print(len(S))
# Prints 34

Replace Text Within a String

Sometimes you want to replace a text inside a string, then you can use the replace() method.

S = 'Hello, World!'
x = S.replace('World', 'Universe')
print(x)
# Prints Hello, Universe!

Split and Join a String

Use split() method to chop up a string into a list of substrings, around a specified delimiter.

# Split the string on comma
S = 'red,green,blue,yellow'
x = S.split(',')
print(x)
# Prints ['red', 'green', 'blue', 'yellow']
print(x[0])
# Prints red

And use join() method to join the list back into a string, with a specified delimiter in between.

# Join the list of substrings
L = ['red', 'green', 'blue', 'yellow']
S = ','.join(L)
print(S)
# Prints red,green,blue,yellow

String Case Conversion

Python provides five methods to perform case conversion on the target string viz. lower(), upper(), capitalize(), swapcase() and title()

S = 'Hello, World!'
print(S.lower())
# Prints hello, world!

S = 'Hello, World!'
print(S.upper())
# Prints HELLO, WORLD!

S = 'Hello, World!'
print(S.capitalize())
# Prints Hello, world!

S = 'Hello, World!'
print(S.swapcase())
# Prints hELLO, wORLD!

S = 'hello, world!'
print(S.title())
# Prints Hello, World!

Check if Substring Contains in a String

To check if a specific text is present in a string, use in operator. The in is a boolean operator, which takes two strings and returns True if the first appears as a substring in the second:

S = 'Hello, World!'
print('Hello' in S)
# Prints True

To search for a specific text within a string, use find() method. It returns the lowest index in the string where substring is found.

# Search for 'Foolish' within a string
S = 'Stay Hungry, Stay Foolish'
x = S.find('Foolish')
print(x)
# Prints 18

Iterate Through a String

To iterate over the characters of a string, use a simple for loop.

# Print each character in a string
S = 'Hello, World!'
for letter in S:
    print(letter, end=' ')
# H e l l o ,   W o r l d !

Python Escape Sequence

You can use quotes inside a string, as long as they don't match the quotes surrounding the string.

S = "We're open"		# Escape single quote
S = "I said 'Wow!'"		# Escape single quotes
S = 'I said "Wow!"'		# Escape double quotes

This is fine for most of the time but what if you want to declare a string with both single and double quotes like:

Bob told me, “Sam said, ‘This won't work.'”

Python will raise a SyntaxError, because both quotation marks are special characters. The solution to avoid this problem is to use the backslash escape character .

Prefixing a special character with \ turns it into an ordinary character. This is called escaping.

S = "Bob told me, \"Sam said, 'This won't work.'\""
print(S)
# Prints Bob told me, "Sam said, 'This won't work.'"
Backslash escape character is used in representing certain special characters like: \n is a newline, \t is a tab. These are known as escape sequences.

S = str('First line.\n\tSecond line.')
print(S)
# First line.
#     Second line.

String Methods

Conversion Functions

capitalize() – Returns the string with the first character capitalized and rest of the characters in lower case.

var = 'PYTHON'
print (var.capitalize())
# Python

lower() – Converts all the characters of the String to lowercase

var = 'FootBall'
print (var.lower())
# football

upper() – Converts all the characters of the String to uppercase

var = 'FootBall'
print (var.upper())
# FOOTBALL

swapcase() – Swaps the case of every character in the String means that lowercase characters got converted to uppercase and vice-versa.

var = 'FootBall'
print (var.swapcase())
# fOOTbALL

title() – Returns the ‘titlecased' version of String, which means that all words start with uppercase and the rest of the characters in words are in lowercase.

var = 'welcome to Python programming'
print (var.title())
# Welcome To Python Programming

count( str[, beg [, end]]) – Returns the number of times substring ‘str' occurs in the range [beg, end] if beg and end index are given else the search continues in full String Search is case-sensitive.

var='Television'
str='e'
print (var.count(str))
# 2
var1='Eagle Eyes'
print (var1.count('e'))
# 2
var2='Eagle Eyes'
print (var2.count('E',0,5))
# 1

Comparison Functions – Part1

islower() – Returns ‘True' if all the characters in the String are in lowercase. If any of the char is in uppercase, it will return False.

var='Python'
print (var.islower())
# False

var='python'
print (var.islower())
# True

isupper() – Returns ‘True' if all the characters in the String are in uppercase. If any of the char is in lowercase, it will return False.

var='Python'
print (var.isupper())
# False

var='PYTHON'
print (var.isupper())
# True

isdecimal() – Returns ‘True' if all the characters in String are decimal. If any character in the String is of other data-type, it will return False.Decimal characters are those from the Unicode category Nd.

num=u'2016'
print (num.isdecimal())
# True

isdigit() – Returns ‘True' for any char for which isdecimal() would return ‘True and some characters in the ‘No' category. If there are any characters other than these, it will return False'.

Precisely, digits are the characters for which Unicode property includes: Numeric_Type=Digit or Numeric_Type=Decimal.

For example, superscripts are digits, but fractions not.

print ('2'.isdigit())
# True

print ('²'.isdigit())
# True

Comparison Functions – II

isnumeric() – Returns ‘True' if all the characters of the Unicode String lie in any one of the categories Nd, No, and NI.

If there are any characters other than these, it will return False.

Precisely, Numeric characters are those for which Unicode property includes: Numeric_Type=Digit, Numeric_Type=Decimal or Numeric_Type=Numeric.

num=u'2016'
print (num.isnumeric())
# True

num=u'year2016'
print (num.isnumeric())
# False

isalpha() – Returns ‘True' if String contains at least one character (non-empty String), and all the characters are alphabetic, ‘False' otherwise.

print ('python'.isalpha())
# True

print ('python3'.isalpha())
# False

isalnum() – Returns ‘True' if String contains at least one character (non-empty String), and all the characters are either alphabetic or decimal digits, ‘False' otherwise.

print ('python'.isalnum())
# True
print ('python3'.isalnum())
# True

Padding Functions

rjust(width[,fillchar]) – Returns string filled with input char while pushing the original content on the right side.

By default, the padding uses a space. Otherwise, ‘fillchar' specifies the filler character.

var='Python'
print (var.rjust(10))
# Python

print (var.rjust(10,'-'))
# ----Python

ljust(width[,fillchar]) – Returns a padded version of String with the original String left-justified to a total of width columns

By default, the padding uses a space. Otherwise, ‘fillchar' specifies the filler character.

var='Python'
print (var.ljust(10))
# Python

print (var.ljust(10,'-'))
# Python----

center(width[,fillchar]) – Returns string filled with the input char while pushing the original content into the center.

By default, the padding uses a space. Otherwise, ‘fillchar' specifies the filler character.

var='Python'
print (var.center(20))
# Python

print (var.center(20,'*'))
# *******Python*******

zfill(width) – Returns string filled with the original content padded on the left with zeros so that the total length of String becomes equal to the input size.

If there is a leading sign (+/-) present in the String, then with this function, padding starts after the symbol, not before it.

var='Python'
print (var.zfill(10))
# 0000Python

var='+Python'
print (var.zfill(10))
# +000Python

Search Functions

find(str [,i [,j]]) – Searches for ‘str' in complete String (if i and j not defined) or in a sub-string of String (if i and j are defined).This function returns the index if ‘str' is found else returns ‘-1'.

Here, i=search starts from this index, j=search ends at this index. See more details – Python String Find()

var="High sky"
str="sky"
print (var.find(str))
# 5

var="High sky"
str="sky"
print (var.find(str,4))
# 5

var="High sky"
str="sky"
print (var.find(str,7))
# -1

index(str[,i [,j]]) – This is same as ‘find' method. The only difference is that it raises the ‘ValueError' exception if ‘str' doesn't exist.

var='High Sky'
str='sky'
print (var.index(str))
# 5

var='High Sky'
str='sky'
print (var.index(str,4))
# 5

var='High Sky'
str='sky'
print (var.index(str,7))
# ValueError: substring not found

rfind(str[,i [,j]]) – This is same as find() just that this function returns the last index where ‘str' is found. If ‘str' is not found, it returns ‘-1'.

var='This is a good example'
str='is'
print (var.rfind(str,0,10))
# 5

print (var.rfind(str,10))
# -1

count(str[,i [,j]]) – Returns the number of occurrences of substring ‘str' in the String. Searches for ‘str' in the complete String (if i and j not defined) or in a sub-string of String (if i and j are defined).

Where: i=search starts from this index, j=search ends at this index.

var='This is a good example'
str='is'
print (var.count(str))
# 2

print (var.count(str,4,10))
# 1

String Substitution Functions

replace(old,new[,count]) – Replaces all the occurrences of substring ‘old' with ‘new' in the String.

If the count is available, then only ‘count' number of occurrences of ‘old' will be replaced with the ‘new' var.

Where old =substring to replace, new =substring

var='This is a good example'
str='was'
print (var.replace('is',str))
# Thwas was a good exampleprint (var.replace('is',str,1))
# Thwas is a good example

split([sep[,maxsplit]]) – Returns a list of substring obtained after splitting the String with ‘sep' as a delimiter.

Where, sep= delimiter, the default is space, maxsplit= number of splits to be done

var = "This is a good example"
print (var.split())
# ['This', 'is', 'a', 'good', 'example']print (var.split(' ', 3))
# ['This', 'is', 'a', 'good example']

splitlines(num) – Splits the String at line breaks and returns the list after removing the line breaks.

Where num = if this is a positive value. It indicates that line breaks will appear in the returned list.

var='Print new line\nNextline\n\nMove again to new line'
print (var.splitlines())
# ['Print new line', 'Nextline', '', 'Move again to new line']print (var.splitlines(1))
# ['Print new line\n', 'Nextline\n', '\n', 'Move again to new line']

join(seq) – Returns a String obtained after concatenating the sequence ‘seq' with a delimiter string.

Where: the seq= sequence of elements to join

seq=('ab','bc','cd')
str='='
print (str.join(seq))
# ab=bc=cd

Misc String Functions

lstrip([chars]) – Returns a string after removing the characters from the beginning of the String.

Where: Chars=this is the character to be trimmed from the String.

The default is whitespace character.

var=' This is a good example '
print (var.lstrip())
# This is a good example
var='*****This is a good example*****'
print (var.lstrip('*'))
# This is a good example**********

rstrip() – Returns a string after removing the characters from the End of the String.

Where: Chars=this is the character to be trimmed from the String. The default is whitespace character.

var=' This is a good example '
print (var.rstrip())
# This is a good example
var='*****This is a good example*****'
print (var.lstrip('*'))
# *****This is a good example

rindex(str[,i [,j]]) – Searches for ‘str' in the complete String (if i and j not defined) or in a sub-string of String (if i and j are defined). This function returns the last index where ‘str' is available.

If ‘str' is not there, then it raises a ValueError exception.

Where: i=search starts from this index, j=search ends at this index.

var='This is a good example'
str='is'
print (var.rindex(str,0,10))
# 5
print (var.rindex(str,10))
# ValueError: substring not found

len(string) – Returns the length of given String

var='This is a good example'
print (len(var))
# 22