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Python Strings

Updated on Jan 10, 2020

Strings in python are contiguous series of characters delimited by single or double quotes. Python doesn't have any separate data type for characters so they are represented as a single character string.

Creating strings #

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>>> name = "tom" # a string
>>> mychar = 'a' # a character

You can also use the following syntax to create strings.

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>>> name1 = str() # this will create empty string object
>>> name2 = str("newstring") # string object containing 'newstring'

name = "tom" # a string
mychar = 'a' # a character

print(name)
print(mychar)

name1 = str() # this will create empty string object
name2 = str("newstring") # string object containing 'newstring'

print(name1)
print(name2)

Strings in Python are Immutable. #

What this means to you is that once a string is created it can't be modified. Let's take an example to illustrate this point.

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>>> str1 = "welcome"
>>> str2 = "welcome"

Here str1 and str2 refer to the same string object "welcome"  which is stored somewhere in memory. You can test whether str1 refers to same object as str2 using the id() function.

What is id?

Every object in python is stored somewhere in memory. We can use id() to get that memory address.

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>>> id(str1)
78965411
>>> id(str2)
78965411

As both str1  and str2  points to the same memory location, hence they both point to the same object.

Let's try to modify str1 object by adding a new string to it.

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>>> str1 += " mike"
>>> str1
welcome mike
>>> id(str1)
>>> 78965579

As you can see now str1  points to totally different memory location, this proves the point that concatenation doesn't modify the original string object instead it creates a new string object. Similarly, Number (i.e int type) is also immutable.

Try it out:

str1 = "welcome"
str2 = "welcome"

print(id(str1),  id(str2))

str1 += " mike"

print(str1)

print(id(str1))

Operations on string #

String index starts from 0, so to access the first character in the string type:

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>>> name[0] #
t

Try it out:

name = "tom"

print(name[0])
print(name[1])

The + operator is used to concatenate string and * operator is a repetition operator for string.

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>>> s = "tom and " + "jerry"
>>> print(s)
tom and jerry

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>>> s = "spamming is bad " * 3
>>> print(s)
'spamming is bad spamming is bad spamming is bad '

Try it out:

s = "tom and " + "jerry"
print(s)

s = "spamming is bad " * 3
print(s)

Slicing string #

You can take subset of string from original string by using [] operator also known as slicing operator.

Syntax: s[start:end]

This will return part of the string starting from index start to index end - 1.

Let's take some examples.

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>>> s = "Welcome"
>>> s[1:3]
el

Some more examples.

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>>> s = "Welcome"
>>>
>>> s[:6]
'Welcom'
>>>
>>> s[4:]
'ome'
>>>
>>> s[1:-1]
'elcom'

Try it out:

s = "Welcome"

print(s[1:3])
print(s[:6])
print(s[4:])
print(s[1:-1])

ord() and chr() Functions #

ord() - function returns the ASCII code of the character.

chr() - function returns character represented by a ASCII number.

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>>> ch = 'b'
>>> ord(ch)
98
>>> chr(97)
'a'
>>> ord('A')
65

Try it out:

ch = 'b'

print(ord(ch))

print(chr(97))

print(ord('A'))

String Functions in Python #

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>>> len("hello")
5
>>> max("abc")
'c'
>>> min("abc")
'a'

Try it out:

print(len("hello"))

print(max("abc"))

print(min("abc"))

in and not in operators #

You can use in and not in operators to check the existence of a string in another string. They are also known as membership operator.

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>>> s1 = "Welcome"
>>> "come" in s1
True
>>> "come" not in s1
False
>>>

Try it out:

s1 = "Welcome"

print("come" in s1)

print("come" not in s1)

String comparison #

You can use ( > , < , <= , <= , == , !=  ) to compare two strings. Python compares string lexicographically i.e using ASCII value of the characters.

Suppose you have str1 as "Mary"  and str2 as "Mac". The first two characters from str1  and str2 ( M and M ) are compared. As they are equal, the second two characters are compared. Because they are also equal, the third two characters (r and c ) are compared. And because r has greater ASCII value than c, str1 is greater than str2.

Here are some more examples:

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>>> "tim" == "tie"
False
>>> "free" != "freedom"
True
>>> "arrow" > "aron"
True
>>> "right" >= "left"
True
>>> "teeth" < "tee"
False
>>> "yellow" <= "fellow"
False
>>> "abc" > ""
True
>>>

Try it out:

print("tim" == "tie")

print("free" != "freedom")

print("arrow" > "aron")

print("right" >= "left")

print("teeth" < "tee")

print("yellow" <= "fellow")

print("abc" > "")

Iterating string using for loop #

String is a sequence type and also iterable using for loop (to learn more about for loop click here ).

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>>> s = "hello"
>>> for i in s:
...     print(i, end="")
hello

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print("my string", end="\n")  # this is default behavior
print("my string", end="")    # print string without a newline 
print("my string", end="foo") # now print() will print foo after every string

Try it out:

s = "hello"
for i in s:
    print(i, end="")

Testing strings #

String class in python has various inbuilt methods which allows to check for different types of strings.

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>>> s = "welcome to python"
>>> s.isalnum()
False
>>> "Welcome".isalpha()
True
>>> "2012".isdigit()
True
>>> "first Number".isidentifier()
False
>>> s.islower()
True
>>> "WELCOME".isupper()
True
>>> "  \t".isspace()
True

Try it out:

s = "welcome to python"

print(s.isalnum())
print("Welcome".isalpha())
print("2012".isdigit())
print("first Number".isidentifier())
print(s.islower())
print("WELCOME".isupper())
print("  \t".isspace())

Searching for Substrings #

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>>> s = "welcome to python"
>>> s.endswith("thon")
True
>>> s.startswith("good")
False
>>> s.find("come")
3
>>> s.find("become")
-1
>>> s.rfind("o")
15
>>> s.count("o")
3
>>>

Try it out:

s = "welcome to python"

print(s.endswith("thon"))

print(s.startswith("good"))

print(s.find("come"))

print(s.find("become"))

print(s.rfind("o"))

print(s.count("o"))

Converting Strings #

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s = "string in python"
>>>
>>> s1 = s.capitalize()
>>> s1
'String in python'
>>>
>>> s2 = s.title()
>>> s2
'String In Python'
>>>
>>> s = "This Is Test"
>>> s3 = s.lower()
>>> s3
'this is test'
>>>
>>> s4 = s.upper()
>>> s4
'THIS IS TEST'
>>>
>>> s5 = s.swapcase()
>>> s5
'tHIS iS tEST'
>>>
>>> s6 = s.replace("Is", "Was")
>>> s6
'This Was Test'
>>>
>>> s
'This Is Test'
>>>

Try it out:

s = "string in python"

s1 = s.capitalize()
print(s1)

s2 = s.title()
print(s2)

s = "This Is Test"
s3 = s.lower()

print(s3)

s4 = s.upper()
print(s4)

s5 = s.swapcase()
print(s5)

s6 = s.replace("Is", "Was")
print(s6)

print(s)

In next chapter we will learn about python lists

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