Skip to content

Strings

Amin Zamani edited this page Mar 23, 2023 · 7 revisions

Strings in python are surrounded by either single quotation marks, or double quotation marks.

'hello' is the same as "hello"

You can display a string literal with the print() function:

print('Hello')

Multiline Strings

You can assign a multiline string to a variable by using three quotes.

You can use three double quotes:

a = """Lorem ipsum dolor sit amet,
consecrate disciplining elit,
sed do usermod tempor incident
ut labor et do lore magna aliquot."""
print(a)

a = '''Lorem ipsum dolor sit amet,
consecrate disciplining elit,
sed do usermod tempor incident
ut labor et do lore magna aliquot.'''
print(a)

String Index

string = "I <3 Cats"
STRINGS:   "I          <      3           C       a       t       s"
INDEXES:    0     1    2      3      4    5       6       7       8 
Negative : -9    -8   -7     -6     -5   -4      -3      -2      -1
msg = "I <3 Cats"
output1 = msg[0]  # output: 'I'
output2 = msg[5]  # output: 'C'

print("hello"[1])  # output: h

Strings are Arrays

Like many other popular programming languages, strings in Python are arrays of bytes representing unicode characters.

However, Python does not have a character data type, a single character is simply a string with a length of 1. Square brackets can be used to access elements of the string.

Get the character at position 1 (remember that the first character has the position 0):

a = "Hello, World!"
print(a[1])

Looping Through a String

Since strings are arrays, we can loop through the characters in a string, with a for loop.

Loop through the letters in the word "banana":

for x in "banana":
    print(x)

String Length

To get the length of a string, use the len() function.

The len() function returns the length of a string:

a = "Hello, World!"
print(len(a))

Check String

To check if a certain phrase or character is present in a string, we can use the keyword in.

Check if "free" is present in the following text:

txt = "The best things in life are free!"
print("free" in txt)  # output: True

Use it in an if statement.Print only if "free" is present:

txt = "The best things in life are free!"
if "free" in txt:
    print("Yes, 'free' is present.")

Check if NOT

To check if a certain phrase or character is NOT present in a string, we can use the keyword not in.

Check if "expensive" is NOT present in the following text:

txt = "The best things in life are free!"
print("expensive" not in txt)

Use it in an if statement, print only if "expensive" is NOT present:

txt = "The best things in life are free!"
if "expensive" not in txt:
    print("No, 'expensive' is NOT present.")

Slicing Strings

You can return a range of characters by using the slice syntax. Specify the start index and the end index, separated by a colon, to return a part of the string.

Get the characters from position 2 to position 5 (not included):

b = "Hello, World!"
print(b[2:5])

I can't use "setitem" in str, so I will get TypeError.

b[1] = 'i'  # TypeError:  'str' objects does not support item assignment
b = b[:1] + 'i' + b[2:]  # Hillo, World!

Slice From the Start

By leaving out the start index, the range will start at the first character:

Get the characters from the start to position 5 (not included):

b = "Hello, World!"
print(b[:5])

Slice To the End

By leaving out the end index, the range will go to the end.

Get the characters from position 2, and all the way to the end:

b = "Hello, World!"
print(b[2:])

Negative Indexing

Use negative indexes to start the slice from the end of the string.

Get the characters:

# From: "o" in "World!" (position -5)
# To, but not included: "d" in "World!" (position -2):

b = "Hello, World!"
print(b[-5:-2])

Upper Case

The upper() method returns the string in upper case.

a = "Hello, World!"
print(a.upper())

Lower Case

The lower() method returns the string in lower case.

a = "Hello, World!"
print(a.lower())

Remove Whitespace

Whitespace is the space before and/or after the actual text, and very often you want to remove this space.

The strip() method removes any whitespace from the beginning or the end:

a = " Hello, World! "
print(a.strip())  # returns "Hello, World!"

msg = '...end...'
msg.strip('.')  # output: end
msg.lstrip('.')  # output: end...
msg.rstrip('.')  # output: ...end

'   hello'.strip()  # output: 'hello'
'   hello   '.strip()  # output: 'hello'
'   hello   \t\n    '.strip()  # output: 'hello'
'   hello   h\t\n   '.strip()  # output: 'hello h'
'---hello-----'.strip('-')  # output: 'hello'

email = '[email protected]     '
email.strip()  # output: '[email protected]'
email.lower()  # output: '[email protected]'

email.strip().lower()  # output: '[email protected]'

Replace String

The replace() method replaces a string with another string.

a = "Hello, World!"
print(a.replace("H", "J"))

Split String

The split() method returns a list where the text between the specified separator becomes the list items.

The split() method splits the string into substrings if it finds instances of the separator:

a = "Hello, World!"
print(a.split(","))  # returns ['Hello', ' World!']

birthday = '03/27/2020'
birthday.split('/')  # return: ['03', '27', '2020']

String Concatenation

To concatenate, or combine, two strings you can use the + operator.

Merge variable a with variable b into variable c :

a = "Hello"
b = "World"
c = a + b
print(c)

To add a space between them, add a " ":

a = "Hello"
b = "World"
c = a + " " + b
print(c)

You can use the * operator to repeat strings:

print('Lucas' * 3)

a = 'hello'
b = 'lucas'
c = a + 5 * '.' + b
print(c)  # hello.....lucas

f strings

f-strings are an easy way to generate strings that contain interpolated# expressions. any code between curly braces {} will be evaluated and then the result will be turned into a string and inserted into the overall string.

f_string = f'there are {24 * 60 * 60} seconds in a day'
print(f_string)

String Format

As we learned in the Python Variables chapter, we cannot combine strings and numbers like this

age = 36
txt = "My name is John, I am " + age
print(txt)

But we can combine strings and numbers by using the format() method!

The format() method takes the passed arguments, formats them, and places them in the string where the placeholders are {} .

Use the format() method to insert numbers into strings:

age = 36
print("My name is John, and I am {}".format(age))

The format() method takes unlimited number of arguments, and are placed into the respective placeholders:

quantity = 3
itemno = 567
price = 49.95
my_order = "I want {} pieces of item {} for {} dollars."
print(my_order.format(quantity, itemno, price))

You can use index numbers {0} to be sure the arguments are placed in the correct placeholders:

quantity = 3
itemno = 567
price = 49.95
my_order = "I want to pay {2} dollars for {0} pieces of item {1}."
print(my_order.format(quantity, itemno, price))

Also, if you want to refer to the same value more than once, use the index number:

age = 36
name = "John"
txt = "His name is {1}. {1} is {0} years old."
print(txt.format(age, name))

You can also use named indexes by entering a name inside the curly brackets {carname}, but then you must use names when you pass the parameter values txt.format(carname = "Ford"):

my_order = "I have a {carname}, it is a {model}."
print(my_order.format(carname="Ford", model="Mustang"))

Escape Character

To insert characters that are illegal in a string, use an escape character.

An escape character is a backslash \ followed by the character you want to insert.

An example of an illegal character is a double quote inside a string that is surrounded by double quotes:

txt = "We are the so-called \"Vikings\" from the north."

#   [Code]	    [Result]
#   \'	            Single Quote
#   \"	            Double Quote
#   \\	            Backslash
#   \n	            New Line
#   \r	            Carriage Return
#   \t	            Tab
#   \b	            Backspace
#   \f	            Form Feed
#   \ooo	    Octal value
#   \xhh	    Hex value

print("Lucas\nSacul")
escape_name = "Lucas\nSacul"

String Methods

Python has a set of built-in methods that you can use on strings. Note: All string methods return new values. They do not change the original string.

#   [Method]                    [Description]
#   capitalize()	        Converts the first character to upper case
#   casefold()	                Converts string into lower case
#   center()	                Returns a centered string
#   count()	                Returns the number of times a specified value occurs in a string
#   encode()	                Returns an encoded version of the string
#   endswith()	                Returns true if the string ends with the specified value
#   expandtabs()	        Sets the tab size of the string
#   find()	                Searches the string for a specified value and returns the position of where it was found
#   format()	                Formats specified values in a string
#   format_map()	        Formats specified values in a string
#   index()	                Searches the string for a specified value and returns the position of where it was found
#   isalnum()	                Returns True if all characters in the string are alphanumeric
#   isalpha()	                Returns True if all characters in the string are in the alphabet
#   isdecimal()	                Returns True if all characters in the string are decimals
#   isdigit()	                Returns True if all characters in the string are digits
#   isidentifier()	        Returns True if the string is an identifier
#   islower()	                Returns True if all characters in the string are lower case
#   isnumeric()	                Returns True if all characters in the string are numeric
#   isprintable()	        Returns True if all characters in the string are printable
#   isspace()	                Returns True if all characters in the string are whitespaces
#   istitle()	                Returns True if the string follows the rules of a title
#   isupper()	                Returns True if all characters in the string are upper case
#   join()	                Joins the elements of an iterable to the end of the string
#   ljust()	                Returns a left justified version of the string
#   lower()	                Converts a string into lower case
#   lstrip()	                Returns a left trim version of the string
#   maketrans()	                Returns a translation table to be used in translations
#   partition()	                Returns a tuple where the string is parted into three parts
#   replace()	                Returns a string where a specified value is replaced with a specified value
#   rfind()	                Searches the string for a specified value and returns the last position of where it was found
#   rindex()	                Searches the string for a specified value and returns the last position of where it was found
#   rjust()	                Returns a right justified version of the string
#   rpartition()	        Returns a tuple where the string is parted into three parts
#   rsplit()	                Splits the string at the specified separator, and returns a list
#   rstrip()	                Returns a right trim version of the string
#   split()	                Splits the string at the specified separator, and returns a list
#   splitlines()	        Splits the string at line breaks and returns a list
#   startswith()	        Returns true if the string starts with the specified value
#   strip()	                Returns a trimmed version of the string
#   swapcase()	                Swaps cases, lower case becomes upper case and vice versa
#   title()	                Converts the first character of each word to upper case
#   translate()         	Returns a translated string
#   upper()	                Converts a string into upper case
#   zfill()	                Fills the string with a specified number of 0 values at the beginning

Comparing Strings

Python use Unicode for encoding characters used in the string data type. The function ord() will return the number for any character.

print('a' == 'a')  # output: True
print('a' != 'b')  # output: True
print('a' < 'w')  # output: True
print('A' < '$')  # output: False
print('aaa' < 'AAA')  # output: False
print('A' < 'a')  # output: True
print('100' < '9')  # output: True

print(ord('a'))  # output: 97
print(ord('A'))  # output: 65
print(ord('B'))  # output: 66
print(ord('C'))  # output: 67
print(ord('9'))  # output: 57
print(ord(' '))  # output: 32

Join

join() is a string method that joins together the elements of an iterable into a single string. whatever string you call it on will be used as a separator.

fruits = ['apple', 'kiwi', 'pear']
" ".join(fruits)  # output: 'apple kiwi pear'
"!!!".join(fruits)  # output: 'apple!!!kiwi!!!pear'
'!'.join('hello')  # output: 'h!e!l!l!o'

Reverse Indexing

my_string = 'hello my name is lucas'
print(my_string[::-1])  # 'sacul si eman ym olleh'

Python

Python Essentials 1 (PCEP)

Introduction to Python and computer programming

Data types, variables, basic I/O operations, and basic operators

Boolean values, conditional execution, loops, lists and list processing, logical and bitwise operations

Clean Code

Algorithms

Django

Django Rest Framework

API

pip

SQLAlchemy

FastAPI

Pytest

TDD

Git

Linux

Docker

Python Testing

Interview Questions

Clone this wiki locally