Strings in Python
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")
print('Hello')
Quotes Inside Quotes​
You can use quotes inside a string, as long as they don't match the quotes surrounding the string:
print("It's alright")
print("He is called 'Johnny'")
print('He is called "Johnny"')
Assign String to a Variable​
Assigning a string to a variable is done with the variable name followed by an equal sign and the string:
a = "Hello"
print(a)
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,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua."""
print(a)
Or three single quotes:
a = '''Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua.'''
print(a)
Note: In the result, the line breaks are inserted at the same position as in the code.
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:
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:
for x in "banana":
print(x)
String Length​
To get the length of a string, use the len() function:
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.
Example​
Check if "free" is present in the following text:
txt = "The best things in life are free!"
print("free" in txt)
Use it in an if statement:
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.
Example​
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:
txt = "The best things in life are free!"
if "expensive" not in txt:
print("No, 'expensive' is NOT present.")
Slicing​
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.
Example​
Get the characters from position 2 to position 5 (not included):
b = "Hello, World!"
print(b[2:5])
Note: The first character has index 0.
Slice From the Start​
By leaving out the start index, the range will start at the first character:
b = "Hello, World!"
print(b[:5])
Slice To the End​
By leaving out the end index, the range will go to the end:
b = "Hello, World!"
print(b[2:])
Negative Indexing​
Use negative indexes to start the slice from the end of the string:
Example​
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])
Modify Strings​
Python has a set of built-in methods that you can use on strings.
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!"
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!']
String Concatenation​
To concatenate, or combine, two strings, you can use the + operator.
Example​
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)
String Format​
As we learned in the Python Variables chapter, we cannot combine strings and numbers like this:
Example​
age = 36
txt = "My name is John, I am " + age
print(txt)
But we can combine strings and numbers by using f-strings or the format() method!
F-Strings​
F-String was introduced in Python 3.6 and is now the preferred way of formatting strings.
To specify a string as an f-string, simply put an f in front of the string literal, and add curly brackets {} as placeholders for variables and other operations.
Example​
Create an f-string:
age = 36
txt = f"My name is John, I am {age}"
print(txt)
Placeholders and Modifiers​
A placeholder can contain variables, operations, functions, and modifiers to format the value.
Example​
Add a placeholder for the price variable:
price = 59
txt = f"The price is {price} dollars"
print(txt)
A placeholder can include a modifier to format the value.
A modifier is included by adding a colon : followed by a legal formatting type, like .2f which means fixed-point number with 2 decimals:
Example​
Display the price with 2 decimals:
price = 59
txt = f"The price is {price:.2f} dollars"
print(txt)
A placeholder can contain Python code, like math operations:
Example​
Perform a math operation in the placeholder, and return the result:
txt = f"The price is {20 * 59} dollars"
print(txt)
Escape Characters​
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:
Example​
You will get an error if you use double quotes inside a string that is surrounded by double quotes:
txt = "We are the so-called "Vikings" from the north."
To fix this problem, use the escape character \":
Example​
The escape character allows you to use double quotes when you normally would not be allowed:
txt = "We are the so-called \"Vikings\" from the north."
Other Escape Characters​
Other escape characters used in Python:
| Code | Result |
|---|---|
\' | Single Quote |
\\ | Backslash |
\n | New Line |
\r | Carriage Return |
\t | Tab |
\b | Backspace |
\f | Form Feed |
\ooo | Octal value |
\xhh | Hex value |
String Methods​
Python has a set of built-in methods that you can use on strings. 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 centeredstring |
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 |
isascii() | Returns True if all characters in the string are ASCII characters |
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 the string contains only whitespace characters |
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 another 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 |