Strings

The string datatype represents textual messages, comprised of any number of alphanumeric characters.

Characteristics

There are three ways to construct a string: by using single quotes, double quotes, or a multi-line string.

Single quotes (') on the extremities:

message = 'HELLO WORLD'
print(message)

HELLO WORLD

Double quotes (") on the extremities:

message = "HELLO WORLD"
print(message)

HELLO WORLD

Multiline string, uses triple quotes (""") on the extremities:

message = """
This is a menu for our trading platform.

To get started, follow these instructions:

  1. __________
  2. ____________
  3. _____________

"""
print(message)


This is a menu for our trading platform.

To get started, follow these instructions:

  1. __________
  2. ____________
  3. _____________

In practice, for single line strings, we can prefer to default to double quotes, because it allows us to use single quotes for contractions, whereas the reverse would break the quoting level:

message = "It's awesome!"
print(message)

It's awesome!

#message = 'It's awesome!' # NOT VALID (BREAKS QUOTING LEVEL)
#print(message)

Operations

In practice, some common operations we perform with strings include concatenation, case manipulation, and substring checking, among others.

Concatenation

message = "HELLO" + "WORLD"
print(message)

message = "HELLO " + "WORLD"
print(message)

message = "HELLO" + " WORLD"
print(message)

message = "HELLO" + " " + "WORLD"
print(message)

HELLOWORLD
HELLO WORLD
HELLO WORLD
HELLO WORLD

Format Strings

As an alternative to concatenation, we can use a format string to dynamically compile a string.

Recall that we are not able to concatenate strings with non-string datatypes such as numbers. To overcome this limitation, we could use string conversion function, or more commonly, a format string.

The format string allows us to inject one or more variables into a string.

To implement a format string, we need the letter f immediately preceding the string. And we need curly braces ({}) inside the string. This allows us to inject a variable inside the curly braces:

price = 4.5
print(f"PRICE: ${price}")

PRICE: $4.5

We can use a formatting directive such as :.2f to format a number to two decimal places, for example to format a number as US Dollars (USD):

price = 4.5
print(f"PRICE: ${price:.2f}")

PRICE: $4.50

A formatting directive such as :, to use a thousands separator:

shares = 120000
print(f"SHARES: {shares:,}")

SHARES: 120,000

Notice, it is possible to inject multiple variables in the format string, and to mix and match formatting directives:

price = 4.5
shares = 120000
stake = price * shares
print(f"PRICE: ${price:.2f} | SHARES: {shares:,} | STAKE: ${stake:,.2f}")

PRICE: $4.50 | SHARES: 120,000 | STAKE: $540,000.00

Case Manipulation

Converting to all uppercase:

"hello WorlD".upper()

'HELLO WORLD'

Converting to all lowercase:

"hello WorlD".lower()

'hello world'

Converting to title case, where the first letter of each word is capitalized:

"hello WorlD".title()

'Hello World'

Substring Checking

We can use the inclusion operator to perform substring checking:

"h" in "hello world"

True

Note this is case sensitive:

"H" in "hello world"

False

Substring Replacement

It is possible to replace all instances of a substring within a larger string:

"hello world. as the world turns.".replace("world", "globe")

'hello globe. as the globe turns.'

Length Checking

A string is like a list of individual characters.

Like a list, we can use the len function to count the number of characters in a string:

len("HELLO WORLD")

String Indexing and Slicing

Similar to list operations, we can use indices to reference a specific character in the string, or a sequence of characters (i.e. string slicing):

"HELLO WORLD"[0]

'H'

"HELLO WORLD"[0:5]

'HELLO'

String Splitting

The split method will split a string on a designated delimiter:

"hello world".split(" ")

['hello', 'world']

"first | second | third".split(" | ")

['first', 'second', 'third']