Python Strings Basics

February 03, 2024•6 min read

Strings in Python are quite versatile and powerful, offering a wide range of operations and methods to manipulate textual data. Understanding strings is fundamental, especially in contexts like finance and trading, where you might deal with stock symbols, financial reports, or trade orders. Let's delve deeper into the nuances of strings.

Defining Strings

Strings in Python can be defined using single, double, or triple quotes. This flexibility allows for easy creation of strings that contain quotes.

single_quoted = 'Buy AAPL'
double_quoted = "Sell GOOGL"
triple_quoted = """Hold MSFT"""

Immutability of Strings

Strings in Python are immutable. This means once a string is created, the characters within it cannot be changed. Any operation that seems to modify a string will actually create a new string.

ticker = "AAPL"
# ticker[0] = "B" would raise an error because strings are immutable.
ticker = "GOOGL"  # This is allowed, creates a new string and assigns it to ticker.

Common String Operations

Python provides a plethora of methods and operations to work with strings, making it a robust tool for text manipulation.

Concatenation

Joining strings together:

company = "Apple Inc."
action = "Buy"
message = action + ": " + company  # Outputs: Buy: Apple Inc.

Length of a String

Finding the number of characters in a string using len():

company = "Apple Inc."
print(len(company))  # Outputs: 10

Case Conversion

Converting strings to upper or lower case, useful for standardizing text data:

ticker = "aapl"
print(ticker.upper())  # Outputs: AAPL

statement = "Buy Low, Sell High."
print(statement.lower())  # Outputs: buy low, sell high.

Stripping Whitespaces

Removing leading and trailing whitespaces, often needed when cleaning data:

raw_data = "  AAPL  "
clean_data = raw_data.strip()
print(clean_data)  # Outputs: AAPL

Finding and Replacing

Locating substrings or replacing parts of the string:

sentence = "Buy low, sell high."
print(sentence.find('low'))  # Outputs: 4 (index where 'low' starts)
print(sentence.replace('low', 'lower'))  # Outputs: Buy lower, sell high.

Splitting and Joining

Breaking a string into a list of substrings or joining a list of strings into a single string:

data = "AAPL,GOOGL,AMZN"
symbols = data.split(',')  # Outputs: ['AAPL', 'GOOGL', 'AMZN']

symbols_list = ['AAPL', 'GOOGL', 'AMZN']
joined_string = ','.join(symbols_list)  # Outputs: AAPL,GOOGL,AMZN

Formatting Strings

Inserting variables into strings. The format() method or f-strings (formatted string literals) make this easy:

ticker = "AAPL"
price = 150
message = "{} price is ${}".format(ticker, price)  # Outputs: AAPL price is $150

# Or using f-strings (Python 3.6+)
message = f"{ticker} price is ${price}"  # Outputs: AAPL price is $150

Operations between strings and integers in Python are quite limited due to the strongly typed nature of the language. This means that you can't directly perform arithmetic operations between strings and integers. Trying to add, subtract, multiply, or divide a string and an integer will result in a TypeError. However, there are a few interactions worth noting:

Concatenation (`+`):

While you can't add a string and an integer in the mathematical sense, you can concatenate a string with an integer if you first convert the integer to a string using the str() function. This is often used to create a combined text message.

age = 25
message = "Your age is " + str(age)
print(message)  # Outputs: Your age is 25

Repetition (`*`):

Multiplying a string by an integer will repeat the string that many times. This is a form of string replication and can be useful in creating repeated patterns or formatting.

separator = "-"
repeated_separator = separator * 10
print(repeated_separator)  # Outputs: ----------

Implicit Conversion (in Print Statements):

Although not a direct operation, when you use a print statement, you can pass integers and strings together, and Python implicitly converts them to a single string.

age = 25
print("Your age is", age)  # Outputs: Your age is 25

Type Conversion:

If you need to perform mathematical operations between a string and an integer, you must convert the string to an integer (if it represents a valid number) using int(), or the integer to a string using str() for concatenation purposes.

number_string = "100"
number_int = 50

# String to Integer Conversion
total = int(number_string) + number_int
print(total)  # Outputs: 150

# Integer to String Conversion (for concatenation)
total_str = number_string + str(number_int)
print(total_str)  # Outputs: 10050

It's important to handle these conversions and operations carefully because if the string does not represent a valid number (e.g., "hello"), converting it to an integer using int() will raise a ValueError.

In summary, direct arithmetic operations between strings and integers are not possible in Python, but with proper conversion and the use of certain operators like + for concatenation and * for repetition, you can achieve the desired interaction between strings and integers.

Indexing

Indexing is used to access individual items in a sequence. Python uses zero-based indexing, meaning the first element is accessed with index 0, the second with index 1, and so on.

Positive Indexing:

Starts from the beginning of the sequence.
The first element has an index of 0, the second element an index of 1, and so on.

Negative Indexing:

Starts from the end of the sequence.
Useful for accessing elements from the end without needing to know the length of the sequence.
The last element has an index of -1, the second to last -2, and so on.

Example:

prices = [100, 105, 110, 115, 120]

# Positive Indexing
print(prices[0])  # Outputs: 100 (first element)
print(prices[3])  # Outputs: 115 (fourth element)

# Negative Indexing
print(prices[-1])  # Outputs: 120 (last element)
print(prices[-2])  # Outputs: 115 (second to last element)

Slicing

Slicing is used to access a subset of a sequence. It's denoted by the colon : symbol inside square brackets [start:stop:step].

Parameters:

Start (optional): The starting index of the slice. If omitted, it defaults to 0 (the beginning of the sequence).
Stop (optional): The ending index of the slice, exclusive. If omitted, it defaults to the length of the sequence.
Step (optional): The amount by which the index increases. If omitted, it defaults to 1.

Characteristics:

The slice includes the start index.
It goes up to, but does not include, the stop index.
The step determines the stride of the slice.