Python for Loops: Basics of Looping Through Lists and Iterables

In programming, we frequently encounter situations where we want to apply the same process sequentially to a collection of data (such as a list or tuple). Python allows you to write these repetitive tasks concisely using the for statement.

Python’s for loop is different from the “counter-based” style found in other languages (like for i = 0; i < 10; i++). Instead, it has an intuitive and powerful mechanism that “extracts elements one by one from a collection (iterable) and processes them.”

This article explains the basic syntax of the for statement and how to loop through iterables like lists.

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Basic Syntax of the for Statement

Python’s for statement is written with the following syntax:

for variable in iterable:
    # Write the process to repeat here
    # The element extracted from the iterable is assigned to the variable

An Iterable is a general term for objects from which elements can be extracted in order, such as lists, tuples, strings, dictionaries, and sets.

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1. Looping Through a List

The most common usage is to perform operations on each element contained in a list. In the following example, we extract server names from a list one by one and perform a connection process (simulated by printing a message).

# List of server names
server_list = ["Web-Server-01", "DB-Server-01", "Cache-Server-01"]

print("--- Maintenance Start ---")

# Assign list elements to the 'server' variable one by one and loop
for server in server_list:
    print(f"Connecting: {server}")
    print(f"Backup created for {server}.")

print("--- Maintenance End ---")

Output:

--- Maintenance Start ---
Connecting: Web-Server-01
Backup created for Web-Server-01.
Connecting: DB-Server-01
Backup created for DB-Server-01.
Connecting: Cache-Server-01
Backup created for Cache-Server-01.
--- Maintenance End ---

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2. Aggregating Numeric Lists

Here is an example of looping through a list of numbers to calculate the total value.

# Daily sales data
daily_sales = [1500, 2300, 1800, 3200, 2100]

total_sales = 0

for sale in daily_sales:
    # Add sales
    total_sales += sale

print(f"Number of data points: {len(daily_sales)}")
print(f"Total Sales: {total_sales}")

Output:

Number of data points: 5
Total Sales: 10900

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3. Looping Through Strings

In Python, a string (str) is also a type of iterable. If you pass a string to a for statement, it extracts one character at a time.

app_code = "A105"

print("Code Analysis:")
for char in app_code:
    # Check if the character is a digit and display
    if char.isdigit():
        print(f"Digit: {char}")
    else:
        print(f"Char: {char}")

Output:

Code Analysis:
Char: A
Digit: 1
Digit: 0
Digit: 5

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Summary

• Python’s for statement is written in the format: for variable in iterable:.
• It extracts elements sequentially from lists, tuples, strings, etc., and executes the process in the block.
• Because you can operate directly on the elements without worrying about index numbers (0, 1, 2…), you can write highly readable code.