Iterate Like a Pro: A Guide to Python Iterables

In Python programming, understanding and using iterables effectively is fundamental to proficient coding. Iterables are objects you can iterate or loop through. They support the sequential traversal of elements within them, making them a critical tool for accessing and manipulating elements in objects or data structures.

This article explores how to properly use Python iterables by focusing on the language’s built-in iterable data types: lists, tuples, dictionaries, strings, and sets. It also explains how to implement custom iterable types and perform advanced operations.

How To Loop Through Python Iterables

In Python, you can iterate through diverse iterable types using a for loop. This enables you to navigate sequences and perform operations on individual items within lists, sets, and dictionaries.

The for keyword in Python deviates from its utility in other object-oriented languages like Java. Python for loops work more like iterator methods. Here are examples to demonstrate loop in iterables:

1. Looping Through a List

Lists are ordered collections of items, allowing for easy iteration using a for loop.

fruits_list = ["Apple", "Mango", "Peach", "Orange", "Banana"]

for fruit in fruits_list:
    print(fruit)

In the code above, fruit acts as an iterator that the loop uses to traverse each list element while simultaneously printing them. The loop terminates after evaluating the last element in the list. The code above should give the following output:

Apple
Mango
Peach
Orange
Banana

2. Iterating Through a Tuple

Tuples are similar to lists but are immutable. You can iterate through them just like lists.

fruits_tuple = ("Apple", "Mango", "Peach", "Orange", "Banana")

for fruit in fruits_tuple:
	print(fruit)

In this example, the for loop iterates through the tuple, and in each iteration, the variable fruit takes on the value of the current element in the tuple. The code should give the following output:

Apple
Mango
Peach
Orange
Banana

3. Looping Through Sets

Sets are unordered collections of unique elements. You can traverse through them using a for loop.

fruits_set = {"Apple", "Mango", "Peach", "Orange", "Banana"}

for fruit in fruits_set:
	print(fruit)

In this example, the for loop iterates through the set. However, since sets are unordered, the order of iteration may not be the same as the order in which elements were defined in the set. In each iteration, the variable fruit takes on the value of the current element in the set. The code should give an output similar to the following (the order may vary):

Mango
Banana
Peach
Apple
Orange

4. Iterating Through Strings

Strings are sequences of characters that you can loop through character by character.

string = "Kinsta"

for char in string:
	print(char)

The code above iterates through the string “Kinsta” and prints each character on a new line. In each iteration, the variable char takes on the value of the current character in the string. The code should give the following output:

K
i
n
s
t
a

5. Traversing a Dictionary

Using the for loop is similar for lists, sets, tuples, and strings — but it’s different for dictionaries since they use key-value pairs to store items. Dictionaries present a unique case for looping, as you can iterate them using different approaches. Here are the different approaches you can use to traverse a Python dictionary:

• Iterating through keys:

  countries_capital = {
      "USA": "Washington D.C.",
      "Australia": "Canberra",
      "France": "Paris",
      "Egypt": "Cairo",
      "Japan": "Tokyo"
  }
  
  for country in countries_capital.keys():
      print(country)

  The code above defines a dictionary called countries_capital, where country names are the keys, and their respective capitals are the values. The for loop iterates through the keys of the countries_capital dictionary using the keys() method. This method returns a view object that displays a list of the keys in the dictionary, which makes it easy to loop through all the keys. In each iteration, the variable country takes on the value of the current key. This code should give the following output:

  USA
  Australia
  France
  Egypt
  Japan

• Iterating through values:

  countries_capital = {
      "USA": "Washington D.C.",
      "Australia": "Canberra",
      "France": "Paris",
      "Egypt": "Cairo",
      "Japan": "Tokyo"
  }
  
  for capital in countries_capital.values():
      print(capital)

  In the code above, the for iterates through the values of the countries_capital dictionary using the values() method. This method returns a view object that displays a list of the values in the dictionary, making it easy to loop through all the values. In each iteration, the variable capital takes on the value of the current value in the list. This code should give the following output:

  Washington D.C.
  Canberra
  Paris
  Cairo
  Tokyo

• Iterating through key-value pairs:

  countries_capital = {
      "USA": "Washington D.C.",
      "Australia": "Canberra",
      "France": "Paris",
      "Egypt": "Cairo",
      "Japan": "Tokyo"
  }
  
  for country, capital in countries_capital.items():
      print(country, ":", capital)

  The code above demonstrates how to iterate through both the keys and values of the countries_capital dictionary using the items() method. The items() method returns a view object that displays a list of key-value tuples in the dictionary. In the for loop, each iteration unpacks a key-value pair from the current element in the list. The variables country and capital are assigned the corresponding key and value, respectively. This code should give the following output:

  USA : Washington D.C.
  Australia : Canberra
  France : Paris
  Egypt : Cairo
  Japan : Tokyo

Advanced Iteration With enumerate() in Python

Another way to iterate over Python iterables while returning both the index and corresponding value of elements is through the enumerate() function. Check out this example:

fruits_list = ["Apple", "Mango", "Peach", "Orange", "Banana"]

for index, fruit in enumerate(fruits_list):
    print(fruit, index)

Here’s the output:

Apple 0
Mango 1
Peach 2
Orange 3
Banana 4

The enumerate function also lets you specify the starting index, besides 0, for the iteration operation. You can modify the example above as follows:

fruits_list = ["Apple", "Mango", "Peach", "Orange", "Banana"]
for index, fruit in enumerate(fruits_list, start = 2):
    print(fruit, index)

Here’s the output:

Apple 2
Mango 3
Peach 4
Orange 5
Banana 6

Note that while this example specifies the starting index of the enumeration, enumerate doesn’t apply a zero-based indexing to the iterable, as is the case with native lists. It simply appends the start value to the first element in the list all the way to the last one.