Using The Len() Function In Python

Table of Contents

• Getting Started With Python’s len()
  • Using len() With Built-in Sequences
  • Using len() With Built-in Collections
  • Exploring len() With Other Built-in Data Types
• Exploring len() Further With Some Examples
  • Verifying the Length of a User Input
  • Ending a Loop Based on the Length of an Object
  • Finding the Index of the Last Item of a Sequence
  • Splitting a List Into Two Halves
• Using the len() Function With Third-Party Libraries
  • NumPy’s ndarray
  • Pandas’ DataFrame
• Using len() on User-Defined Classes
• Conclusion
• Frequently Asked Questions

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Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Python's len() Function

The len() function in Python is a powerful and efficient tool used to determine the number of items in objects, such as sequences or collections. You can use len() with various data types, including strings, lists, dictionaries, and third-party types like NumPy arrays and pandas DataFrames. Understanding how len() works with different data types helps you write more efficient and concise Python code.

Using len() in Python is straightforward for built-in types, but you can extend it to your custom classes by implementing the .__len__() method. This allows you to customize what length means for your objects. For example, with pandas DataFrames, len() returns the number of rows. Mastering len() not only enhances your grasp of Python’s data structures but also empowers you to craft more robust and adaptable programs.

By the end of this tutorial, you’ll understand that:

• The len() function in Python returns the number of items in an object, such as strings, lists, or dictionaries.
• To get the length of a string in Python, you use len() with the string as an argument, like len("example").
• To find the length of a list in Python, you pass the list to len(), like len([1, 2, 3]).
• The len() function operates in constant time, O(1), as it accesses a length attribute in most cases.

In this tutorial, you’ll learn when to use the len() Python function and how to use it effectively. You’ll discover which built-in data types are valid arguments for len() and which ones you can’t use. You’ll also learn how to use len() with third-party types like ndarray in NumPy and DataFrame in pandas, and with your own classes.

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Getting Started With Python’s len()

The function len() is one of Python’s built-in functions. It returns the length of an object. For example, it can return the number of items in a list. You can use the function with many different data types. However, not all data types are valid arguments for len().

You can start by looking at the help for this function:

Python >>> help(len) Help on built-in function len in module builtins: len(obj, /) Return the number of items in a container.