Write a Python Function to Check if an Object is a List | Rustcode

Write a Python Function to Check if an Object is a List

Sometimes, you need to know if a value in your Python program is really a list. Maybe you're building a function that should only work with lists, or you want to avoid errors when looping. Python makes it easy to check the type of an object. Here’s how you can write a simple, reusable function to check if something is a list, with clear explanations and examples.

Why Check if an Object is a List?

Definition: Type checking means verifying whether a variable is a specific type, like a list, tuple, or string.
Use Cases: Writing safe functions, avoiding runtime errors, and supporting flexible code that can handle different types.
Benefits: Prevents bugs, makes your code more robust, and helps with debugging.

01. Using `isinstance()` (Recommended)

The isinstance() function is the most Pythonic way to check if an object is a list. It returns True if the object is a list, and False otherwise.

def is_list(obj):
    return isinstance(obj, list)

Example Usage:

print(is_list([1, 2, 3]))      # True
print(is_list("hello"))        # False
print(is_list((1, 2, 3)))      # False
print(is_list(42))             # False

Output:

True
False
False
False

Explanation:

isinstance(obj, list) checks if obj is a list or a subclass of list.
Works even if you define your own class that inherits from list.
Recommended for most real-world Python code.

02. Using `type()` for Exact Match

If you want to check that an object is exactly a list (not a subclass), you can use type():

def is_exact_list(obj):
    return type(obj) is list

Example Usage:

print(is_exact_list([1, 2, 3]))          # True
class MyList(list): pass
mylist = MyList()
print(is_exact_list(mylist))             # False

Output:

True
False

Explanation:

type(obj) is list checks for the exact type, not subclasses.
Use this if you want to exclude custom list-like classes.

03. Function Examples and Outputs

Here’s a function that prints a message based on the type of the input:

def describe_object(obj):
    if isinstance(obj, list):
        print("It's a list!")
    else:
        print("Not a list.")

describe_object([10, 20])
describe_object("Python")
describe_object({'a': 1})

Output:

It's a list!
Not a list.
Not a list.

Explanation:

The function uses isinstance() for flexible type checking.
Helps you debug or branch your code based on input type.

04. Handling Other Sequence Types

If you want your function to work with any sequence (like lists, tuples, or strings), you can check for collections.abc.Sequence:

from collections.abc import Sequence

def is_sequence(obj):
    return isinstance(obj, Sequence) and not isinstance(obj, str)

Example Usage:

print(is_sequence([1, 2, 3]))   # True
print(is_sequence((1, 2, 3)))   # True
print(is_sequence("abc"))       # False (excluded string)

Output:

True
True
False

Explanation:

This approach is useful if you want to accept lists and tuples, but not strings.
Gives your code more flexibility for different sequence types.

05. Comparison Table: Ways to Check for a List in Python

Method	Checks Subclasses	Checks Only Lists	Best For
isinstance(obj, list)	Yes	No	General use, flexible code
type(obj) is list	No	Yes	Exact type match
collections.abc.Sequence	Yes	No	Accepting lists, tuples, etc.

Conclusion

Checking if an object is a list in Python is simple and reliable. Use isinstance(obj, list) for most cases, or type(obj) is list if you want to be strict. For even more flexibility, check for sequence types using collections.abc.Sequence. This helps you write safer, more robust, and more flexible Python code.

Tip: Type checking is your friend when writing functions that need to handle different kinds of data!