Use and meaning of "in" in an if statement?

The keyword in in an if statement is commonly used in Python (and some other languages) to check membership — that is, whether a particular value exists within a collection like a list, tuple, dictionary, or string. It makes conditions shorter and more readable compared to writing loops or extra logic.

Here’s what the in keyword means and how it’s used:

Membership Test:

 Checks if an element exists in a sequence (list, tuple, set, string, or dictionary keys).

 fruits = ["apple", "banana", "mango"]

if "apple" in fruits:

    print("Yes, apple is in the list")

String Check:

 You can use in to see if a substring exists within a string.

 if "world" in "Hello world":

    print("Substring found")

Dictionary Keys:

 With dictionaries, in checks only the keys by default.

 student = {"name": "Alice", "age": 21}

if "name" in student:

    print("Key exists in dictionary")

Negation with not in:

 You can also check if something is not present.

 if "grape" not in fruits:

    print("Grape is not in the list")

Why use it?

• Short and expressive syntax.
  
• Improves code readability compared to manual loops.
  
• Works across different collection types.
  

In short, using in within an if statement is a clean, Pythonic way to check for existence or membership in collections and strings.