python – Use and meaning of “in” in an if statement?

In Python, the in keyword is used to check for membership. It is commonly used in an if statement to determine if a specific value exists within a sequence, such as a list, tuple, string, set, or dictionary keys.

Meaning of in in an if Statement

The in keyword checks whether an element is present in a collection (e.g., list, string, dictionary). If the element is found, it returns True; otherwise, it returns False.

Usage Examples

1. Checking in a List:

   fruits = ["apple", "banana", "cherry"]
   if "banana" in fruits:
       print("Banana is in the list!")
   else:
       print("Banana is not in the list.")
   

   Output:

   Banana is in the list!
   

2. Checking in a String:

   text = "Hello, world!"
   if "world" in text:
       print("The word 'world' is in the string!")
   

   Output:

   The word 'world' is in the string!
   

3. Checking in a Dictionary:

   person = {"name": "Alice", "age": 25}
   if "name" in person:
       print("The key 'name' exists in the dictionary!")
   

   Output:

   The key 'name' exists in the dictionary!
   

4. Checking in a Set:

   numbers = {1, 2, 3, 4}
   if 3 in numbers:
       print("3 is in the set!")
   

   Output:

   3 is in the set!
   

5. Negating with not in: You can use not in to check if an element is absent from a collection:

   vowels = "aeiou"
   if "z" not in vowels:
       print("The letter 'z' is not a vowel.")
   

   Output:

   The letter 'z' is not a vowel.
   

How It Works

• When in is used in an if statement, Python checks each element in the sequence to see if it matches the value.
• For strings, it checks substrings.
• For dictionaries, in checks keys by default.

Key Points

• in is case-sensitive. For example, "A" is not equal to "a" in strings.
• Works with all iterables, including lists, strings, tuples, dictionaries, and sets.
• For dictionaries, in checks only the keys, not the values.

Example with Explanation

data = {"apple": 5, "banana": 8}
if "apple" in data:
    print("Key 'apple' exists in the dictionary.")
if 8 in data.values():
    print("The value 8 exists in the dictionary.")

Output:

Key 'apple' exists in the dictionary.
The value 8 exists in the dictionary.

Here:

• in data checks for keys in the dictionary.
• in data.values() checks for values in the dictionary.

This makes the in keyword highly versatile and commonly used in Python programming.