How to check if a given key already exists in a dictionary in Python?

To check if a key exists in a Python dictionary, use the in keyword. Here’s a detailed explanation with examples:

Method 1: Using in (Recommended)

The simplest and most efficient way to check for a key’s existence:

my_dict = {"name": "Alice", "age": 30, "city": "Paris"}

# Check if "age" exists
if "age" in my_dict:
    print("Key 'age' exists!")  # Output: Key 'age' exists!

# Check if "country" exists
if "country" not in my_dict:
    print("Key 'country' does NOT exist!")  # Output: Key 'country' does NOT exist!

Why this works:
The in keyword checks for the presence of a key in the dictionary’s keys (average O(1) time complexity due to hash tables).

Method 2: Using dict.get()

The get() method returns None (or a default value) if the key is absent:

value = my_dict.get("name")
if value is not None:
    print(f"Key 'name' exists with value: {value}")  # Output: Key 'name' exists with value: Alice

# Check with a default value (e.g., "Key not found")
value = my_dict.get("country", "Key not found")
print(value)  # Output: Key not found

Limitations:

• Fails if the key exists but its value is explicitly None (e.g., my_dict = {"key": None}).

Method 3: Using try–except (EAFP Principle)

Use this if you plan to handle the key’s absence as an exception (Easier to Ask for Forgiveness than Permission):

try:
    value = my_dict["country"]
    print(f"Key 'country' exists: {value}")
except KeyError:
    print("Key 'country' does NOT exist!")  # Output: Key 'country' does NOT exist!

Edge Cases & Notes

1. Key with None Value:

   my_dict = {"key": None}
   print("key" in my_dict)  # Output: True (key exists, even though value is None)

2. Avoid dict.has_key() (Python 2 Only):
   The deprecated has_key() method works in Python 2 but not in Python 3.
3. Check for Values (Not Keys):
   Use in my_dict.values() to check if a value exists (slower, O(n) time):

   if "Paris" in my_dict.values():
       print("Value 'Paris' exists!")  # Output: Value 'Paris' exists!

Summary

Method	Use Case	Time Complexity
key in my_dict	Fastest and most Pythonic	O(1)
my_dict.get(key)	When you need a default value	O(1)
try–except	When accessing the key is part of logic	O(1)

Best Practice: Use in for simple checks. Reserve get() or try–except for cases where you need to handle missing keys gracefully.