How to Find the First Key in a Dictionary in Python

In Python, dictionaries store key-value pairs, and you may sometimes need to retrieve the first key. The approach depends on the Python version you are using. Here’s how you can do it efficiently.

Python Dictionary Basics

Consider the following dictionary for demonstration:

prices = {
    "banana": 4,
    "apple": 2,
    "orange": 1.5,
    "pear": 3
}

Method 1: Using next(iter())

From Python 3.7 onward (and Python 3.6 for CPython), dictionaries maintain insertion order. You can fetch the first key with:

first_key = next(iter(prices))
print(first_key)  # Output: banana

This method is efficient as it avoids creating unnecessary intermediate structures.

Method 2: Using list()

A straightforward but less efficient approach involves converting keys to a list:

first_key = list(prices.keys())[0]
print(first_key)  # Output: banana

While easy to understand, this creates a temporary list in memory, making it slower for large dictionaries.

Method 3: Using a for Loop

A simple loop can also fetch the first key:

for key in prices:
    print(key)  # Output: banana
    break

This method is intuitive and does not require additional imports or conversions.

Method 4: Using Unpacking

For Python 3.5 and later, unpacking offers a concise way to retrieve the first key:

first_key, *rest = prices
print(first_key)  # Output: banana

This technique is especially useful when you also want to keep the remaining keys.

Handling Python 3.6 and Earlier

In versions before 3.7, dictionary keys are not guaranteed to maintain order. To ensure order, use collections.OrderedDict:

from collections import OrderedDict

prices = OrderedDict([
    ("banana", 4),
    ("apple", 2),
    ("orange", 1.5),
    ("pear", 3),
])

first_key = next(iter(prices))
print(first_key)  # Output: banana

Retrieving Both Key and Value

If you need both the first key and its corresponding value, use .items() with next():

first_key, first_value = next(iter(prices.items()))
print(first_key, first_value)  # Output: banana 4

Alternatively, unpack both directly:

(first_key, first_value), *rest = prices.items()
print(first_key, first_value)  # Output: banana 4

Choose the method based on your requirements and Python version:

• Use next(iter(dict)) for simplicity and efficiency in Python 3.7 and later.
• Opt for list(dict.keys())[0] if readability is your priority, but be cautious about performance.
• A for loop is a universal and beginner-friendly option.
• Unpacking is a neat alternative for Python 3.5+ users who want to retrieve the first key and store the rest.

With these techniques, you can efficiently retrieve the first key in any dictionary.