Ordereddict vs dict in Python 3.7: Key Differences and Usage

In Python 3.7, the built-in dict preserves insertion order as an official language feature, similar to collections.OrderedDict. However, OrderedDict still offers extra methods like move_to_end() and guarantees order in older Python versions.

Quick Comparison

This table summarizes the main differences between dict and OrderedDict in Python 3.7.

Feature	dict (Python 3.7+)	OrderedDict
Order Preservation	Yes, guaranteed insertion order	Yes, guaranteed insertion order
Performance	Faster for most operations	Slightly slower due to extra features
Extra Methods	No special order methods	Yes, e.g., move_to_end(), popitem(last=True)
Compatibility	Only Python 3.7 and later	Works in all Python versions (2.7+ and 3.x)
Use Case	General purpose with order	When order-specific methods or older Python support needed

Key Differences

Starting with Python 3.7, the built-in dict officially preserves the order in which keys are inserted. This means that when you iterate over a dict, keys come out in the same order they were added. This was an implementation detail in Python 3.6 but became a language guarantee in 3.7.

On the other hand, collections.OrderedDict was created to provide ordered dictionaries before this feature was in dict. It not only preserves order but also offers extra methods like move_to_end() to change the position of keys and a popitem() method that can pop items from either end.

Performance-wise, dict is generally faster because it is a built-in type optimized in C. OrderedDict has some overhead due to maintaining a doubly linked list internally to track order and support its extra methods.

Code Comparison

Here is how you create and use a regular dict in Python 3.7 to preserve insertion order.

python

my_dict = {'apple': 1, 'banana': 2, 'cherry': 3}

for key in my_dict:
    print(key, my_dict[key])

Output

apple 1
banana 2
cherry 3

OrderedDict Equivalent

This example shows the same task using OrderedDict with an extra method to move a key to the end.

python

from collections import OrderedDict

my_odict = OrderedDict([('apple', 1), ('banana', 2), ('cherry', 3)])

my_odict.move_to_end('banana')

for key in my_odict:
    print(key, my_odict[key])

Output

apple 1
cherry 3
banana 2

When to Use Which

Choose dict in Python 3.7 and later for most cases since it preserves order and is faster. It is perfect for general use when you just need to keep keys in insertion order.

Use OrderedDict if you need special order-related methods like move_to_end() or popitem(last=False), or if you need compatibility with Python versions before 3.7.

Key Takeaways

Python 3.7+ dict preserves insertion order officially and is faster.

OrderedDict offers extra methods to manipulate order not available in dict.

Use dict for general ordered mappings in modern Python.

Use OrderedDict for older Python versions or when order-specific methods are needed.