Python OrderedDict

Python’s `OrderedDict`, available from the `collections` module, is a dictionary subclass that maintains the insertion order of items. This feature is helpful when you need a dictionary that remembers the sequence of keys and values as they were added. Although standard Python dictionaries (`dict`) now also maintain order (starting from Python 3.7), `OrderedDict` offers additional functionality, such as reordering elements and moving items to the beginning or end of the dictionary.

1. Creating an OrderedDict

from collections import OrderedDict

# Create an OrderedDict
ordered_dict = OrderedDict()
ordered_dict["apple"] = 1
ordered_dict["banana"] = 2
ordered_dict["cherry"] = 3

print("OrderedDict:", ordered_dict)

Output:
OrderedDict: OrderedDict([('apple', 1), ('banana', 2), ('cherry', 3)])

2. Converting a Regular Dictionary to an OrderedDict

# Convert a dictionary to an OrderedDict
regular_dict = {"grape": 4, "orange": 5, "melon": 6}
ordered_dict = OrderedDict(regular_dict)

print("OrderedDict from regular dict:", ordered_dict)

Output:
OrderedDict from regular dict: OrderedDict([('grape', 4), ('orange', 5), ('melon', 6)])

3. Accessing Items in an OrderedDict

# Accessing and modifying values
print("Original cherry value:", ordered_dict["cherry"])

# Modify a value
ordered_dict["cherry"] = 10
print("Updated OrderedDict:", ordered_dict)

Output:
Original cherry value: 3
Updated OrderedDict: OrderedDict([('grape', 4), ('orange', 5), ('melon', 6), ('cherry', 10)])

4. Moving Items with move_to_end()

# Move an item to the end
ordered_dict.move_to_end("grape")
print("After moving grape to the end:", ordered_dict)

# Move an item to the beginning
ordered_dict.move_to_end("cherry", last=False)
print("After moving cherry to the beginning:", ordered_dict)

Output:
After moving grape to the end: OrderedDict([('orange', 5), ('melon', 6), ('cherry', 10), ('grape', 4)])
After moving cherry to the beginning: OrderedDict([('cherry', 10), ('orange', 5), ('melon', 6), ('grape', 4)])

5. Using popitem() with OrderedDict

# Remove the last item
last_item = ordered_dict.popitem()
print("Popped last item:", last_item)
print("OrderedDict after popping last item:", ordered_dict)

# Remove the first item
first_item = ordered_dict.popitem(last=False)
print("Popped first item:", first_item)
print("OrderedDict after popping first item:", ordered_dict)

Output:
Popped last item: ('grape', 4)
OrderedDict after popping last item: OrderedDict([('cherry', 10), ('orange', 5), ('melon', 6)])
Popped first item: ('cherry', 10)
OrderedDict after popping first item: OrderedDict([('orange', 5), ('melon', 6)])

6. Comparing OrderedDicts

# Define two OrderedDicts
od1 = OrderedDict({"apple": 1, "banana": 2})
od2 = OrderedDict({"banana": 2, "apple": 1})

# Compare the two OrderedDicts
print("Are od1 and od2 equal?", od1 == od2)

# Create an identical OrderedDict to od1
od3 = OrderedDict({"apple": 1, "banana": 2})
print("Are od1 and od3 equal?", od1 == od3)

Output:
Are od1 and od2 equal? False
Are od1 and od3 equal? True

7. Reversing an OrderedDict

# Reverse an OrderedDict
reversed_od = OrderedDict(reversed(ordered_dict.items()))
print("Reversed OrderedDict:", reversed_od)

Output:
Reversed OrderedDict: OrderedDict([('melon', 6), ('orange', 5)])

8. Iterating Over an OrderedDict

# Iterate over keys and values in an OrderedDict
for key, value in ordered_dict.items():
    print(f"Key: {key}, Value: {value}")

Output:
Key: orange, Value: 5
Key: melon, Value: 6

9. Clearing an OrderedDict

# Clear all items from the OrderedDict
ordered_dict.clear()
print("OrderedDict after clearing:", ordered_dict)

Output:
OrderedDict after clearing: OrderedDict()

10. Preserving Order in JSON Output with OrderedDict

import json

# Create an OrderedDict
ordered_dict = OrderedDict([("name", "Alice"), ("age", 25), ("country", "USA")])

# Convert OrderedDict to JSON
json_output = json.dumps(ordered_dict)
print("JSON output with OrderedDict:", json_output)

Output:
JSON output with OrderedDict: {"name": "Alice", "age": 25, "country": "USA"}

Summary