How to truncate a Python dictionary at a given length?

Truncating a dictionary in Python means limiting it to a fixed number of key-value pairs.

From version Python 3.7 onwards, dictionaries maintain insertion order, which makes it easy to slice and rebuild a smaller dictionary using the first N items. This is helpful when we want to reduce data size or process only a portion of the dictionary.

In this article, we explore different methods to truncate a Python dictionary at a given length.

Using itertools.islice() Method

The itertools.islice() method truncates a dictionary by slicing its items without loading all items into memory. This method is memory-efficient for large dictionaries.

Syntax

itertools.islice(sequence, stop)
# or
itertools.islice(sequence, start, stop, step)

Parameters

• sequence ? The item we want to iterate
• stop ? Stop the iteration at this position
• start ? Where the iteration begins (optional)
• step ? Skip items by this amount (optional)

Note: The islice() method does not support negative values for start, stop, or step.

Example 1: Truncate to First N Items

Here we truncate a dictionary to keep only the first 2 key-value pairs ?

import itertools

# Creating a Dictionary with 4 key-value pairs
product = {
    "Product": "Mobile",
    "Model": "XUT",
    "Units": 120,
    "Available": "Yes"
}

print("Original Dictionary:")
print(product)

# Truncating to first 2 items
truncated = dict(itertools.islice(product.items(), 2))

print("\nTruncated Dictionary:")
print(truncated)

Original Dictionary:
{'Product': 'Mobile', 'Model': 'XUT', 'Units': 120, 'Available': 'Yes'}

Truncated Dictionary:
{'Product': 'Mobile', 'Model': 'XUT'}

Example 2: Truncate in a Range

We can specify start and stop positions to extract items from a specific range ?

import itertools

# Creating a Dictionary with 6 key-value pairs
product = {
    "Product": "Mobile",
    "Model": "XUT",
    "Units": 120,
    "Available": "Yes",
    "Grade": "A",
    "Rating": "5"
}

print("Original Dictionary:")
print(product)

# Extract items from position 2 to 4 (indices 2, 3)
truncated = dict(itertools.islice(product.items(), 2, 4))

print("\nTruncated Dictionary (range 2-4):")
print(truncated)

Original Dictionary:
{'Product': 'Mobile', 'Model': 'XUT', 'Units': 120, 'Available': 'Yes', 'Grade': 'A', 'Rating': '5'}

Truncated Dictionary (range 2-4):
{'Units': 120, 'Available': 'Yes'}

Example 3: Truncate with Step

Use the step parameter to skip items while truncating ?

import itertools

product = {
    "Product": "Mobile",
    "Model": "XUT", 
    "Units": 120,
    "Available": "Yes",
    "Grade": "A",
    "Rating": "5"
}

print("Original Dictionary:")
print(product)

# Start at index 1, stop at 5, step by 2 (take every 2nd item)
truncated = dict(itertools.islice(product.items(), 1, 5, 2))

print("\nTruncated Dictionary (step=2):")
print(truncated)

Original Dictionary:
{'Product': 'Mobile', 'Model': 'XUT', 'Units': 120, 'Available': 'Yes', 'Grade': 'A', 'Rating': '5'}

Truncated Dictionary (step=2):
{'Model': 'XUT', 'Available': 'Yes'}

Using List Slicing with items()

Another approach converts dictionary items to a list, slices it, then converts back to a dictionary. This method is simple but creates a temporary list in memory.

Syntax

dict(list(dictionary.items())[:n])

Where n is the number of key-value pairs to retain.

Example

Here we truncate a dictionary to keep only the first 2 items ?

# Original dictionary
data = {'name': 'Alice', 'age': 30, 'city': 'New York', 'country': 'USA'}

print("Original Dictionary:")
print(data)

# Truncate to first 2 items
truncated = dict(list(data.items())[:2])

print("\nTruncated Dictionary:")
print(truncated)

Original Dictionary:
{'name': 'Alice', 'age': 30, 'city': 'New York', 'country': 'USA'}

Truncated Dictionary:
{'name': 'Alice', 'age': 30}

Comparison of Methods

Conclusion

Use itertools.islice() for memory-efficient truncation of large dictionaries. For simple cases with small dictionaries, list slicing provides cleaner syntax. Both methods preserve insertion order in Python 3.7+.