How to Use np.savez for Multiple Arrays in NumPy

Use np.savez to save multiple arrays into one compressed file by passing arrays as keyword arguments or positional arguments. Each array can be accessed later by its name when loading the file with np.load. This method helps organize and store multiple arrays efficiently in one file.

Syntax

The basic syntax of np.savez is:

• np.savez(file, *args, **kwargs)

Where:

• file: The filename or file object to save the arrays to (usually ends with .npz).
• *args: Arrays passed without names, saved with default names like arr_0, arr_1, etc.
• **kwargs: Arrays passed with names, saved with those names for easy access later.

python

np.savez(file, *args, **kwargs)

Example

This example shows how to save three arrays into one file using np.savez with named arrays, then load and access them.

python

import numpy as np

# Create sample arrays
array1 = np.array([1, 2, 3])
array2 = np.array([4, 5, 6])
array3 = np.array([7, 8, 9])

# Save arrays with names into a .npz file
np.savez('arrays.npz', first=array1, second=array2, third=array3)

# Load the .npz file
loaded = np.load('arrays.npz')

# Access arrays by their names
first_array = loaded['first']
second_array = loaded['second']
third_array = loaded['third']

print('First array:', first_array)
print('Second array:', second_array)
print('Third array:', third_array)

Output

First array: [1 2 3]
Second array: [4 5 6]
Third array: [7 8 9]

Common Pitfalls

Common mistakes when using np.savez include:

• Not using keyword arguments, which causes arrays to be saved with default names like arr_0, making it harder to remember which is which.
• Forgetting to add the .npz extension, which can confuse file handling.
• Trying to save non-array objects, which np.savez does not support.

Always use keyword arguments for clarity and include the .npz extension in the filename.

python

import numpy as np

# Wrong way: positional arguments without names
array1 = np.array([1, 2])
array2 = np.array([3, 4])
np.savez('wrong_save.npz', array1, array2)

loaded_wrong = np.load('wrong_save.npz')
print('Default names:', loaded_wrong.files)

# Right way: keyword arguments with names
np.savez('right_save.npz', first=array1, second=array2)
loaded_right = np.load('right_save.npz')
print('Named arrays:', loaded_right.files)

Output

Default names: ['arr_0', 'arr_1']
Named arrays: ['first', 'second']

Quick Reference

Tips for using np.savez effectively:

• Use keyword arguments to name arrays for easy access.
• Always use the .npz file extension.
• Load saved files with np.load and access arrays by their names.
• Use np.savez_compressed if you want to save space with compression.

Key Takeaways

Use np.savez with keyword arguments to save multiple arrays with names in one file.

Always include the .npz extension when saving files with np.savez.

Load arrays with np.load and access them by the names used during saving.

Positional arguments save arrays with default names like arr_0, which can be confusing.

For smaller file size, consider using np.savez_compressed instead of np.savez.