Module 6 · Lesson 3

Saving & Loading Arrays

np.save / np.load write a single array to a binary .npy file.
np.savez / np.load write several named arrays to one .npz archive (loading returns an NpzFile you index like a dict).
np.savetxt / np.loadtxt write a plain-text, CSV-like format — human readable, but slower and loses exact dtype information.

All three normally take a file path, but they’re equally happy with an in-memory file-like object (io.BytesIO for the binary formats, io.StringIO for text) — handy for round-tripping without touching disk, which is exactly what the exercises below do. Remember to seek(0) on the buffer after writing and before reading, since writing leaves the buffer’s position at the end.

arr = np.array([1, 2, 3, 4])

buf = io.BytesIO()
np.save(buf, arr)
buf.seek(0)
loaded = np.load(buf)
print("single array round-trip:", loaded)

buf2 = io.BytesIO()
np.savez(buf2, first=arr, second=arr * 10)
buf2.seek(0)
archive = np.load(buf2)
print("multi-array round-trip:", archive["first"], archive["second"])

Output:

single array round-trip: [1 2 3 4]
multi-array round-trip: [1 2 3 4] [10 20 30 40]

Write roundtrip_npy(arr) that writes arr to an in-memory io.BytesIO buffer with np.save, reads it back with np.load, and returns the loaded array.

💡 Hint

buf = io.BytesIO(), then np.save(buf, arr), then buf.seek(0) before np.load(buf).

✅ Solution

def roundtrip_npy(arr):
    buf = io.BytesIO()
    np.save(buf, arr)
    buf.seek(0)
    return np.load(buf)

Write roundtrip_npz(a, b) that saves a and b into one in-memory .npz archive under the keys "a" and "b" with np.savez, reloads it, and returns a plain dict {"a": ..., "b": ...} of the loaded arrays.

💡 Hint

np.savez(buf, a=a, b=b), buf.seek(0), then np.load(buf) returns an NpzFile — index it with ["a"] and ["b"].

✅ Solution

def roundtrip_npz(a, b):
    buf = io.BytesIO()
    np.savez(buf, a=a, b=b)
    buf.seek(0)
    archive = np.load(buf)
    return {"a": archive["a"], "b": archive["b"]}

Saving & Loading Arrays

Round-trip a single array

Round-trip two named arrays