sliceview 0.1.1

Zero-copy, composable slice views for Python sequences

sliceview

Zero-copy, composable slice views for Python sequences.

sliceview lets you work with windows into lists, tuples, strings, or any Sequence without copying the underlying data. It is the proof-of-concept library accompanying the Python discussion on Slice Views for Python Sequences.

from sliceview import sliceview

data = list(range(1_000_000))

# O(1) — no copy
window = sliceview(data)[50_000:60_000]

# Compose slices — still O(1), still no copy
every_third = sliceview(data)[::3][100:200]

# In-place windowed update
sv = sliceview(data)
sv[0:5] = [10, 20, 30, 40, 50]

# Sliding window without creating new objects
view = sliceview(data, 0, 1000)
for _ in range(10):
    process(view)
    view.advance(1000)

Why?

In Python today, seq[a:b] copies the data. For large pipelines — text processing, audio, genomics, sorting algorithms — those copies dominate both time and memory. memoryview solves this for bytes-like objects; sliceview targets generic sequences of Python objects.

Inspired by Go slices, NumPy views, and memoryview.

Installation

pip install sliceview

Features

Feature	Details
Zero-copy slicing	sv[a:b:c] returns a new sliceview in O(1)
Composable	sv[2:][::3][5:10] chains correctly with no intermediate copies
Live view	Mutations to the base are immediately visible through the view
Write-through	sv[i] = x and sv[a:b] = iterable forward to the base
Sliding window	sv.advance(n) shifts the window in-place — no new object
Any sequence	Works with list, tuple, str, array, or your own type

API

sliceview(base, start=None, stop=None, step=None)

Create a view over base. start may be a slice object.

sv = sliceview(my_list)           # full view
sv = sliceview(my_list, 10, 20)   # [10:20]
sv = sliceview(my_list, slice(10, 20, 2))  # [10:20:2]

Indexing and slicing

sv[i]        # element access — maps to base[start + i*step]
sv[a:b:c]    # returns a new sliceview (O(1))
sv[i] = x    # write-through to the base
sv[a:b] = it # slice assignment (delegates to base)

sv.advance(n) -> self

Shift the view's window forward by n index positions (negative to retreat). Returns self for chaining. Useful for sliding-window algorithms:

view = sliceview(samples, 0, window_size)
while True:
    result = process(view)
    if view.advance(window_size)._start >= len(samples):
        break

sv.tolist() / sv.copy()

Materialise the view as a new list (explicit copy).

sv.base

The underlying sequence the view points into.

Semantics

• len(sv) reflects the current base length, so appending to the base is immediately visible.
• sv[:] returns a new sliceview pointing at the same base — O(1).
• Hashing: sliceview is intentionally unhashable.
• Equality: compares element-wise to any Sequence.
• Immutable bases: sv[i] = x raises TypeError if the base does not support __setitem__.

Design notes and open questions

This library implements the core proposal from the Python discussion. Deliberately left out to keep the scope focused:

• view() builtin shortcut (consensus in the thread was it's unnecessary)
• __sliceview__ dunder (motivation unclear until adoption data exists)
• Multidimensional views (NumPy is the right tool for that)
• ABC / Protocol additions to collections.abc

Feedback welcome — please open an issue or join the discussion thread.

License

MIT