Two Pointer Palindrome Check in Python

from bisect import bisect_left

# Two Pointer Palindrome Check in Python
#
# PURPOSE
# A palindrome reads the same in both directions. Symmetric comparisons detect the first
# disagreement without reversing or copying the sequence.
#
# METHOD
# Compare the two ends, reject on a mismatch, and move inward after a match. Passing every
# symmetric pair proves the sequence is a palindrome.
#
# COMPLEXITY
# O(n) time and O(1) auxiliary space.
#
# CONTRACT
# Input is an integer sequence, so token and character encoding are outside this example.
# Returns 1 or 0; empty input is a palindrome.
# Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
# counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
# Sorted order, correct tree topology, and caller-owned data-structure invariants are
# preconditions; checking them on every query would change query complexity.
# Scalar and shape checks are included. Value-range checks are included where needed to
# prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
# workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
# uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
# partly updated after arithmetic failure. These are single-threaded, in-memory
# implementations.
#
# LANGUAGE DETAILS
# Python 3.9+: automatic lengths, native lists, arbitrary-precision arithmetic, and
# bisect_left for LIS. Invalid scalar/shape inputs raise ValueError. Integer element types
# and sorted/tree invariants remain caller preconditions.
#
# EXAMPLE
# Expected return: 1.
# Run: python example.py
# Tests for this exact source artifact run before publication. The accompanying source
# archive includes unit cases, runtime results, and source hashes.

def alg_require(condition):
    if not condition:
        raise ValueError("invalid algorithm input; see contract")
def alg_size(n):
    alg_require(0 <= n <= 4000000)
    return n

def is_palindrome(a: list[int]) -> int:
    alg_require(isinstance(a, (list, tuple)) and len(a) <= 1000000)
    n: int = len(a)
    alg_require(n >= 0 and n <= 1000000)
    alg_require(len(a) == n)
    left: int = 0
    right: int = n - 1
    while left < right:
        if a[left] != a[right]:
            return 0
        left += 1
        right -= 1
    return 1

a = [1, 2, 1]
result = is_palindrome(a)
print(result)