[YashAnand69]

Fixes #318.

UART.readline() consumed LF before adding it to the returned bytes. A blank line therefore returned None, indistinguishable from a timeout, and CRLF lines lost their LF. Append the byte before checking for the line terminator, preserving complete line framing without consuming the next line.

This restores the documented CircuitPython-style newline contract. Complete lines now include LF; partial timeout results, raw reads, polling and constructor/parity handling are unchanged.

Validation: five framing regressions fail on unchanged main while five controls pass. All ten focused cases and 33 existing hardware-independent tests pass with the fix. Tests exercise the actual UART status/read command path with controlled firmware replies and clock. Configured source/test Black, Flake8, Bandit and pydocstyle, isolated wheel/sdist builds and Sphinx pass (seven existing docs warnings). Same-selection busio coverage remains 48.148%; this is not a whole-package or physical-hardware coverage claim.

Prepared with Codex assistance and reviewed against the method documentation and CircuitPython/pySerial line-reading contracts. No physical UART integration was performed. This read-path fix is separate from #317's constructor parity handling.

Summary by Sourcery

Restore complete UART line framing by retaining newline terminators while preserving timeout and raw-read behavior.

Bug Fixes:

• Preserve LF terminators in UART line reads, including blank lines and CRLF sequences, without consuming bytes from subsequent lines.

Tests:

• Add hardware-independent UART framing tests covering complete lines, partial timeouts, raw reads, readinto, byte limits, and zero-length reads.

Summary by CodeRabbit

• Bug Fixes
  • UART line reads now include the terminating newline, making it easier to preserve complete lines when reading serial data.
  • Reading consecutive lines preserves each line ending, while blank lines remain distinct from cases where no data is available.
  • Reads that time out still return any data received so far, and byte-limited reads continue to respect the requested size.

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Reviewer's Guide

Fix UART readline framing by appending bytes before checking for LF, so complete lines retain their newline terminators without consuming subsequent data. Add focused transport-level tests covering line framing, timeout semantics, raw reads, binary data, byte limits, and buffer preservation.

Sequence diagram for UART readline framing

sequenceDiagram
    participant Caller
    participant UART
    participant Transport

    Caller->>UART: readline()
    UART->>Transport: _read_byte()
    Transport-->>UART: byte
    UART->>UART: data.append(char)
    alt char == 0xA
        UART-->>Caller: complete line including LF
    else char != 0xA
        UART->>Transport: _read_byte()
        Transport-->>UART: next byte
        UART-->>Caller: line or timeout result
    end

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File-Level Changes

Change	Details	Files
Preserve LF terminators when reading complete UART lines.	Append each received byte before evaluating the newline boundary. Ensure blank lines return b"\n", CRLF lines retain both terminators, and the next line remains buffered.	pslab/bus/busio.py
Add hardware-independent regression coverage for UART framing and read behavior.	Mock the actual UART status/read command path with controlled buffering and deterministic timeout progression. Cover blank, LF, CRLF, binary, partial-timeout, successive-line, raw-read, readinto, zero-length, and no-data cases. Verify command counts and unread receive-buffer contents to detect over-reading.	tests/test_busio_uart_readline.py

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#318	Ensure UART.readline() retains the LF newline terminator in returned complete lines, including LF, CRLF, and binary-data lines.	✅
#318	Make blank lines such as b"\n" return the complete line rather than None, while preserving the distinction between a successful line read and a timeout.	✅
#318	Preserve existing UART read behavior outside line termination handling, including partial-line timeout results, successive-line framing, raw reads, readinto, and zero-length reads.	✅

Possibly linked issues

• CircuitPython UART readline drops LF and returns None for blank lines #318: The PR directly fixes issue CircuitPython UART readline drops LF and returns None for blank lines #318 by appending LF before termination checks and adding regression tests.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 69bcc38a-15b8-411f-87ea-602118183837

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📥 Commits

Reviewing files that changed from the base of the PR and between 08de0d0 and 51bc003.

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📒 Files selected for processing (2)

• pslab/bus/busio.py
• tests/test_busio_uart_readline.py

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Included review availability: This review used your included allowance. Your plan provides up to 4 included reviews per hour; 3 remain after this review.

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📝 Walkthrough

Walkthrough

UART._read_with_timeout now includes the received newline byte in its result. Hardware-free tests cover newline framing, partial reads, empty input, and byte-limited reads.

Changes

UART line-read behavior

Layer / File(s)	Summary
Include newline bytes in UART reads pslab/bus/busio.py, tests/test_busio_uart_readline.py	UART._read_with_timeout appends and counts each received byte before checking for a newline. Tests cover blank lines, LF and CRLF endings, successive lines, partial data on timeout, read(), readinto(), and read(0).

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Low

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Merge Risk | ⚪ Minimal · up to 51bc0

Merge Risk: ⚪ Minimal · up to 51bc0

UART line reads retain newline terminators. No actionable merge-blocking issue is established; merge after normal checks.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to 51bc0

The change restores complete line framing without expanding device access or privileges. No material security risk was found in the changed behavior; hardware integration and downstream application behavior were not validated.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The demonstrated exposure change is confined to bytes returned to existing UART.readline() callers. The inspected change introduces no new entrypoint, device authority, or sensitive sink; downstream application interpretation of the retained terminator remains unvalidated.

Trust Boundaries and Controls

• observed — The routed Clock and UARTTransport declarations are test-local helpers. Their command responses and clock substitution are applied through the pytest fixture, not added to production connection handling.

Resilience and Maintainability Implications

• inferred — The change introduces no retained transaction state or new cleanup requirement. Read interruption and unsynchronized concurrent access remain pre-existing limitations rather than PR-introduced failure-containment concerns.

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Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 5.56% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 18 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: preserving newline terminators in UART.readline().
Linked Issues check		Issue #318 requires UART.readline() to retain LF, distinguish a blank LF line from timeout, preserve CRLF framing, and avoid consuming following data. The reviewed change in pslab/bus/busio.py app…
Out of Scope Changes check		The changes are limited to the UART read path and tests for the #318 framing contract. The tests also verify unchanged raw-read, timeout, byte-limit, and zero-length behavior. No unrelated production …

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