f.writelines(...) is absent

with open(p, "w") as f:
    f.writelines(["a\n", "b\n"])     # pxx: TPyFile has no method writelines

Found 2026-08-15 by a CPython differential sweep of a script-shaped program (open/write/read/readlines/iterate/append/split/format). Everything else in that probe agreed exactly; writelines was the only wall, and it is loud.

Fix

TPyFile.writelines(const v: Variant) — a variant parameter for the reason the write row beside it documents (the call site picks an overload by name and ARITY, so a dynamically-typed argument needs a variant-typed entry), and the sequence comes from pyseq_of_obj, so a list, a tuple, a cursor, a range or a user __iter__ all work by the one chain. No separator is added — CPython's writelines does not, the caller's strings carry their own newlines, and adding one would be a plausible wrong answer rather than an error.

Also found, NOT fixed here

f.writelines(str(i) + "\n" for i in xs) — a BARE generator expression as a METHOD-call argument — fails with undefined variable (i). The diversion that handles this (PyBareGenExprAhead, at the ordinary-call site in parser.inc) is not on the method-call path, so "".join(x for x in xs) and friends have the same hole. Filed as [[bug-nilpy-bare-genexpr-as-a-method-argument-does-not-parse]].

Gate

test/test_nilpy_file_writelines.npy (+.expected, in the Makefile), byte-identical to CPython: a list of newline-terminated strings, an EMPTY sequence, a comprehension result, and a TUPLE of strings with no newlines at all (proving no separator is inserted). gate.sh quick GREEN. Pinned (compiler/builtin/**).