posix: always seek to EOF when opening a file in append mode Python 3 already does this, so skip it there. Consider the program: #include <stdio.h> int main() { FILE *f = fopen("narf", "w"); fprintf(f, "narf\n"); fclose(f); f = fopen("narf", "a"); printf("%ld\n", ftell(f)); fprintf(f, "troz\n"); printf("%ld\n", ftell(f)); return 0; } on macOS, FreeBSD, and Linux with glibc, this program prints 5 10 but on musl libc (Alpine Linux and probably others) this prints 0 10 By my reading of https://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html this is technically correct, specifically: > Opening a file with append mode (a as the first character in the > mode argument) shall cause all subsequent writes to the file to be > forced to the then current end-of-file, regardless of intervening > calls to fseek(). in other words, the file position doesn't really matter in append-mode files, and we can't depend on it being at all meaningful unless we perform a seek() before tell() after open(..., 'a'). Experimentally after a .write() we can do a .tell() and it'll always be reasonable, but I'm unclear from reading the specification if that's a smart thing to rely on. This matches what we do on Windows and what Python 3 does for free, so let's just be consistent. Thanks to Yuya for the idea.

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#!/usr/bin/env python

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# Filter output by pyflakes to control which warnings we check

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from __future__ import absolute_import, print_function

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import re

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import sys

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lines = []

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for line in sys.stdin:

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    # We blacklist tests that are too noisy for us

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    pats = [

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        r"undefined name 'WindowsError'",

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        r"redefinition of unused '[^']+' from line",

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        # for cffi, allow re-exports from pure.*

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        r"cffi/[^:]*:.*\bimport \*' used",

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        r"cffi/[^:]*:.*\*' imported but unused",

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    ]

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    keep = True

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    for pat in pats:

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        if re.search(pat, line):

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            keep = False

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            break # pattern matches

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    if keep:

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        fn = line.split(':', 1)[0]

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        f = open(fn)

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        data = f.read()

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        f.close()

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        if 'no-' 'check-code' in data:

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            continue

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        lines.append(line)

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for line in lines:

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    sys.stdout.write(line)

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print()