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"""

A script that replaces an old file with a new one, only if the contents

actually changed. If not, the new file is simply deleted.

This avoids wholesale rebuilds when a code (re)generation phase does not

actually change the in-tree generated code.

"""

from __future__ import annotations

import contextlib

import os

import os.path

import sys

TYPE_CHECKING = False

if TYPE_CHECKING:

import typing

from collections.abc import Iterator

from io import TextIOWrapper

_Outcome: typing.TypeAlias = typing.Literal['created', 'updated', 'same']

@contextlib.contextmanager

def updating_file_with_tmpfile(

filename: str,

tmpfile: str | None = None,

) -> Iterator[tuple[TextIOWrapper, TextIOWrapper]]:

"""A context manager for updating a file via a temp file.

The context manager provides two open files: the source file open

for reading, and the temp file, open for writing.

Upon exiting: both files are closed, and the source file is replaced

with the temp file.

"""

# XXX Optionally use tempfile.TemporaryFile?

if not tmpfile:

tmpfile = filename + '.tmp'

elif os.path.isdir(tmpfile):

tmpfile = os.path.join(tmpfile, filename + '.tmp')

with open(filename, 'rb') as infile:

line = infile.readline()

if line.endswith(b'\r\n'):

newline = "\r\n"

elif line.endswith(b'\r'):

newline = "\r"

elif line.endswith(b'\n'):

newline = "\n"

else:

raise ValueError(f"unknown end of line: {filename}: {line!a}")

with open(tmpfile, 'w', newline=newline) as outfile:

with open(filename) as infile:

yield infile, outfile

update_file_with_tmpfile(filename, tmpfile)

def update_file_with_tmpfile(

filename: str,

tmpfile: str,

create: bool = False,

) -> _Outcome:

try:

targetfile = open(filename, 'rb')

except FileNotFoundError:

if not create:

raise # re-raise

outcome: _Outcome = 'created'

os.replace(tmpfile, filename)

else:

with targetfile:

old_contents = targetfile.read()

with open(tmpfile, 'rb') as f:

new_contents = f.read()

# Now compare!

if old_contents != new_contents:

outcome = 'updated'

os.replace(tmpfile, filename)

else:

outcome = 'same'

os.unlink(tmpfile)

return outcome

if __name__ == '__main__':

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('--create', action='store_true')

parser.add_argument('--exitcode', action='store_true')

parser.add_argument('filename', help='path to be updated')

parser.add_argument('tmpfile', help='path with new contents')

args = parser.parse_args()

kwargs = vars(args)

setexitcode = kwargs.pop('exitcode')

outcome = update_file_with_tmpfile(**kwargs)

if setexitcode:

if outcome == 'same':

sys.exit(0)

elif outcome == 'updated':

sys.exit(1)

elif outcome == 'created':

sys.exit(2)

else:

raise NotImplementedError

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