Draft the initial version

This is inspired by Daniel Holth's module httpfile and is based on
the discussion over https://github.com/pypa/pip/issues/7819

.travis.yml

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+branches:
+  only:
+    - master
+    - /^\d+(\.\d+)+((a|b|rc)\d+)?(\.post\d+)?(\.dev\d+)?$/
+
+language: python
+
+install: pip install tox
+script: tox

lazip.py

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+# Lazy ZIP over HTTP
+# Copyright (C) 2020  Nguyễn Gia Phong
+#
+# This file is part of lazip.
+#
+# lazip is free software: you can redistribute it and/or modify it
+# under the terms of the GNU Lesser General Public License as published
+# by the Free Software Foundation, either version 3 of the License,
+# or (at your option) any later version.
+#
+# lazip is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with palace.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Lazy ZIP over HTTP"""
+
+__version__ = '0.0.1'
+__all__ = ['Lazip']
+
+from bisect import bisect_left, bisect_right
+from contextlib import contextmanager
+from tempfile import NamedTemporaryFile
+from typing import Dict, Iterator, List, Optional, Tuple
+from zipfile import BadZipFile, ZipFile
+
+from pip._internal.network.utils import response_chunks
+from pip._internal.utils.wheel import pkg_resources_distribution_for_wheel
+from requests import Session
+from requests.models import CONTENT_CHUNK_SIZE, Response
+
+HEADERS: Dict[str, str] = {'Accept-Encoding': 'identity'}
+
+
+def init_range(stop: int, size: int) -> Iterator[Tuple[int, int]]:
+    """Return an iterator of intervals to fetch a file reversedly."""
+    start = stop - size
+    while start > 0:
+        yield start, stop-1
+        stop = start
+        start -= size
+    yield 0, stop-1
+
+
+class Lazip:
+    """File-like object mapped to a ZIP file over HTTP.
+
+    This uses HTTP range requests to lazily fetch the file's content,
+    which is supposed to be fed to ZipFile.
+    """
+
+    def __init__(self, session: Session, url: str,
+                 chunk_size: int = CONTENT_CHUNK_SIZE) -> None:
+        head = session.head(url, headers=HEADERS)
+        head.raise_for_status()
+        assert head.status_code == 200
+        self.session, self.url, self.chunk_size = session, url, chunk_size
+        self.length = int(head.headers['Content-Length'])
+        self.file = NamedTemporaryFile()
+        self.file.truncate(self.length)
+        self.left: List[int] = []
+        self.right: List[int] = []
+        self.check_zip('bytes' in head.headers.get('Accept-Ranges', 'none'))
+
+    def __enter__(self) -> 'Lazip':
+        self.file.__enter__()
+        return self
+
+    def __exit__(self, *exc) -> Optional[bool]:
+        return self.file.__exit__(*exc)
+
+    @property
+    def name(self):
+        """File name."""
+        return self.file.name
+
+    def seekable(self):
+        """Return whether random access is supported, which is True."""
+        return True
+
+    @contextmanager
+    def stay(self) -> Iterator[None]:
+        """Return a context manager keeping the position.
+
+        At the end of the block, seek back to original position.
+        """
+        pos = self.tell()
+        try:
+            yield
+        finally:
+            self.seek(pos)
+
+    def check_zip(self, range_request: bool):
+        """Check and download until the file is a valid ZIP."""
+        if not range_request:
+            end = self.length - 1
+            self.download(0, end)
+            self.left, self.right = [0], [end]
+            return
+        for start, end in init_range(self.length, self.chunk_size):
+            self.download(start, end)
+            with self.stay():
+                try:
+                    ZipFile(self)
+                except BadZipFile:
+                    pass
+                else:
+                    break
+
+    def stream_response(self, start: int, end: int,
+                        base_headers: Dict[str, str] = HEADERS) -> Response:
+        """Return HTTP response to a range request from start to end."""
+        headers = {'Range': f'bytes={start}-{end}'}
+        headers.update(base_headers)
+        return self.session.get(self.url, headers=headers, stream=True)
+
+    def merge(self, start: int, end: int,
+              left: int, right: int) -> Iterator[Tuple[int, int]]:
+        """Return an iterator of intervals to be fetched.
+
+        Args:
+            start (int): Start of needed interval
+            end (int): End of needed interval
+            left (int): Index of first overlapping downloaded data
+            right (int): Index after last overlapping downloaded data
+        """
+        lslice, rslice = self.left[left:right], self.right[left:right]
+        i = start = min(start, min(lslice, default=start))
+        end = min(end, min(rslice, default=end))
+        for j, k in zip(lslice, rslice):
+            if j > i: yield i, j-1
+            i = k + 1
+        if i <= end: yield i, end
+        self.left[left:right], self.right[left:right] = [start], [end]
+
+    def download(self, start: int, end: int):
+        """Download bytes from start to end inclusively."""
+        with self.stay():
+            i, j = bisect_left(self.right, start), bisect_right(self.left, end)
+            for start, end in self.merge(start, end, i, j):
+                response = self.stream_response(start, end)
+                response.raise_for_status()
+                self.seek(start)
+                for chunk in response_chunks(response, self.chunk_size):
+                    self.file.write(chunk)
+
+    def read(self, size: int = -1) -> bytes:
+        """Read up to size bytes from the object and return them.
+
+        As a convenience, if size is unspecified or -1,
+        all bytes until EOF are returned.  Fewer than
+        size bytes may be returned if EOF is reached.
+        """
+        start = self.tell()
+        stop = start + size if 0 <= size <= self.length-start else self.length
+        self.download(start, stop-1)
+        return self.file.read(size)
+
+    def seek(self, offset: int, whence: int = 0) -> int:
+        """Change stream position and return the new absolute position.
+
+        Seek to offset relative position indicated by whence:
+        * 0: Start of stream (the default).  pos should be >= 0;
+        * 1: Current position - pos may be negative;
+        * 2: End of stream - pos usually negative.
+        """
+        return self.file.seek(offset, whence)
+
+    def tell(self) -> int:
+        """Return the current possition."""
+        return self.file.tell()
+
+    def close(self) -> None:
+        """Close the file."""
+        self.file.close()
+
+
+if __name__ == '__main__':
+    url = ('https://files.pythonhosted.org/packages/17/d9/'
+           'ff8955ce17c080c956cd5eed9c2da4de139d5eeabb9f9ebf2d981acef31d/'
+           'brutalmaze-0.9.2-py3-none-any.whl')
+    with Lazip(Session(), url) as wheel:
+        print(pkg_resources_distribution_for_wheel(
+            ZipFile(wheel), 'brutalmaze', wheel.name).requires())

pyproject.toml

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+[build-system]
+requires = ['flit_core >=2,<3']
+build-backend = 'flit_core.buildapi'
+
+[tool.flit.metadata]
+module = 'lazip'
+author = 'Nguyễn Gia Phong'
+author-email = 'mcsinyx@disroot.org'
+home-page = 'https://github.com/McSinyx/lazip'
+requires = ['pip', 'requests']
+description-file = 'README.md'
+classifiers = [
+    'Development Status :: 1 - Planning',
+    'Intended Audience :: Developers',
+    'License :: OSI Approved :: GNU Lesser General Public License v3 or later (LGPLv3+)',
+    'Natural Language :: English',
+    'Operating System :: OS Independent',
+    'Programming Language :: Python',
+    'Programming Language :: Python :: 3 :: Only',
+    'Topic :: Internet :: WWW/HTTP',
+    'Typing :: Typed']
+requires-python = '>=3.6'
+keywords = 'zip,http,range,lazy'
+license = 'LGPLv3+'

tox.ini

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+[tox]
+envlist = py
+minversion = 3.3
+isolated_build = true
+
+[testenv]
+deps =
+    flake8-builtins
+    isort[requirements]
+commands =
+    flake8
+    isort -c --diff
+
+[flake8]
+hang-closing = True
+ignore = W503, E125, E225, E226, E227, E701, E704
+; See https://github.com/PyCQA/pycodestyle/issues/906
+;max-doc-length = 72