#!/usr/bin/env python
# cardinal_pythonlib/compression.py
"""
===============================================================================
Original code copyright (C) 2009-2022 Rudolf Cardinal (rudolf@pobox.com).
This file is part of cardinal_pythonlib.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
https://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
===============================================================================
Compression functions.
"""
import gzip
# =============================================================================
# gzip compression/decompression
# =============================================================================
[docs]def gzip_string(text: str, encoding: str = "utf-8") -> bytes:
"""
Encodes a string, then compresses it with gzip.
When you send data over HTTP and wish to compress it, what should you do?
- Use HTTP ``Content-Encoding``;
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding.
- This is defined in HTTP/1.1; see https://www.ietf.org/rfc/rfc2616.txt.
- The gzip format is the most broadly supported, according to
https://en.wikipedia.org/wiki/HTTP_compression.
- This format is defined in https://www.ietf.org/rfc/rfc1952.txt.
- The gzip format has a header; see above and
https://en.wikipedia.org/wiki/Gzip.
- Python's :func:`gzip.compress` writes to a memory file internally and
writes the header.
- So the work in the most popular answer here is unnecessary:
https://stackoverflow.com/questions/8506897/how-do-i-gzip-compress-a-string-in-python
- All we need is conversion of the string to bytes (via the appropriate
encoding) and then :func:`gzip.compress`.
- The requestor should also set HTTP ``Accept-Encoding`` if it wants
compressed data back. See RFC2616 again (as above).
Args:
text:
a string to compress
encoding:
encoding to use when converting string to bytes prior to
compression
Returns:
bytes: gzip-compressed data
Test code:
.. code-block:: python
import io
import gzip
teststring = "Testing; one, two, three."
testbytes = teststring.encode("utf-8")
gz1 = gzip.compress(testbytes)
out1 = io.BytesIO()
with gzip.GzipFile(fileobj=out1, mode="w") as gzfile1:
gzfile1.write(testbytes)
gz2 = out1.getvalue()
print(len(gz1) == len(gz2)) # False
print(gz1 == gz2) # False
# ... but the difference is probably in the timestamp bytes!
""" # noqa
data = text.encode(encoding)
return gzip.compress(data)
[docs]def gunzip_string(zipped: bytes, encoding: str = "utf-8") -> str:
"""
Reverses :func:`gzip_string`, i.e. un-gzip-compresses it, then decodes it
into a string.
Args:
zipped:
zipped data
encoding:
encoding that was used for the string prior to compression
Returns:
str: text
Raises:
- :exc:`OsError` if the data wasn't gzipped
- :exc:`UnicodeDecodeError` if the decompressed data won't decode
"""
data = gzip.decompress(zipped)
return data.decode(encoding)