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Incrementally decode bytes into strings and lines

Project description

An incremental decoder of bytes into characters and lines

The DecodeAccumulator class

The DecodeAccumulator class implements an incremental decoder: an object that may be fed bytes (one or several at a time) as they are e.g. read from a network stream or a subprocess's output, and that adds to a result string as soon as enough bytes have been accumulated to produce a character in the specified encoding.

Note that DecodeAccumulator objects are immutable value objects: the add() method does not modify its invocant, but returns a new DecodeAccumulator object instead.

Sample usage:

while True:
    bb = subprocess.stdout.read(1024)
    if len(bb) == 0:
        break
    acc = acc.add(bb)
    assert(not acc.done)
    if acc.splitter.lines:
        # at least one full line was produced
        (acc, lines) = acc.pop_lines()
        print('\n'.join(lines)

if acc.buf:
    print('Leftover bytes left in the buffer!', file=sys.stderr)

if acc.splitter.buf:
    print('Incomplete line: ' + acc.splitter.buf)

final = acc.add(None)
assert(final.splitter.buf == '')
assert(final.splitter.done)
assert(final.done)
if acc.splitter.buf:
    assert(len(final.splitter.lines) == len(acc.splitter.lines) + 1)

The splitter classes: UniversalNewlines, FixedEOLSplitter, NullSplitter

The decode_acc.newlines module provides three classes that may be used to split a text string into lines in different ways. The UniversalNewlines class does its best to simulate the "universal newlines" behavior of file objects. The FixedEOLSplitter class uses a specified string as a line terminator to split on. The NullSplitter class does not do any splitting.

Sample usage:

spl = newlines.UniversalNewlines()
for char in input_string:
    spl = spl.add(char)
spl.add(None)

for (idx, line) in enumerate(spl.lines):
    print('line {idx}: {line}'.format(idx=idx, line=line))

The UTF-8 environment setup functions: detect_utf8_locale(), get_utf8_env()

The decode_acc.util module provides two functions that are useful for setting up an environment in which to run child processes.

The detect_utf8_locale() function runs the external locale command to obtain a list of the supported locale names, and then picks a suitable one to use so that programs are more likely to output valid UTF-8 characters and language-neutral messages. It prefers the C base locale, but if neither C.UTF-8 nor C.utf8 is available, it will fall back to a list of other locale names that are likely to be present on the system.

The get_utf8_env() function invokes detect_utf8_locale() and then returns a dictionary similar to os.environ, but with LC_ALL set to the obtained locale name and LANGUAGE set to an empty string so that recent versions of the gettext library do not choose a different language to output messages in. If a dictionary is passed as the env parameter, get_utf8_env() uses it as a base instead of the value of os.environ.

The configuration classes: Config, ConfigProc

The decode_acc.util module also provides two dataclasses that may be used as base classes for program configuration data, usually runtime settings obtained from command-line options. Their main purpose, however, or rather the main purpose of the ConfigProc class, is to provide automated detection of the UTF-8 locale to use and supply helper methods similar to the functions and classes provided by the subprocess module for starting child processes in an UTF-8 environment.

The Config class has a single field, the verbose boolean flag. It also provides the diag() method that will check the verbose flag and output the specified message to the standard error stream (in order not to intersperse program output and diagnostic messages on the standard output stream) if requested.

The ConfigProc class extends the Config class with three methods, check_call(), check_output(), and Popen(), that accept exactly the same arguments as the corresponding symbols from the subprocess module, but provide default values for some of them. In particular:

  • env uses the environment returned by get_utf8_env()

  • encoding uses the string returned by detect_utf8_locale()

  • shell is set to False unless specified

Sample usage:

import dataclasses

from decode_acc import util as d_util

@dataclasses.dataclass(frozen=True)
class Config(d_util.ConfigProc):
    """Runtime configuration for the hello program."""

    target: str


...
cfg = Config(target="world", verbose=True)
...
cfg.diag(f"About to greet {cfg.target}")
cfg.check_call(["printf", "--", "Hello, %s!\\n", cfg.target])

License and copyright

Copyright (c) 2018 - 2020  Peter Pentchev <roam@ringlet.net>
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.

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