Das deutsche Python-Forum

• Zugriff auf Zeitzonen durch das neue zoneinfo-Modul
• Leichteres Zusammenführen von Wörterbüchern durch den Union-Operator (|)
• Zusatzinfos bei Type-Hints nun möglich durch Annotated
• Umstellung auf einen PEG-Parser (bisher wurde LL(1) genutzt) für weitere moderne Konstrukte in der Python-Syntax
• Einführung von removeprefix() und removesuffix() für Strings
• Nutzung von Builtin-Typen für Annotations (typing.List[int] -> list[int])

from pathlib import Path

desktop = Path.home() / "Desktop"
file1 = desktop / "test_file_1.txt"
file2 = desktop / "test_file_2.txt"


with (
    file1.open("w") as fd1,
    file2.open("w") as fd2,
):
    fd1.write("Hello World 1\n")
    fd2.write("Hello World 2\n")

PS C:\Users\Admin\Desktop> py -3.8 .\peg_py.py
File ".\peg_py.py", line 9
file1.open("w") as fd1,
^
SyntaxError: invalid syntax
PS C:\Users\Admin\Desktop> py -3.9 .\peg_py.py
PS C:\Users\Admin\Desktop>

The preferred way of wrapping long lines is by using Python’s implied line continuation inside parentheses, brackets and braces. Long lines can be broken over multiple lines by wrapping expressions in parentheses. These should be used in preference to using a backslash for line continuation.

The match statement uses a soft keyword, and it is one of the first major Python features to take advantage of the capabilities of the new PEG parser.

The difference between hard and soft keywords is that hard keywords are always reserved words, even in positions where they make no sense (e.g. x = class + 1), while soft keywords only get a special meaning in context. Since PEP 617 the parser backtracks, that means that on different attempts to parse a code fragment it could interpret a soft keyword differently.

For example, suppose the parser encounters the following input:

Code: Alles auswählen

match [x, y]:

The parser first attempts to parse this as an expression statement. It interprets match as a NAME token, and then considers [x, y] to be a double subscript. It then encounters the colon and has to backtrack, since an expression statement cannot be followed by a colon. The parser then backtracks to the start of the line and finds that match is a soft keyword allowed in this position. It then considers [x, y] to be a list expression. The colon then is just what the parser expected, and the parse succeeds.

import sys
from collections import defaultdict
from itertools import chain
from pprint import pprint
from subprocess import DEVNULL, CalledProcessError, check_output
from graphlib import TopologicalSorter


def get_deps(package):
    packages = []
    cmd = ("apt-cache", "depends", package)
    stdout = check_output(cmd, encoding="utf8", stderr=DEVNULL)
    try:
        for line in stdout.splitlines():
            if "Depends:" in line:
                _, p = line.split(":", maxsplit=1)
                if "<" in p:
                    continue
                packages.append(p.strip())
    except CalledProcessError:
        return []
    else:
        return packages


def main(p):
    deps = defaultdict(set)
    deps[p] = set(get_deps(p))
    while missing := (set(chain.from_iterable(deps.values())) - deps.keys()):
        for dep in missing:
            if dep not in deps:
                deps[dep] = set(get_deps(dep))
    # remove cyclic redundancies
    for name, p_deps in deps.items():
        for sub in p_deps:
            if sub in deps and name in deps[sub]:
                print("Removing:", name)
                deps[sub].remove(name)
    return dict(deps)


if __name__ == "__main__":
    if len(sys.argv) != 2:
        print(sys.executable, sys.argv[0], "package-name")
        sys.exit(1)
    package = sys.argv[1].strip().lower()
    deps = main(package)
    tps = TopologicalSorter(deps)
    tps.prepare()
    while tps.is_active():
        to_install = tps.get_ready()
        print(to_install)
        for install in to_install:
            tps.done(install)

andre@DESKTOP-F29NT09:/mnt/c/Users/Admin$ python dependencies2.py python3
Removing: libc6
Removing: libc6
('mime-support', 'libcrypt1', 'gcc-10-base')
('libgcc-s1',)
('libc6',)
('libdb5.3', 'libbz2-1.0', 'liblzma5', 'libexpat1', 'libsqlite3-0', 'libzstd1', 'libtinfo6', 'libffi7', 'libmpdec2', 'libuuid1', 'zlib1g', 'libacl1', 'libpcre2-8-0')
('libncursesw6', 'libselinux1')
('tar',)
('dpkg',)
('install-info', 'perl-base')
('readline-common', 'debconf')
('libreadline8', 'libssl1.1')
('libpython3.8-minimal',)
('python3.8-minimal', 'libpython3.8-stdlib')
('python3-minimal', 'python3.8', 'libpython3-stdlib')
('python3',)