Function to export a requirements.txt from poetry

export.py

from poetry.console.application import Application
from poetry.semver import parse_constraint, VersionUnion


def export_requirements(tag=False):
    app = Application()

    installed_repo = app.poetry.locker.locked_repository(True)

    output = []
    for package in installed_repo.packages:
        if package.source_type:
            name_version = '{}+{}@{}#egg={}'.format(
                package.source_type,
                package.source_url,
                package.version
                if (tag and package.version)
                else package.source_reference,
                package.pretty_name,
            )
        else:
            name_version = '{}=={}'.format(
                package.pretty_name, package.version
            )

        markers = []

        if 'platform' in package.requirements:
            platform = package.requirements['platform']
            if platform.startswith('!='):
                platform = platform[2:]
                operator = '!='
            else:
                operator = '=='
            markers.append("sys_platform {} '{}'".format(operator, platform))

        if 'python' in package.requirements:
            versions = parse_python_versions(package.requirements['python'])
            markers.append(versions)

        markers_delimiter = '; ' if markers else ''

        output.append(
            '{}{}{}'.format(
                name_version, markers_delimiter, " and ".join(markers)
            )
        )

    return '\n'.join(output)


def parse_python_versions(python):
    _and = []
    versions = parse_constraint(python)
    if isinstance(versions, VersionUnion):
        _or = []
        for version_range in versions.ranges:
            _or.append(range_to_requirements_text(version_range))
        if len(_or) > 1:
            _and.append('(' + ') or ('.join(_or) + ')')
        else:
            _and.append(_or)
    else:
        _and.append(range_to_requirements_text(versions))

    return ' and '.join(_and)


def range_to_requirements_text(version_range):
    if version_range.min:
        if version_range.max:
            text = "python_version >= '{}' and python_version < '{}'".format(
                version_range.min, version_range.max
            )
        else:
            text = "python_version > '{}'".format(version_range.min)
    else:
        text = "python_version < '{}'".format(version_range.max)

    return text