Environment Markers (Platform-Specific Dependencies)

Environment markers (PEP 508) allow you to conditionally include dependencies based on the target environment. Common markers include python_version, sys_platform, os_name, and implementation_name.

requirements.txt with markers:

gunicorn>=21.2 ; sys_platform != "win32"
uvloop>=0.19 ; sys_platform != "win32"
winloop>=0.1 ; sys_platform == "win32"
dataclasses>=0.6 ; python_version < "3.7"
typing-extensions>=4.8 ; python_version < "3.12"
tomli>=2.0 ; python_version < "3.11"

Converted to pyproject.toml:

[project]
dependencies = [
    "gunicorn>=21.2 ; sys_platform != 'win32'",
    "uvloop>=0.19 ; sys_platform != 'win32'",
    "winloop>=0.1 ; sys_platform == 'win32'",
    "dataclasses>=0.6 ; python_version < '3.7'",
    "typing-extensions>=4.8 ; python_version < '3.12'",
    "tomli>=2.0 ; python_version < '3.11'",
]

Available environment markers:

Marker	Description	Example Values
python_version	Python major.minor	"3.11", "3.12"
python_full_version	Full Python version	"3.11.7"
sys_platform	OS platform string	"linux", "win32", "darwin"
os_name	OS name	"posix", "nt"
platform_machine	CPU architecture	"x86_64", "aarch64"
implementation_name	Python implementation	"cpython", "pypy"

Common patterns:

• Cross-platform servers: Use gunicorn on Linux/macOS, waitress on Windows
• Backport packages: Include tomli only for Python < 3.11 (when tomllib was added to stdlib)
• Performance packages: Install uvloop only on non-Windows platforms

Markers are preserved verbatim during conversion. The only difference is quoting style: requirements.txt uses double quotes inside markers, while pyproject.toml strings (already in double quotes) typically use single quotes inside markers.