PEP 496 – Environment Markers

PEP: 496
Title: Environment Markers
Author: James Polley <jp at jamezpolley.com>
BDFL-Delegate: Nick Coghlan <ncoghlan at gmail.com>
Status: Rejected
Type: Informational
Created: 03-Jul-2015

PEP Status
Abstract
Rationale
Examples
Micro-language
- Strings
- Version numbers
References
Copyright

PEP Status

After this PEP was initially drafted, PEP 508 was developed and submitted to fully specify the dependency declaration syntax, including environment markers. As a result, this PEP ended up being rejected in favour of the more comprehensive PEP 508.

Abstract

An environment marker describes a condition about the current execution environment. They are used to indicate when certain dependencies are only required in particular environments, and to indicate supported platforms for distributions with additional constraints beyond the availability of a Python runtime.

Environment markers were first specified in PEP-0345 1. PEP-0426 2 (which would replace PEP-0345) proposed extensions to the markers. When 2.7.10 was released, even these extensions became insufficient due to their reliance on simple lexical comparisons, and thus this PEP has been born.

Rationale

Many Python packages are written with portability in mind.

For many packages this means they aim to support a wide range of Python releases. If they depend on libraries such as argparse - which started as external libraries, but later got incorporated into core - specifying a single set of requirements is difficult, as the set of required packages differs depending on the version of Python in use.

For other packages, designing for portability means supporting multiple operating systems. However, the significant differences between them may mean that particular dependencies are only needed on particular platforms (relying on pywin32 only on Windows, for example)”

Environment Markers attempt to provide more flexibility in a list of requirements by allowing the developer to list requirements that are specific to a particular environment.

Examples

Here are some examples of such markers inside a requirements.txt:

pywin32 >=1.0 ; sys_platform == 'win32'
unittest2 >=2.0,<3.0 ; python_version == '2.4' or python_version == '2.5'
backports.ssl_match_hostname >= 3.4 ; python_version < '2.7.9' or (python_version >= '3.0' and python_version < '3.4')

And here’s an example of some conditional metadata included in setup.py for a distribution that requires PyWin32 both at runtime and buildtime when using Windows:

setup(
  install_requires=["pywin32 > 1.0 : sys.platform == 'win32'"],
  setup_requires=["pywin32 > 1.0 : sys.platform == 'win32'"]
  )

Micro-language

The micro-language behind this is as follows. It compares:

strings with the == and in operators (and their opposites)
version numbers with the <, <=, >=, and < operators in addition to those supported for strings

The usual boolean operators and and or can be used to combine expressions, and parentheses are supported for grouping.

The pseudo-grammar is

MARKER: EXPR [(and|or) EXPR]*
EXPR: ("(" MARKER ")") | (STREXPR|VEREXPR)
STREXPR: STRING [STRCMPOP STREXPR]
STRCMPOP: ==|!=|in|not in
VEREXPR: VERSION [VERCMPOP VEREXPR]
VERCMPOP: (==|!=|<|>|<=|>=)

SUBEXPR is either a Python string (such as 'win32') or one of the Strings marker variables listed below.

VEREXPR is a PEP-0440 3 version identifier, or one of the Version number marker variables listed below. Comparisons between version numbers are done using PEP-0440 semantics.

Strings

os_name: os.name
sys_platform: sys.platform
platform_release: platform.release()
implementation_name: sys.implementation.name
platform_machine: platform.machine()
platform_python_implementation: platform.python_implementation()

If a particular string value is not available (such as sys.implementation.name in versions of Python prior to 3.3), the corresponding marker variable MUST be considered equivalent to the empty string.

If a particular version number value is not available (such as sys.implementation.version in versions of Python prior to 3.3) the corresponding marker variable MUST be considered equivalent to 0

Version numbers

python_version: platform.python_version()[:3]
python_full_version: see definition below
platform_version: platform.version()
implementation_version: see definition below

The python_full_version and implementation_version marker variables are derived from sys.version_info and sys.implementation.version respectively, in accordance with the following algorithm:

def format_full_version(info):
    version = '{0.major}.{0.minor}.{0.micro}'.format(info)
    kind = info.releaselevel
    if kind != 'final':
        version += kind[0] + str(info.serial)
    return version

python_full_version = format_full_version(sys.version_info)
implementation_version = format_full_version(sys.implementation.version)