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PEP 508 -- Dependency specification for Python Software Packages

PEP: 508
Title: Dependency specification for Python Software Packages
Author: Robert Collins <rbtcollins at hp.com>
BDFL-Delegate: Donald Stufft <donald at stufft.io>
Discussions-To: distutils-sig < distutils-sig at python.org >
Status: Active
Type: Informational
Created: 11-Nov-2015
Post-History: 05-Nov-2015, 16-Nov-2015
Resolution: https://mail.python.org/pipermail/distutils-sig/2015-November/027868.html

Abstract

This PEP specifies the language used to describe dependencies for packages. It draws a border at the edge of describing a single dependency - the different sorts of dependencies and when they should be installed is a higher level problem. The intent is to provide a building block for higher layer specifications.

The job of a dependency is to enable tools like pip [1] to find the right package to install. Sometimes this is very loose - just specifying a name, and sometimes very specific - referring to a specific file to install. Sometimes dependencies are only relevant in one platform, or only some versions are acceptable, so the language permits describing all these cases.

The language defined is a compact line based format which is already in widespread use in pip requirements files, though we do not specify the command line option handling that those files permit. There is one caveat - the URL reference form, specified in PEP-440 [4] is not actually implemented in pip, but since PEP-440 is accepted, we use that format rather than pip's current native format.

Motivation

Any specification in the Python packaging ecosystem that needs to consume lists of dependencies needs to build on an approved PEP for such, but PEP-426 [3] is mostly aspirational - and there are already existing implementations of the dependency specification which we can instead adopt. The existing implementations are battle proven and user friendly, so adopting them is arguably much better than approving an aspirational, unconsumed, format.

Specification

Examples

All features of the language shown with a name based lookup:

requests [security,tests] >= 2.8.1, == 2.8.* ; python_version < "2.7"

A minimal URL based lookup:

pip @ https://github.com/pypa/pip/archive/1.3.1.zip#sha1=da9234ee9982d4bbb3c72346a6de940a148ea686

Concepts

A dependency specification always specifies a distribution name. It may include extras, which expand the dependencies of the named distribution to enable optional features. The version installed can be controlled using version limits, or giving the URL to a specific artifact to install. Finally the dependency can be made conditional using environment markers.

Grammar

We first cover the grammar briefly and then drill into the semantics of each section later.

A distribution specification is written in ASCII text. We use a parsley [6] grammar to provide a precise grammar. It is expected that the specification will be embedded into a larger system which offers framing such as comments, multiple line support via continuations, or other such features.

The full grammar including annotations to build a useful parse tree is included at the end of the PEP.

Versions may be specified according to the PEP-440 [4] rules. (Note: URI is defined in std-66 [5] ):

version_cmp   = wsp* '<' | '<=' | '!=' | '==' | '>=' | '>' | '~=' | '==='
version       = wsp* ( letterOrDigit | '-' | '_' | '.' | '*' | '+' | '!' )+
version_one   = version_cmp version wsp*
version_many  = version_one (wsp* ',' version_one)*
versionspec   = ( '(' version_many ')' ) | version_many
urlspec       = '@' wsp* <URI_reference>

Environment markers allow making a specification only take effect in some environments:

marker_op     = version_cmp | (wsp* 'in') | (wsp* 'not' wsp+ 'in')
python_str_c  = (wsp | letter | digit | '(' | ')' | '.' | '{' | '}' |
                 '-' | '_' | '*' | '#' | ':' | ';' | ',' | '/' | '?' |
                 '[' | ']' | '!' | '~' | '`' | '@' | '$' | '%' | '^' |
                 '&' | '=' | '+' | '|' | '<' | '>' )
dquote        = '"'
squote        = '\\''
python_str    = (squote (python_str_c | dquote)* squote |
                 dquote (python_str_c | squote)* dquote)
env_var       = ('python_version' | 'python_full_version' |
                 'os_name' | 'sys_platform' | 'platform_release' |
                 'platform_system' | 'platform_version' |
                 'platform_machine' | 'platform_python_implementation' |
                 'implementation_name' | 'implementation_version' |
                 'extra' # ONLY when defined by a containing layer
                 )
marker_var    = wsp* (env_var | python_str)
marker_expr   = marker_var marker_op marker_var
              | wsp* '(' marker wsp* ')'
marker_and    = marker_expr wsp* 'and' marker_expr
              | marker_expr
marker_or     = marker_and wsp* 'or' marker_and
                  | marker_and
marker        = marker_or
quoted_marker = ';' wsp* marker

Optional components of a distribution may be specified using the extras field:

identifer_end = letterOrDigit | (('-' | '_' | '.' )* letterOrDigit)
identifier    = letterOrDigit identifier_end*
name          = identifier
extras_list   = identifier (wsp* ',' wsp* identifier)*
extras        = '[' wsp* extras_list? wsp* ']'

Giving us a rule for name based requirements:

name_req      = name wsp* extras? wsp* versionspec? wsp* quoted_marker?

And a rule for direct reference specifications:

url_req       = name wsp* extras? wsp* urlspec wsp+ quoted_marker?

Leading to the unified rule that can specify a dependency.:

specification = wsp* ( url_req | name_req ) wsp*

Whitespace

Non line-breaking whitespace is mostly optional with no semantic meaning. The sole exception is detecting the end of a URL requirement.

Names

Python distribution names are currently defined in PEP-345 [2] . Names act as the primary identifier for distributions. They are present in all dependency specifications, and are sufficient to be a specification on their own. However, PyPI places strict restrictions on names - they must match a case insensitive regex or they won't be accepted. Accordingly, in this PEP we limit the acceptable values for identifiers to that regex. A full redefinition of name may take place in a future metadata PEP. The regex (run with re.IGNORECASE) is:

^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$

Extras

An extra is an optional part of a distribution. Distributions can specify as many extras as they wish, and each extra results in the declaration of additional dependencies of the distribution when the extra is used in a dependency specification. For instance:

requests[security]

Extras union in the dependencies they define with the dependencies of the distribution they are attached to. The example above would result in requests being installed, and requests own dependencies, and also any dependencies that are listed in the "security" extra of requests.

If multiple extras are listed, all the dependencies are unioned together.

Versions

See PEP-440 [4] for more detail on both version numbers and version comparisons. Version specifications limit the versions of a distribution that can be used. They only apply to distributions looked up by name, rather than via a URL. Version comparison are also used in the markers feature. The optional brackets around a version are present for compatibility with PEP-345 [2] but should not be generated, only accepted.

Environment Markers

Environment markers allow a dependency specification to provide a rule that describes when the dependency should be used. For instance, consider a package that needs argparse. In Python 2.7 argparse is always present. On older Python versions it has to be installed as a dependency. This can be expressed as so:

argparse;python_version<"2.7"

A marker expression evaluates to either True or False. When it evaluates to False, the dependency specification should be ignored.

The marker language is inspired by Python itself, chosen for the ability to safely evaluate it without running arbitrary code that could become a security vulnerability. Markers were first standardised in PEP-345 [2] . This PEP fixes some issues that were observed in the design described in PEP-426 [3] .

Comparisons in marker expressions are typed by the comparison operator. The <marker_op> operators that are not in <version_cmp> perform the same as they do for strings in Python. The <version_cmp> operators use the PEP-440 [4] version comparison rules when those are defined (that is when both sides have a valid version specifier). If there is no defined PEP-440 behaviour and the operator exists in Python, then the operator falls back to the Python behaviour. Otherwise an error should be raised. e.g. the following will result in errors:

"dog" ~= "fred"
python_version ~= "surprise"

User supplied constants are always encoded as strings with either ' or " quote marks. Note that backslash escapes are not defined, but existing implementations do support them. They are not included in this specification because they add complexity and there is no observable need for them today. Similarly we do not define non-ASCII character support: all the runtime variables we are referencing are expected to be ASCII-only.

The variables in the marker grammar such as "os_name" resolve to values looked up in the Python runtime. With the exception of "extra" all values are defined on all Python versions today - it is an error in the implementation of markers if a value is not defined.

Unknown variables must raise an error rather than resulting in a comparison that evaluates to True or False.

Variables whose value cannot be calculated on a given Python implementation should evaluate to 0 for versions, and an empty string for all other variables.

The "extra" variable is special. It is used by wheels to signal which specifications apply to a given extra in the wheel METADATA file, but since the METADATA file is based on a draft version of PEP-426, there is no current specification for this. Regardless, outside of a context where this special handling is taking place, the "extra" variable should result in an error like all other unknown variables.

Marker Python equivalent Sample values
os_name os.name posix , java
sys_platform sys.platform linux , linux2 , darwin , java1.8.0_51 (note that "linux" is from Python3 and "linux2" from Python2)
platform_machine platform.machine() x86_64
platform_python_implementation platform.python_implementation() CPython , Jython
platform_release platform.release() 3.14.1-x86_64-linode39 , 14.5.0 , 1.8.0_51
platform_system platform.system() Linux , Windows , Java
platform_version platform.version() #1 SMP Fri Apr 25 13:07:35 EDT 2014 Java HotSpot(TM) 64-Bit Server VM, 25.51-b03, Oracle Corporation Darwin Kernel Version 14.5.0: Wed Jul 29 02:18:53 PDT 2015; root:xnu-2782.40.9~2/RELEASE_X86_64
python_version platform.python_version()[:3] 3.4 , 2.7
python_full_version platform.python_version() 3.4.0 , 3.5.0b1
implementation_name sys.implementation.name cpython
implementation_version see definition below 3.4.0 , 3.5.0b1
extra An error except when defined by the context interpreting the specification. test

The implementation_version marker variable is derived from sys.implementation.version :

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

if hasattr(sys, 'implementation'):
    implementation_version = format_full_version(sys.implementation.version)
else:
    implementation_version = "0"

Backwards Compatibility

Most of this PEP is already widely deployed and thus offers no compatibility concerns.

There are however a few points where the PEP differs from the deployed base.

Firstly, PEP-440 direct references haven't actually been deployed in the wild, but they were designed to be compatibly added, and there are no known obstacles to adding them to pip or other tools that consume the existing dependency metadata in distributions - particularly since they won't be permitted to be present in PyPI uploaded distributions anyway.

Secondly, PEP-426 markers which have had some reasonable deployment, particularly in wheels and pip, will handle version comparisons with python_full_version "2.7.10" differently. Specifically in 426 "2.7.10" is less than "2.7.9". This backward incompatibility is deliberate. We are also defining new operators - "~=" and "===", and new variables - platform_release , platform_system , implementation_name , and implementation_version which are not present in older marker implementations. The variables will error on those implementations. Users of both features will need to make a judgement as to when support has become sufficiently widespread in the ecosystem that using them will not cause compatibility issues.

Thirdly, PEP-345 required brackets around version specifiers. In order to accept PEP-345 dependency specifications, brackets are accepted, but they should not be generated.

Rationale

In order to move forward with any new PEPs that depend on environment markers, we needed a specification that included them in their modern form. This PEP brings together all the currently unspecified components into a specified form.

The requirement specifier was adopted from the EBNF in the setuptools pkg_resources documentation, since we wish to avoid depending on a de facto, vs PEP specified, standard.

Complete Grammar

The complete parsley grammar:

wsp           = ' ' | '\t'
version_cmp   = wsp* <'<' | '<=' | '!=' | '==' | '>=' | '>' | '~=' | '==='>
version       = wsp* <( letterOrDigit | '-' | '_' | '.' | '*' | '+' | '!' )+>
version_one   = version_cmp:op version:v wsp* -> (op, v)
version_many  = version_one:v1 (wsp* ',' version_one)*:v2 -> [v1] + v2
versionspec   = ('(' version_many:v ')' ->v) | version_many
urlspec       = '@' wsp* <URI_reference>
marker_op     = version_cmp | (wsp* 'in') | (wsp* 'not' wsp+ 'in')
python_str_c  = (wsp | letter | digit | '(' | ')' | '.' | '{' | '}' |
                 '-' | '_' | '*' | '#' | ':' | ';' | ',' | '/' | '?' |
                 '[' | ']' | '!' | '~' | '`' | '@' | '$' | '%' | '^' |
                 '&' | '=' | '+' | '|' | '<' | '>' )
dquote        = '"'
squote        = '\\''
python_str    = (squote <(python_str_c | dquote)*>:s squote |
                 dquote <(python_str_c | squote)*>:s dquote) -> s
env_var       = ('python_version' | 'python_full_version' |
                 'os_name' | 'sys_platform' | 'platform_release' |
                 'platform_system' | 'platform_version' |
                 'platform_machine' | 'platform_python_implementation' |
                 'implementation_name' | 'implementation_version' |
                 'extra' # ONLY when defined by a containing layer
                 ):varname -> lookup(varname)
marker_var    = wsp* (env_var | python_str)
marker_expr   = marker_var:l marker_op:o marker_var:r -> (o, l, r)
              | wsp* '(' marker:m wsp* ')' -> m
marker_and    = marker_expr:l wsp* 'and' marker_expr:r -> ('and', l, r)
              | marker_expr:m -> m
marker_or     = marker_and:l wsp* 'or' marker_and:r -> ('or', l, r)
                  | marker_and:m -> m
marker        = marker_or
quoted_marker = ';' wsp* marker
identifer_end = letterOrDigit | (('-' | '_' | '.' )* letterOrDigit)
identifier    = < letterOrDigit identifier_end* >
name          = identifier
extras_list   = identifier:i (wsp* ',' wsp* identifier)*:ids -> [i] + ids
extras        = '[' wsp* extras_list?:e wsp* ']' -> e
name_req      = (name:n wsp* extras?:e wsp* versionspec?:v wsp* quoted_marker?:m
                 -> (n, e or [], v or [], m))
url_req       = (name:n wsp* extras?:e wsp* urlspec:v wsp+ quoted_marker?:m
                 -> (n, e or [], v, m))
specification = wsp* ( url_req | name_req ):s wsp* -> s
# The result is a tuple - name, list-of-extras,
# list-of-version-constraints-or-a-url, marker-ast or None


URI_reference = <URI | relative_ref>
URI           = scheme ':' hier_part ('?' query )? ( '#' fragment)?
hier_part     = ('//' authority path_abempty) | path_absolute | path_rootless | path_empty
absolute_URI  = scheme ':' hier_part ( '?' query )?
relative_ref  = relative_part ( '?' query )? ( '#' fragment )?
relative_part = '//' authority path_abempty | path_absolute | path_noscheme | path_empty
scheme        = letter ( letter | digit | '+' | '-' | '.')*
authority     = ( userinfo '@' )? host ( ':' port )?
userinfo      = ( unreserved | pct_encoded | sub_delims | ':')*
host          = IP_literal | IPv4address | reg_name
port          = digit*
IP_literal    = '[' ( IPv6address | IPvFuture) ']'
IPvFuture     = 'v' hexdig+ '.' ( unreserved | sub_delims | ':')+
IPv6address   = (
                  ( h16 ':'){6} ls32
                  | '::' ( h16 ':'){5} ls32
                  | ( h16 )?  '::' ( h16 ':'){4} ls32
                  | ( ( h16 ':')? h16 )? '::' ( h16 ':'){3} ls32
                  | ( ( h16 ':'){0,2} h16 )? '::' ( h16 ':'){2} ls32
                  | ( ( h16 ':'){0,3} h16 )? '::' h16 ':' ls32
                  | ( ( h16 ':'){0,4} h16 )? '::' ls32
                  | ( ( h16 ':'){0,5} h16 )? '::' h16
                  | ( ( h16 ':'){0,6} h16 )? '::' )
h16           = hexdig{1,4}
ls32          = ( h16 ':' h16) | IPv4address
IPv4address   = dec_octet '.' dec_octet '.' dec_octet '.' Dec_octet
nz            = ~'0' digit
dec_octet     = (
                  digit # 0-9
                  | nz digit # 10-99
                  | '1' digit{2} # 100-199
                  | '2' ('0' | '1' | '2' | '3' | '4') digit # 200-249
                  | '25' ('0' | '1' | '2' | '3' | '4' | '5') )# %250-255
reg_name = ( unreserved | pct_encoded | sub_delims)*
path = (
        path_abempty # begins with '/' or is empty
        | path_absolute # begins with '/' but not '//'
        | path_noscheme # begins with a non-colon segment
        | path_rootless # begins with a segment
        | path_empty ) # zero characters
path_abempty  = ( '/' segment)*
path_absolute = '/' ( segment_nz ( '/' segment)* )?
path_noscheme = segment_nz_nc ( '/' segment)*
path_rootless = segment_nz ( '/' segment)*
path_empty    = pchar{0}
segment       = pchar*
segment_nz    = pchar+
segment_nz_nc = ( unreserved | pct_encoded | sub_delims | '@')+
                # non-zero-length segment without any colon ':'
pchar         = unreserved | pct_encoded | sub_delims | ':' | '@'
query         = ( pchar | '/' | '?')*
fragment      = ( pchar | '/' | '?')*
pct_encoded   = '%' hexdig
unreserved    = letter | digit | '-' | '.' | '_' | '~'
reserved      = gen_delims | sub_delims
gen_delims    = ':' | '/' | '?' | '#' | '(' | ')?' | '@'
sub_delims    = '!' | '$' | '&' | '\\'' | '(' | ')' | '*' | '+' | ',' | ';' | '='
hexdig        = digit | 'a' | 'A' | 'b' | 'B' | 'c' | 'C' | 'd' | 'D' | 'e' | 'E' | 'f' | 'F'

A test program - if the grammar is in a string grammar :

import os
import sys
import platform

from parsley import makeGrammar

grammar = """
    wsp ...
    """
tests = [
    "A",
    "A.B-C_D",
    "aa",
    "name",
    "name>=3",
    "name>=3,<2",
    "name [fred,bar] @ http://foo.com ; python_version=='2.7'",
    "name[quux, strange];python_version<'2.7' and platform_version=='2'",
    "name; os_name=='a' or os_name=='b'",
    # Should parse as (a and b) or c
    "name; os_name=='a' and os_name=='b' or os_name=='c'",
    # Overriding precedence -> a and (b or c)
    "name; os_name=='a' and (os_name=='b' or os_name=='c')",
    # should parse as a or (b and c)
    "name; os_name=='a' or os_name=='b' and os_name=='c'",
    # Overriding precedence -> (a or b) and c
    "name; (os_name=='a' or os_name=='b') and os_name=='c'",
    ]

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

if hasattr(sys, 'implementation'):
    implementation_version = format_full_version(sys.implementation.version)
    implementation_name = sys.implementation.name
else:
    implementation_version = '0'
    implementation_name = ''
bindings = {
    'implementation_name': implementation_name,
    'implementation_version': implementation_version,
    'os_name': os.name,
    'platform_machine': platform.machine(),
    'platform_python_implementation': platform.python_implementation(),
    'platform_release': platform.release(),
    'platform_system': platform.system(),
    'platform_version': platform.version(),
    'python_full_version': platform.python_version(),
    'python_version': platform.python_version()[:3],
    'sys_platform': sys.platform,
}

compiled = makeGrammar(grammar, {'lookup': bindings.__getitem__})
for test in tests:
    parsed = compiled(test).specification()
    print("%s -> %s" % (test, parsed))

References

[1] pip, the recommended installer for Python packages ( http://pip.readthedocs.org/en/stable/ )
[2] ( 1 , 2 , 3 ) PEP-345, Python distribution metadata version 1.2. ( https://www.python.org/dev/peps/pep-0345/ )
[3] ( 1 , 2 ) PEP-426, Python distribution metadata. ( https://www.python.org/dev/peps/pep-0426/ )
[4] ( 1 , 2 , 3 , 4 ) PEP-440, Python distribution metadata. ( https://www.python.org/dev/peps/pep-0440/ )
[5] The URL specification. ( https://tools.ietf.org/html/rfc3986 )
[6] The parsley PEG library. ( https://pypi.python.org/pypi/parsley/ )
Source: https://github.com/python/peps/blob/master/pep-0508.txt