Distutils/DiscussionOverview/FilePrefixes

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Rationale

It is not possible to retrieve the installation paths of data, or other, files for all installation schemes supported by distutils right now. I propose the inclusion of a PREFIX file within the .egg-info directory that holds information on all prefixes set at installation time and a suitable API within pkgutil.

Proposal

PREFIX

The RECORD file is a CSV file, composed of records, one line per prefix. The csv module is used to read the file, with these options:

field delimiter : ,
quoting char : "
line terminator : os.linesep (so \r\n or \n)

Each record is composed of two elements:

prefix identifier
- Prefix identifiers differentiate between the various prefixes used within the installation process.
  Valid identifiers must adhere to the regular expression ^[a-z_]+, i.e any combination of lowercase ASCII characters and the underscore.
path
- The path for the prefix identifier for this distribution.
  A / seperated path will be used regardless of the target system. The path definition can be either absolute or prefixed with one of the identifiers defined below. An absolute path is only used if the installation location is not relative to sys.prefix ($base) or sys.exec_prefix ($platbase).

Identifiers

The list of standard identifiers comprises:

$base - Base prefix
$platbase - Platform specific base prefix
$purelib - Pure Python distribution
$platlib - Non-pure Python distribution (i.e one with extensions)
$scripts - Executable Python scripts
$data - Data files

Example

Standard scheme installation on posix:

base,/usr
platbase,/usr
purelib,$base/lib/pythonX.Y/site-packages
platlib,$platbase/lib/pythonX.Y/site-packages
scripts,$base/bin
data,$base/share

User scheme installation on posix:

base,/home/sirrobin/.local
platbase,/home/sirrobin/.local
purelib,$base/lib/pythonX.Y/site-packages
platlib,$platbase/lib/pythonX.Y/site-packages
scripts,$base/bin
data,share

Custom installation scheme:

base,/home/sirrobin/.local
platbase,/home/sirrobin/.local
purelib,$base/lib/pythonX.Y/site-packages
platlib,$platbase/lib/pythonX.Y/site-packages
scripts,/usr/local/bin
data,/usr/local/share/

Default values

Tables summarising the default values on different operating systems:

posix

Prefix identifier	Default value: Standard scheme	Default value: User scheme	Environment variable	Command line option
`$base`	`sys.prefix`	`~/.local`	`$PYDIST_BASE`	`--prefix`
`$platbase`	`sys.exec_prefix`	`~/.local`	`$PYDIST_PLATBASE`	`--exec-prefix`
`$purelib`	`$base/lib/pythonX.Y/site-packages`	`$base/lib/pythonX.Y/site-packages`	`$PYDIST_PURELIB`	`--install-purelib`
`$platlib`	`$platbase/lib/pythonX.Y/site-packages`	`$platbase/lib/pythonX.Y/site-packages`	`$PYDIST_PLATLIB`	`--install-platlib`
`$scripts`	`$base/bin`	`$base/bin`	`$PYDIST_SCRIPTS`	`--install-scripts`
`$data`	`$base/share`	`$base/share`	`$PYDIST_DATA`	`--install-data`

nt

Prefix identifier	Default value: Standard scheme	Default value: User scheme	Environment variable	Command line option
`$base`	`sys.prefix`	`os.environ.get('%APPDATA%')`	`$PYDIST_BASE`	`--prefix`
`$platbase`	`sys.exec_prefix`	`os.environ.get('%APPDATA%')`	`$PYDIST_PLATBASE`	`--exec-prefix`
`$purelib`	`$base/Python`	`$base/Python/PythonXY/site-packages`	`$PYDIST_PURELIB`	`--install-purelib`
`$platlib`	`$platbase/Python`	`$platbase/Python/PythonXY/site-packages`	`$PYDIST_PLATLIB`	`--install-platlib`
`$scripts`	`$base/Scripts`	`$base/Python/Scripts`	`$PYDIST_SCRIPTS`	`--install-scripts`
`$data`	`$base/Data`	`$base/Python/Python26`	`$PYDIST_DATA`	`--install-data`

mac

os2

ce

java

riscos

API

The Distribution class will get a new attribute prefixes which holds a dictionary mapping prefix identifiers to their absolute paths.

Usage example:

>>> foo_dist = pkgutil.get_distribution('foo')
>>> foo_dist.prefixes['$base']
'/usr'
>>> foo_dist.prefixes['$data']
'/usr/share'
>>> ni_wish = open(os.path.join(foo_dist.prefixes.get('$data'), 'foo', 'shrubbery.jpg'))
>>> unladen = open(os.path.join(foo_dist.prefixes['$data'], 'foo', 'european.swallow'))