Updated readme with a clear explanation of the problem statement.

This commit is contained in:
Elf M. Sternberg 2016-06-17 07:53:27 -07:00
parent 1b614cb103
commit df657c6b46
3 changed files with 112 additions and 6 deletions

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**polyloader** is a python module to hook into Python's import machinery Synopsis
and insert your own syntax parser/recognizer. Importlib uses filename --------
suffixes to recognize which compiler to use, but is internally
hard-coded to only recognize ".py" as a valid suffix. **Polyloader** is a python module that enables the discovery and loading
of heterogenous source code packages. This discovery and loading is
critical to the functioning of other programming languages that use the
Python AST and Python VM, languages such as Hy, Doge, and Mochi.
Problem Statement
-----------------
The Python module loader system is hard-coded to prevent the discovery
of heterogenous source code packages. From Python 2.6 through the
current (as of this writing) Python 3.5, the import mechanism allowed
for the creation of file finders and importers that would transform
Python's import syntax into a *path,* assert whether or not that path
could be made to correspond to a *syntax object*, and then attempt to
*load* that syntax object as a Python module. Python *packages*,
however, are assumed to be uniformly made up of Python syntax objects,
be they **.py** source files, **.pyc/.pyo** bytecode, or **.so/.dll**
files with an exposed Python-to-C API. In Python 2 these suffixes are
hard-coded into the source in the **imp** builtin module; in Python 3
these suffixes are constants defined in a private section of
**importlib**; in either case, they are unavailable for modification.
This lack of access to the extensions list prevents the *discovery* of
heterogenous source code packages.
The discovery mechanism is outlined in Python's pkgutil module; features
such as **pkgutil.iter_modules** do not work with heterogenous source
code, which in turn means that one cannot write, for one important
example, Django management commands in an alternative syntax.
**polyloader** is a python module that intercepts calls to the default
finder, loader, and package module iterator, and if the path resolves to
an alternative syntax, provide the appropriate finder, loader and
iterator. **polyloader** is different from traditional importlib shims
in that it directly affects the root loader, and thus allows for the
discovery and importation of suffixes not listed in Python's defaults.
To use: To use:
------- -------

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docs/notes.txt Normal file
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Python 2.7
iter_modules (iter_importers) ->
calls iter_importer_modules for each importer in iter_importers
iter_importers (meta_path, get_importer) ->
returns every importer in sys.meta_path + map(get_importer, sys.path)
get_importer(path):
returns a filtered list of sys.path_hooks for importers that can
handle this path; if there is no match, returns ImpImporter(),
which supplies a module iterator (ImpImporter.iter_modules) that
relies on getmodulename.
* A path_hook is a function of (path -> Maybe importer)
iter_modules(path, get_importer, prefix) ->
calls iter_importer_modules for each importer returned by path.map(get_importer)
iter_importer_modules(importer) ->
returns list of (filename, ispkg) for each module understood by the importer
* The method called depends on the class of the importer
* The default is a generic call for "no specific importer"
* For FILES, iter_import_modules returns a list of files whose
extensions match those in imp.get_suffixes(), which is hard-
coded into the interpreter.
* MEANING: Unless your importer can handle heterogenous module
suffixes, SourceFiles.iter_importer_modules can only find
homogeonous modules.
This relationship issue holds for Python 2.6 as well.
Python 3.3
The same issue holds, although now most of the extensions have been
moved to importlib._bootstrap.
It is the relationship between
importlib.machinery.FileFinder
and
_iter_file_finder_modules
That's killing us.

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@ -47,7 +47,30 @@ class PolyLoader(pkgutil.ImpLoader):
return module return module
class PolyFinder(pkgutil.ImpImporter): # Problem to be solved: pkgutil.iter_modules depends upon
# get_importer, which requires that we uses path_hooks, not meta_path.
# This is acceptable (see: https://pymotw.com/2/sys/imports.html), but
# then it depends upon the inspect get_modulename, which in turn is
# dependent upon the __builtin__.imp.get_suffixes(), which excludes
# anything other than the builtin-recognizes suffixes. The
# arrangement, as of Python 2.7, excludes heterogenous packages from
# being locatable by pkgutil.iter_modules.
#
# iter_modules use of the simplegeneric protocol makes things even
# harder, as the order in which finders are loaded is not available at
# runtime.
#
# Possible solutions: We provide our own pkgutils, which in turn hacks
# the iter_modules; or we provide our own finder and ensure it gets
# found before the native one.
We actually want to have multiple
# SourceFileFinders, each of which either recognizes the file to be
class PolyFinder(object):
def __init__(self, path = None):
self.path = path
def find_on_path(self, fullname): def find_on_path(self, fullname):
fls = ["%s/__init__.%s", "%s.%s"] fls = ["%s/__init__.%s", "%s.%s"]
dirpath = "/".join(fullname.split(".")) dirpath = "/".join(fullname.split("."))