66 lines
2.6 KiB
ReStructuredText
66 lines
2.6 KiB
ReStructuredText
Synopsis
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--------
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**Polyloader** is a python module that extends the Python `import`
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statement to enable the discovery and loading of heterogenous source
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code packages.
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Say What?
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---------
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The ``import`` statement is how the Python interpreter finds a module
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written in Python and loads it, turning it into variables, executable
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functions, constructable classes, and other Python objects, and then
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exposes those objects to the currently running program.
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The ``import`` statement has long been extensible so that things other
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than Python code could be imported, but this feature has always had two
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limitations:
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1. It's annoyingly hard to write an importer. (Believe me. Polyloader
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*is* one!)
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2. For filesystem-based modules (which is 99% of them) Python's importer
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only understands one loader type per directory. It's not possible to
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store code or data written in something other than Python in the same
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directory with Python module code and load it via ``import``.
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The former requires a certain degree of abstraction and thought. For
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the latter, most people ignore the problem and load module configuration
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files written in JSON or YAML or whatever directly. This is fine,
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except when you want to write in one of Python's extended languages like
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Hy or Coconut in a framework like Django, Flask or Glitch.
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**Polyloader** eliminates these limitations.
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What's the real problem?
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------------------------
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The real problem is that Python's traditional extensions, ``.py``,
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``.pyc/.pyo``, and ``.so/.dll`` files, are hard-coded in Python. In
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Python 2, they're in the ``imp`` builtin; In Python 3, they're defined
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in a private section of `importlib`. Either way, they're not accessible
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for modification and extension.
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This problem is made harder by the ``pkglib`` module, which is part of
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Python's standard library. This module uses ``inspect.getmoduleinfo``,
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which again only recognizes the usual extensions. Which means you can't
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list multilingual modules either; this hampers the development of Django
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management commands in a syntax other than Python.
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What the solution?
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------------------
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At its heart, the Python import system runs two different internal
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mechanisms to figure out what the *import string* (the dotted terms
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after the word "import") "means." Each mechanism has one or more
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*finders*, and the first finder to report "I have a *loader* that knows
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what that import string means" wins.
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The very last finder is for the filesystem. The solution is to get in
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front of that finder with one that can handle all the other syntax
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loaders *and* knows how to fall back on the last one for those files the
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last one handles.
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That's what ``polyloader`` does.
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