Python 3.x is patched in a way that integrates `.hy` source files into
Pythons default `importlib` machinery. In Python 2.7, a PEP-302 "importer"
and "loader" is implemented according to the standard `import` logic (via
`pkgutil` and later pure-Python `imp` package code).
In both cases, the entry-point for the loaders is through `sys.path_hooks` only.
As well, the import semantics have been updated all throughout to utilize
`importlib` and follow aspects of PEP-420. This, along with some light
patches, should allow for basic use of `runpy`, `py_compile` and `reload`.
In all cases, if a `.hy` file is shadowed by a `.py`, Hy will silently use
`.hy`.
Auto-promotion now occurs in only two cases: when we start the compiler and when we expand a macro. It's fully recursive so even a non-model nested in a model will be promoted.
This change fixes some regressions induced by the stricter type checks of the pattern-matching compiler.
It's rarely useful, because it catches all exceptions, but it doesn't let you do anything other than return `None`. You can still get the same effect with `(except [])`.
HyKeywords are no longer an instances of string with a particular
prefix, but a completely separate object.
This means keywords no longer trip isinstance str checks, adding a
little bit of type safety to the compiler.
It also means that HyKeywords evaluate to themselves.
Closes#1352
Python Class AST expects a body which is a list of ast.Expr. Force
every entry to be stored as a statement. This means we'll preserve
print statements.
Python also doesn't construct docstrings in classes by setting a
__doc__ attribute, it does it by inspecting the first ast.Expr node of
the class. But this means we can remove the special handler for it.
