imphook is a simple, small single-file module and a tool which allows to easily and selectively override Python import machinery for particular file types (including Python source files).
As an example of usage, recently on the python-ideas mailing list, there was a discussion about "arrow functions" for Python (as a syntactic alternative to lambda). A prototype implementation using "imphook" was posted: https://email@example.com/thread/VOVHZUN...
The project is available from:
PyPI: https://pypi.org/project/imphook/ (package includes only the module itself).
Github: https://github.com/pfalcon/python-imphook (assorted usage examples are included in the git repository)
Below is a documentation snapshot:
imphook - Simple and clear import hooks for Python ==================================================
The ``imphook`` module allows to easily define per file type import hooks, i.e. overload or extend import processing for particular file types, without affecting processing of other file types, and at the same time, while ensuring that new processing integrates as seamlessly as possible with normal Python import rules.
Besides the Python-level API to install import hooks, the module also provides command-line interface to run an existing Python script or module with one or more import hooks preloaded (i.e. without modifying existing script source code).
Some but not all things you can easily do using ``imphook`` (most of these require additional modules to do the heavy lifting, ``imphook`` just allows to plug it seamlessly into the Python import system):
* Override importing of (all or some) .py files, to support new syntax or semantics in them. * Import files written using a DSL (domain-specific language) as if they were Python modules. E.g., config or data files. * Import modules written in other language(s), assuming you have an interpreter(s) for them. * Import binary files, e.g. Java or LLVM bytecode.
``imphook`` works both with new, lightweight legacy-free Python API, as promoted by the `Pycopy https://github.com/pfalcon/pycopy`_ Python dialect (the original source of the "easy import hooks" idea), and CPython (the older, reference Python implementation), and with other Python implementations which are CPython-compatible.
Quick Start -----------
Make sure that you already installed ``imphook`` using::
pip3 install -U imphook
Below is a complete example of an import hook module to load ``key = value`` style config files::
def hook(modname, filename): with open(filename) as f: # Create a module object which will be the result of import. mod = type(imphook)(modname) for l in f: k, v = [x.strip() for x in l.split("=", 1)] setattr(mod, k, v) return mod
Save this as the ``mod_conf.py`` file, and add the two following files to test it:
var1 = 123 var2 = hello
import example_settings as settings
python3 -m imphook -i mod_conf example_conf.py
As you can see, the ``example_conf.py`` is able to import ``example_settings.conf`` as if it were a normal Python module.
Besides copy-pasting the above and other examples, you can also clone the Git repository of ``imphook``, which contains various ready-to-use examples::
API to install hooks and hook structure ---------------------------------------
The API of the module consists of one function: `imphook.add_import_hook(hook, ext_tuple)`. *hook* is a name of hook function. *ext_tuple* is a tuple of file extensions the hook function should handle (the leading dot should be included). More often than not, you will want to handle just one extension, so don't forget to use the usual Python syntax with a trailing comma for 1-element tuple, e.g.: ``(".ext",)``. Python modules may not contain a dot (``"."``) in their names (they are used to separate subpackages), so the extension you register may contain multiple dots, e.g. ``".foo.bar"``, with filename ``my_module.foo.bar`` matching it.
It is possible to call `imphook.add_import_hook(hook, ext_tuple)` multiple times to install multiple hooks. The hooks are installed in the stack-like fashion, the last installed will be called first. It is possible to install multiple hooks for the same file extension, and earlier installed hooks may still be called in this case, because a hook function may skip processing a particular file, and let other hooks to take a chance, with default processing happening if no hook handled the import.
The signature and template of the actual hook function is::
def my_hook(modname, filename): # Return None if you don't want to handle `filename`. # Otherwise, load `filename`, create a Python module object, # with name `modname`, populate it based on the loaded file # contents, and return it.
The *modname* parameter is a full module name of the module to import, in the usual dot-separated notation, e.g. ``my_module`` or ``pkg.subp.mod``. For relative imports originated from within a package, this name is already resolved to full absolute name. The *modname* should be used to create a module object with the given name.
The *filename* parameter is a full pathname (with extension) of the file which hook should import. This filename is known to exist, so you may proceed to open it directly. You may skip processing this file by returning ``None`` from the hook, then other hooks may be tried, and default processing happens otherwise (e.g. ``.py`` files are loaded as usual, or ImportError raised for non-standard extensions). For package imports, the value of *filename* ends with ``/__init__.py``, and that is the way to distinguish module vs package imports.
If the hook proceeds with the file, it should load it by whatever means suitable for the file type. File types which are not natively supported by Python would require installing and using other extension modules (beyond ``imphook``). After loading the file, the hook should create an empty Python module object which will be the result of the import. There are a few ways to do that:
* The baseline is to call a module type as a constructor. To get a module type, just apply the usual ``type()`` function to an existing (imported) module. You'll definitely have ``imphook`` itself imported, which leads us to::
mod = type(imphook)(modname)
The parameter to constructor is the name of module to create, as passed to the hook. * If the above looks too magic for you, you can import symbolic name for module type from the ``types`` module::
from types import ModuleType mod = ModuleType(modname)
* Finally, you may use the ``imp`` module, which may be as well the clearest (to the newbie) way of doing it::
import imp mod = imp.new_module(modname)
But mind that the ``imp`` module is considered deprecated.
Of the choices above, the first is the most efficient - no need to import additional modules, and it's just one line. And once you saw and were explained what it does, it shouldn't be a problem to remember and recognize it later.
Once the module object is created as discussed above, you should populate it. A way to do that is by using ``setattr()`` builtin to set a particular attribute of a module to a particular value. Attributes usually represent variables with data values, but may be also functions and classes.
Finally, you just return the populated module object.
In case you want to perform custom transformation on the Python source, the process is usually somewhat different, where you transform a representation of the source, and then execute it in the context of a new module, which causes it to be populated.
Using import hooks in your applications ---------------------------------------
There are 2 ways to use import hook(s) in you Python programs: either preloading them before starting your program using ``imphook`` command-line runner (next section) or load them explicitly at the startup of your application. Crucial thing to remember that import hooks apply: a) for imports only; b) for imports appearing after the hook was installed.
The main file of our application is normally *not imported*, but executed directly. This leads to the following pattern in structuring your application source files:
* Have a "startup file", which is the one which user will actually run, so name it appropriately. In that file, you load import hooks and perform other baseline system-level initialization. * The main functionality of your application is contained in seperate module(s). The startup script imports such a main module and executes it (e.g., by calling a function from it).
You already grasped how that works - as the "main" module is *imported*, whatever hooks the "startup" script installed, will apply to it.
This pattern is actually officially encoded in the structure of Python *packages*. And any non-trivial Python application will likely be a package with a few sub-modules in it, so you can as well structure your applications this way (as a package), even if they start simple (only one submodule initially). So, if you try to "run" a package, what actually gets run is ``__main__`` submodule in that package. That's exactly the "startup" file we discussed above. It installs the import hooks, and imports a submodule with actual application's functionality.
Credits and licensing ---------------------
``imphook`` is (c) `Paul Sokolovsky https://github.com/pfalcon`_ and is released under the MIT license.