imphook 0.5.2

Simple and clear import hooks for Python - import anything as if it were a Python module

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 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:

import imphook

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

imphook.add_import_hook(hook, (".conf",))

Save this as the mod_conf.py file, and add the two following files to test it:

example_settings.conf:

var1 = 123
var2 = hello

example_conf.py:

import example_settings as settings

print(settings.var1)
print(settings.var2)

Now run:

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:

git clone https://github.com/pfalcon/python-imphook

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 and is released under the MIT license.