How to import a module given its name as string?
I'm writing a Python application that takes as a command as an argument, for example:
$ python myapp.py command1
I want the application to be extensible, that is, to be able to add new modules that implement new commands without having to change the main application source. The tree looks something like:
myapp/ __init__.py commands/ __init__.py command1.py command2.py foo.py bar.py
So I want the application to find the available command modules at runtime and execute the appropriate one.
Python defines an __import__ function, which takes a string for a module name:
__import__(name, globals=None, locals=None, fromlist=(), level=0)
The function imports the module name, potentially using the given globals and locals to determine how to interpret the name in a package context. The fromlist gives the names of objects or submodules that should be imported from the module given by name.
So currently I have something like:
command = sys.argv try: command_module = __import__("myapp.commands.%s" % command, fromlist=["myapp.commands"]) except ImportError: # Display error message command_module.run()
This works just fine, I'm just wondering if there is possibly a more idiomatic way to accomplish what we are doing with this code.
Note that I specifically don't want to get in to using eggs or extension points. This is not an open-source project and I don't expect there to be "plugins". The point is to simplify the main application code and remove the need to modify it each time a new command module is added.
With Python older than 2.7/3.1, that's pretty much how you do it.
You can use
exec if you want to as well.
__import__ you can import a list of modules by doing this:
>>> moduleNames = ['sys', 'os', 're', 'unittest'] >>> moduleNames ['sys', 'os', 're', 'unittest'] >>> modules = map(__import__, moduleNames)
Ripped straight from Dive Into Python.
The recommended way for Python 2.7 and 3.1 and later is to use
Import a module. The name argument specifies what module to import in absolute or relative terms (e.g. either pkg.mod or ..mod). If the name is specified in relative terms, then the package argument must be set to the name of the package which is to act as the anchor for resolving the package name (e.g. import_module('..mod', 'pkg.subpkg') will import pkg.mod).
my_module = importlib.import_module('os.path')
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Note: imp is deprecated since Python 3.4 in favor of importlib
As mentioned the imp module provides you loading functions:
imp.load_source(name, path) imp.load_compiled(name, path)
I've used these before to perform something similar.
In my case I defined a specific class with defined methods that were required. Once I loaded the module I would check if the class was in the module, and then create an instance of that class, something like this:
import imp import os def load_from_file(filepath): class_inst = None expected_class = 'MyClass' mod_name,file_ext = os.path.splitext(os.path.split(filepath)[-1]) if file_ext.lower() == '.py': py_mod = imp.load_source(mod_name, filepath) elif file_ext.lower() == '.pyc': py_mod = imp.load_compiled(mod_name, filepath) if hasattr(py_mod, expected_class): class_inst = getattr(py_mod, expected_class)() return class_inst
Nowadays you should use importlib.
Import a source file
The docs actually provide a recipe for that, and it goes like:
import sys import importlib.util file_path = 'pluginX.py' module_name = 'pluginX' spec = importlib.util.spec_from_file_location(module_name, file_path) module = importlib.util.module_from_spec(spec) spec.loader.exec_module(module) # check if it's all there.. def bla(mod): print(dir(mod)) bla(module)
This way you can access the members (e.g, a function "
hello") from your module
pluginX.py -- in this snippet being called
module -- under its namespace; E.g,
If you want to import the members (e.g, "
hello") you can include
pluginX in the in-memory list of modules:
sys.modules[module_name] = module from pluginX import hello hello()
Import a package
Importing a package (e.g.,
pluginX/__init__.py) under your current dir is actually straightforward:
import importlib pkg = importlib.import_module('pluginX') # check if it's all there.. def bla(mod): print(dir(mod)) bla(pkg)
If you want it in your locals:
>>> mod = 'sys' >>> locals()['my_module'] = __import__(mod) >>> my_module.version '2.6.6 (r266:84297, Aug 24 2010, 18:46:32) [MSC v.1500 32 bit (Intel)]'
same would work with