Python: Dynamic “from” import

Question

So i'm trying to turn a bunch of "from x import x" statements, that look like this:

from class_foo import class_foo

into something dynamic. I'm trying to pass a path to a directory and have it import all of the modules therein.

def dynamicImport(dirPath):
    filez = os.listdir(dirPath)
    for file in filez:
        if "class" in file:
            oname = file[:-3] #cut off the file extension, trivial

            imp_statement = "from " + oname + " import " + oname
            #when I print imp_statement, I can verify it's being concatenated correctly

            exec(imp_statement)

When I run this function and pass it a path, the statement strings are being created properly and it produces no errors , yet then later I'll try to access one of the imported objects, and this happens:

foo = class_foo()

NameError: name 'class_foo' is not defined

Clearly I'm doing something wrong. Any help would be appreciated.

Answer 1

You're exec ing your import statement in your function's local namespace, so that's where the names are defined. This namespace goes away when the function ends, leaving you with nothing. What you probably want is something like exec imp_statement in globals() .

Why not just use __import__() instead of string-munging? Then you get a reference to your module. You can then fish out the class reference using getattr() on the module object and insert that into globals() (or just pass a dictionary back to the caller, who can then do globals().update() with it).

import sys, os

def getClasses(directory):
    classes = {}
    oldcwd = os.getcwd()
    os.chdir(directory)   # change working directory so we know import will work
    for filename in os.listdir(directory):
        if filename.endswith(".py"):
            modname = filename[:-3]
            classes[modname] = getattr(__import__(modname), modname)
    os.setcwd(oldcwd)
    return classes

globals().update(getClasses(r"C:\plugin_classes"))

Something like that. Or rather than updating globals() with your modules, which could clobber a global variable you care about, just leave the classes in the dictionary and reference them from there:

classes = getClasess(r"C:\plugin_classes")
for clas in classes.itervalues():
    instance = clas(1, 2, 3)       # instantiate
    instance.dosomething_cool(42)  # call method

Answer 2

Python >= 2.7 has importlib (you can pip install importlib to use importlib in earlier versions of python)

module = importlib.import_module("path.to.module")
MyClass = module.MyClass

Answer 3

It's been a long time since I've worked with Python. But I think your problem might lay in the fact that "oname" is a string. The line from class_foo import class_foo is not a string. One painstaking option would be to have your code make a whole new .py file that would have all of the imports. So you would write all of your current file plus the new imports to, basically, a text file ending in .py

Answer 4

看一下__import__函数