如何在python中导入部分模块？

Question

I need to use a python module (available in some library). The module looks like this:

class A:
  def f1():
  ...

print "Done"
...

I need only the functionality of class A. However, when I import the module, the code at bottom (print and others) gets executed. Is there a way to avoid that? Essentially I need to import part of a module: "from module1 import A" which should import only A. Is it possible?

Answer 1

Yes, sure:

from module1 import A

Is the general syntax. For example:

from datetime import timedelta

The code at the bottom should be protected from running at import time by being wrapped like so:

if __name__ == "__main__":
  # Put code that should only run when the module
  # is used as a stand-alone program, here.
  # It will not run when the module is imported.

Answer 2

If you're only annoyed by the print statements, you could redirect the code's output to somewhere invisible, like explained in one comment of this post: http://coreygoldberg.blogspot.com/2009/05/python-redirect-or-turn-off-stdout-and.html

sys.stdout = open(os.devnull, 'w')
# now doing the stuff you need
...

# but do not forget to come back!
sys.stdout = sys.__stdout__

Documentation: http://docs.python.org/library/sys.html#sys.stdin

But if you want to deactivate file modifications, or time-consuming code, the only thing that comes to my mind is some dirty trick: copy the objects you need in another file, then import it (but I do not recommend it!).

Answer 3

In addition to @unwind's answer the usual way of doing this is to protect the code in the module that should only be run if the module is used directly with:

if __name__ == "__main__":
    <code to only execute if module called directly>

That way you can import the module normally.