Error when using importlib.util to check for library

问题

I'm trying to use the importlib library to verify whether the nmap library is installed on the computer executing the script in Python 3.5.2

I'm trying to use importlib.util.find_spec("nmap") but receive the following error.

>>> import importlib
>>> importlib.util.find_spec("nmap")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'importlib' has no attribute 'util'

Can someone tell me where I'm going wrong?

EDIT

I was able to get the function to work using the following code.

#!/usr/bin/pythonw

import importlib
from importlib import util

#check to see if nmap module is installed
find_nmap = util.find_spec("nmap")
if find_nmap is None:
    print("Error")

回答1:

Try this:

from importlib import util
util.find_spec("nmap")

I intend to investigate, but honestly I don't know why one works and the other doesn't. Also, observe the following interactive session:

>>> import importlib
>>> importlib.util
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'importlib' has no attribute 'util'
>>> from importlib import util
>>> util
<module 'importlib.util' from '/usr/lib/python3.5/importlib/util.py'>
>>> importlib.util
<module 'importlib.util' from '/usr/lib/python3.5/importlib/util.py'>

So...yeah. I am sure this makes perfect sense to someone, but not to me. I will update once I figure it out.

Update:

Comparing this to something like:

>>> import datetime
>>> datetime
<module 'datetime' from '/usr/lib/python3.5/datetime.py'>
>>> datetime.datetime
<class 'datetime.datetime'>

I think the difference is that in this case the first datetime is a module and the second is a class, while in the importlib.util case both are modules. So perhaps module.module is not OK unless the code from both modules has been loaded, while module.class is OK, because the class code is loaded when the module is imported.

Update #2

Nope, it seems like in many cases module.module is fine. For example:

>>> import urllib
>>> urllib
<module 'urllib' from '/usr/lib/python3.5/urllib/__init__.py'>
>>> urllib.error
<module 'urllib.error' from '/usr/lib/python3.5/urllib/error.py'>

So perhaps it is something specific to importlib.

Update #3

As @kfb pointed out in the comments, it does seem to be related to importlib specifically. See the following comment from the __init__.py for importlib:

# Until bootstrapping is complete, DO NOT import any modules that attempt
# to import importlib._bootstrap (directly or indirectly). Since this
# partially initialised package would be present in sys.modules, those
# modules would get an uninitialised copy of the source version, instead
# of a fully initialised version (either the frozen one or the one
# initialised below if the frozen one is not available).

importlib/util.py does import importlib._bootstrap so I would assume that this is realted. If my understanding is correct, when you do import importlib the submodules will be initialized, but are not initialized for the importlib module object that you have imported. At this point, if you do dir(importlib) you will not see util. Interestingly, after you have tried to access importlib.util and gotten an AttributeError, util (along with the other submodules) gets loaded/initialized, and now you can access importlib.util!

>>> import importlib
>>> dir(importlib)
['_RELOADING', '__all__', '__builtins__', '__cached__', '__doc__', '__file__', '__import__', '__loader__', '__name__', '__package__', '__path__', '__spec__', '_bootstrap', '_bootstrap_external', '_imp', '_r_long', '_w_long', 'find_loader', 'import_module', 'invalidate_caches', 'reload', 'sys', 'types', 'warnings']
>>> importlib.util
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'importlib' has no attribute 'util'
>>> importlib.util
<module 'importlib.util' from '/usr/lib/python3.5/importlib/util.py'>
>>> dir(importlib)
['_RELOADING', '__all__', '__builtins__', '__cached__', '__doc__', '__file__', '__import__', '__loader__', '__name__', '__package__', '__path__', '__spec__', '_bootstrap', '_bootstrap_external', '_imp', '_r_long', '_w_long', 'abc', 'find_loader', 'import_module', 'invalidate_caches', 'machinery', 'reload', 'sys', 'types', 'util', 'warnings']

来源：https://stackoverflow.com/questions/39660934/error-when-using-importlib-util-to-check-for-library