Question

Let’s say A is a package directory, B is a module within the directory, and X is a function or variable written in B. How can I import X using the __import__() syntax? Using scipy as an example:

What I want:

from scipy.constants.constants import yotta

What doesn’t work:

>>> __import__("yotta", fromlist="scipy.constants.constants")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named yotta

>>> __import__("yotta", fromlist=["scipy.constants.constants"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named yotta

>>> __import__("yotta", fromlist=["scipy","constants","constants"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named yotta

>>> __import__("scipy.constants.constants.yotta", fromlist=["scipy.constants.constats"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named yotta

Any suggestions would be much appreciated.

Answer 1

The python import statement performs two tasks: loading the module and makeing it available in the namespace.

import foo.bar.baz 

will provide the name foo in the namespace, not baz, so __import__ will give you foo

foo = __import__('foo.bar.baz')

On the other hand

from foo.bar.baz import a, b

does not make a module available, but what the import statement needs to perform the assignmaents is baz. this corresponds to

_tmp_baz = __import__('foo.bar.baz', fromlist=['a', 'b'])
a = _tmp_baz.a
b = _tmp_baz.b

without making the temporary visible, of course.

the __import__ function does not enforce the presence of a and b, so when you want baz you can just give anything in the fromlist argument to put __import__ in the “from input” mode.

So the solution is the following. Assuming ‘yotta’ is given as a string variable, I have used getattr for attribute access.

yotta = getattr(__import__('scipy.constants.constants', 
                           fromlist=['yotta']), 
                'yotta')