dynamic module import?

[Lawson Hanson]

Is it possible to import a module of Python code
where I do not know the name of the module
until run-time?

The Python statement:

from someModule import *

requires that "someModule" be the name of the module,
but what I would like is to be able to define a value
for "someModule" ... and provided that such a module
exists (in an extended "sys.path" directory), then
import from the specified module at run-time

If I have a module called "dummy.py" in my own "myModules"
directory (just below my HOME directory in Linux)

If I do this:

import os, sys

myModDir = os.environ["HOME"] + "/myModules"

sys.path.append(myModDir)

modName = "%s/%s" % (myModDir, "dummy")

from modName import *

I get the following error:

ImportError: No module named modName

So is there any way to get Python to import the named module
without just doing "from dummy import *", because I will
not know that the user wants to use the "dummy" module
until run-time ... I'm trying to import control data
for different run scenarios which will be defined in
differently named Python modules which the user will
specify at run-time with a command-line option

And help with this would be most appreciated

Best regards,

Lawson Hanson

 

[Chris Rebert]

Is it possible to import a module of Python code
where I do not know the name of the module
until run-time?

Yes. Use the __import__() built-in function. See
http://docs.python.org/library/functions.html#__import__ for details.

Cheers,
Chris

 

[Steven D'Aprano]

modu = "os"
exec("from " + modu + " import *")

"from module import *" is generally frowned upon, although it does
occasionally have its uses.

By the way, I think your computer's clock is set wrong. My newsclient is
reporting that you sent your post at 4:30pm today -- and I'm writing this
at 11am. Perhaps your timezone is set wrong?

Regards,

 

[alex23]

modu = "os"
exec("from " + modu + " import *")

And of course, there's the usual disclaimer that this should be only
used in circumstances where you can guarantee the contents of 'modu'
aren't ever going to be anything like:

modu = "os import system; system('rm *'); from os "

Another reason to prefer __import__ over exec is speed:

$ /usr/lib/python2.5/timeit.py -n 100000 'from os import system'
100000 loops, best of 3: 2.12 usec per loop

$ /usr/lib/python2.5/timeit.py -n 100000 '__import__("os", fromlist=
["system"])'
100000 loops, best of 3: 2.72 usec per loop

$ /usr/lib/python2.5/timeit.py -n 100000 'm = "os"; exec("from " + m +
" import system")'
100000 loops, best of 3: 25.1 usec per loop

 

[Eduardo Lenz]

Is it possible to import a module of Python code
where I do not know the name of the module
until run-time?

The Python statement:

from someModule import *

requires that "someModule" be the name of the module,
but what I would like is to be able to define a value
for "someModule" ... and provided that such a module
exists (in an extended "sys.path" directory), then
import from the specified module at run-time

If I have a module called "dummy.py" in my own "myModules"
directory (just below my HOME directory in Linux)

If I do this:

import os, sys

myModDir = os.environ["HOME"] + "/myModules"

sys.path.append(myModDir)

modName = "%s/%s" % (myModDir, "dummy")

from modName import *

I get the following error:

ImportError: No module named modName

So is there any way to get Python to import the named module
without just doing "from dummy import *", because I will
not know that the user wants to use the "dummy" module
until run-time ... I'm trying to import control data
for different run scenarios which will be defined in
differently named Python modules which the user will
specify at run-time with a command-line option

And help with this would be most appreciated

Best regards,

Lawson Hanson

modu = "os"
exec("from " + modu + " import *")

[]'s
Lenz.

--

Eduardo Lenz Cardoso
Dr. Eng.
Associate Professor

State University of Santa Catarina
Department of Mechanical Engineering
89223-100 - Joinville-SC - Brasil

Tel: +55 47 4009-7971 - Fax: +55 47 4009-7940
E-mail: (e-mail address removed)
---------------------------------------------

 

[Steven D'Aprano]

And of course, there's the usual disclaimer that this should be only
used in circumstances where you can guarantee the contents of 'modu'
aren't ever going to be anything like:

modu = "os import system; system('rm *'); from os "

Another reason to prefer __import__ over exec is speed:

Both very good points, but consider that you're not comparing apples with
apples.

from os import system
system

del system

exec "from %s import system" % "os"
system

del system

__import__("os", fromlist=["system"])

system

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'system' is not defined

I mention this only to be pedantic, because I agree with your point that
exec can introduce security issues, and be significantly slower.

 

[Eduardo Lenz]

"from module import *" is generally frowned upon, although it does
occasionally have its uses.

By the way, I think your computer's clock is set wrong. My newsclient is
reporting that you sent your post at 4:30pm today -- and I'm writing this
at 11am. Perhaps your timezone is set wrong?

Regards,

OK,

my point was about the exec statement, not the import * (it was just an
example). Well, my clock is OK here..maybe the time zone is wrong. I will
check this out. Thanks for the advice.

[]'s
Lenz.

--

Eduardo Lenz Cardoso
Dr. Eng.
Associate Professor

State University of Santa Catarina
Department of Mechanical Engineering
89223-100 - Joinville-SC - Brasil

Tel: +55 47 4009-7971 - Fax: +55 47 4009-7940
E-mail: (e-mail address removed)
---------------------------------------------

 

[alex23]

Both very good points, but consider that you're not comparing apples with
apples.

__import__("os", fromlist=["system"]) <module 'os' from '/usr/lib/python2.5/os.pyc'
system

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'system' is not defined

I must confess I've rarely had a need to use __import__ and don't
think I've ever used the fromlist arg. I'm confused, though, because
the docstring states:

The fromlist should be a list of names to emulate ``from name
import ...''

But it also states that __import__ always returns a module, so I'm
utterly confused as to the purpose of fromlist, or how to inject the
specified entries into the calling namespace. If anyone could explain
this for me, I'd really appreciate it.

I mention this only to be pedantic, because I agree with your point that
exec can introduce security issues, and be significantly slower.

Correcting misinformation isn't pedantry, especially when I've learned
something

 

[alex23]

[a nice concise explanation on __import__ & fromlist]

Cheers, Duncan, that explained it perfectly.