Boštjan Mejak

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Jul 21, 2015, 1:54:22 PM7/21/15

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def tryToImport(module):

try:

import module

except ImportError:

print("The import failed.")

As you can see from my code above, the function tryToImport() takes a 'module' argument which is then taken as a module to be imported. But I always get "The import failed." message. Why does my code not work? No module can be imported this way, even the built-in Python modules. How can I refactor my code to make this work?

Werner

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Jul 21, 2015, 2:15:11 PM7/21/15

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maybe using python's importlib

Werner

Boštjan Mejak

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Jul 21, 2015, 4:41:06 PM7/21/15

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I was thinking something along the same lines. What about, instead of using the importlib, doing __import__(module)?

Boštjan Mejak

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Jul 21, 2015, 4:53:34 PM7/21/15

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My suggestion works like a charm. But one refinement is needed: __import__("module"). The argument, i.e. the name of the module to be imported, needs to be enclosed in parentheses.

But now I am baffled. Why import module doesn't work and __import__("module") does work? What is the difference here?

Boštjan Mejak

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Jul 21, 2015, 5:02:52 PM7/21/15

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Oops, forget what I said about "enclosed in parentheses". No parentheses needed. My bad.

So, anyway, why is the attempt with the import statement not working and is working by using the __import__() function?

Boštjan Mejak

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Jul 21, 2015, 5:09:31 PM7/21/15

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Ah, my code doesn't work at all. Fucking shit! The module that I import via importer.tryToImport("sys") doesn't actually import the sys module. What the fuck should I do?!

tjohnson

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Jul 21, 2015, 6:30:57 PM7/21/15

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__import__(module) cannot be used as a drop-in replacement for import module, because it returns the imported module rather than initializing it as a variable. This link may be helpful: http://effbot.org/zone/import-string.htm

There are several alternatives to using the import statement or the __import__() function:

You can use exec(), which is probably undesirable.
You can use the imp module.
You can use the importlib module (as Werner suggested).

These links may also be helpful:

Boštjan Mejak

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Jul 22, 2015, 6:13:31 AM7/22/15

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I am trying and trying and I can't actually import the module name given as an argument to my function tryToImport(). The import module does not work, then __import__(module) doesn't work either, neither does module = __import__(module). Also tried importlib.import_module(module) and that also doesn't work. Nothing on this world can't import a module, just the import statement.

Boštjan Mejak

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Jul 22, 2015, 6:22:41 AM7/22/15

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def tryToImport(module):

try:

import importlib

importlib.import_module(module)

except ImportError:

print("The import failed.")

Try to create the exact same function, as is above, in your interpreter and then do this:

tryToImport("wx")

You will notice that everything went smoothly, until you do this:

wx

Now what the hell is this? A NameError I get? But I just imported wx with no errors. Well, my friend, that's my hell that I am going through.

tjohnson

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Jul 22, 2015, 7:15:35 AM7/22/15

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I understand your problem. All of the Python functions that import modules (__import__, importlib.import_module, and imp.load_source) will only return the module as an object. They will NOT add it to the global namespace. Try using this code for your function (obtained from this StackOverflow answer):

def tryToImport(module):
    try:
        globals()[module] = __import__(module)
    except:

print("The import failed.")

Boštjan Mejak

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Jul 22, 2015, 1:55:59 PM7/22/15

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Your code doesn't work. Try importing sys using your code in the interactive interpreter and then type sys and hit Enter. You'll get a NameError as if sys wasn't imported at all.

I'm struggling with this for days now.

Tim Roberts

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Jul 22, 2015, 3:59:07 PM7/22/15

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Boštjan Mejak wrote:

Your code doesn't work. Try importing sys using your code in the interactive interpreter and then type sys and hit Enter. You'll get a NameError as if sys wasn't imported at all.

I'm struggling with this for days now.

Days? Seriously? Consider this:

def one(): x = 3 one() print( x )
That fails in exactly the same way for exactly the same reason. Can you see why?

-- 
Tim Roberts, ti...@probo.com
Providenza & Boekelheide, Inc.

Cody

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Jul 22, 2015, 4:00:37 PM7/22/15

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Hi,

Not sure why you want to try and conditionally import the sys module. It will always exist never going to fail. Anyway your problem just appears to be a fundamental misunderstanding of between what a variable is and the value that is assigned to the variable..

The import functions return an instance of the module object, this instance not automatically bound to a name in the global namespace like is done when using the normal import keyword statement.

Also unlike the "import" statement ( import foo) the import function takes a string as an argument not an unbound variable ( import_module("moduleName") vs import moduleName ) the later keyword statement has special hook that calls the import function and then assigns the loaded object to the named variable in the global namespace.

You, probably would have been able to catch this mistake in your earlier attempts if you had actually looked at the message in the exception that was generated as it would have have said something an failed to import "module" where "module" was literally the name of the module it tried to import and not the _value_ of the string variable you had passed in.

Output from interactive session:

* First note that import_module returns the module as a local variable (I assigned the "sys" module to variable 'x')

* dir shows that it is the contents of the sys module

* Then assign x to "sys" in globals, there will now be a global "sys" variable that references the sys module

* dir(sys) now shows that the global "sys" is the sys module

Python 2.7.9 (default, Dec 10 2014, 12:24:55) [MSC v.1500 32 bit (Intel)] on win

32

Type "help", "copyright", "credits" or "license" for more information.

>>> import importlib

>>> x = importlib.import_module("sys")

>>> dir(x)

['__displayhook__', '__doc__', '__excepthook__', '__name__', '__package__', '__s

tderr__', '__stdin__', '__stdout__', '_clear_type_cache', '_current_frames', '_g

etframe', '_mercurial', 'api_version', 'argv', 'builtin_module_names', 'byteorde

r', 'call_tracing', 'callstats', 'copyright', 'displayhook', 'dllhandle', 'dont_

write_bytecode', 'exc_clear', 'exc_info', 'exc_type', 'excepthook', 'exec_prefix

', 'executable', 'exit', 'flags', 'float_info', 'float_repr_style', 'getcheckint

erval', 'getdefaultencoding', 'getfilesystemencoding', 'getprofile', 'getrecursi

onlimit', 'getrefcount', 'getsizeof', 'gettrace', 'getwindowsversion', 'hexversi

on', 'long_info', 'maxint', 'maxsize', 'maxunicode', 'meta_path', 'modules', 'pa

th', 'path_hooks', 'path_importer_cache', 'platform', 'prefix', 'ps1', 'ps2', 'p

y3kwarning', 'setcheckinterval', 'setprofile', 'setrecursionlimit', 'settrace',

'stderr', 'stdin', 'stdout', 'subversion', 'version', 'version_info', 'warnoptio

ns', 'winver']

>>> globals()["sys"] = x

>>> dir(sys)