CODEWITHSUNDEEP
pythonpython-import
theta
theta

Reputation: 25631

Importing module from string variable using "__import__" gives different results than a normal import statement

I'm working on a documentation (personal) for nested matplotlib (MPL) library, which differs from MPL own provided, by interested submodule packages. I'm writing Python script which I hope will automate document generation from future MPL releases.

I selected interested submodules/packages and want to list their main classes from which I'll generate list and process it with pydoc.

The problem is that I can't find a way to instruct Python to load a submodule from a string. Here is an example of what I tried:

import matplotlib.text as text
x = dir(text)

i = __import__('matplotlib.text')
y = dir(i)

j = __import__('matplotlib')
z = dir(j)

And here is a 3-way comparison of above lists through pprint:

enter image description here

I don't understand what's loaded in y object - it's base matplotlib plus something else, but it lacks information that I wanted and that is main classes from matplotlib.text package. It's the top blue coloured part on screenshot (x list).

Upvotes: 268

Views: 238761

Answers (8)

Velda
Velda

Reputation: 617

from <mymodule> import x, y, z

Since I do not see a safe solution mimicking from <mymodule> import x, y, z, where mymodule is given as user input, this is what I suggest:

mymodule = input()

# pseudocode: from <mymodule> import x, y, z
symbols = ['x', 'y', 'z']
mymodule_ = __import__(mymodule, fromlist=symbols)
for s in symbols:
    globals()[s] = eval("mymodule_.%s" % s)

Explanation:

One cannot subscript a module in Python (e.g., mymodule_[symbols[1]]), therefore need for an eval/exec function. We do not want a user input there as a safety measure though. Thus it is done in two steps: first, import the module; second, expose the symbols.

Upvotes: 0

gecco
gecco

Reputation: 18850

importlib.import_module is what you are looking for. It returns the imported module.

import importlib

# equiv. of your `import matplotlib.text as text`
text = importlib.import_module('matplotlib.text')

You can thereafter access anything in the module as text.myclass, text.myfunction, etc.

Upvotes: 119

mzjn
mzjn

Reputation: 51002

The __import__ function can be a bit hard to understand.

If you change

i = __import__('matplotlib.text')

to

i = __import__('matplotlib.text', fromlist=[''])

then i will refer to matplotlib.text.

In Python 3.1 or later, you can use importlib:

import importlib

i = importlib.import_module("matplotlib.text")

Some notes

  • If you're trying to import something from a sub-folder e.g. ./feature/email.py, the code will look like importlib.import_module("feature.email")

  • Before Python 3.3 you could not import anything if there was no __init__.py in the folder with file you were trying to import (see caveats before deciding if you want to keep the file for backward compatibility e.g. with pytest).

Upvotes: 361

Danilo Matrangolo Marano
Danilo Matrangolo Marano

Reputation: 952

You can also use exec built-in function that execute any string as a Python code.

In [1]: module = 'pandas'
   ...: function = 'DataFrame'
   ...: alias = 'DF'

In [2]: exec(f"from {module} import {function} as {alias}")

In [3]: DF
Out[3]: pandas.core.frame.DataFrame

For me this was the most readable way to solve my problem.

Upvotes: 3

Serge Stroobandt
Serge Stroobandt

Reputation: 31568

Module auto-install & import from list

Below script works fine with both submodules and pseudo submodules.

# PyPI imports
import pkg_resources, subprocess, sys

modules   = {'lxml.etree', 'pandas', 'screeninfo'}
required  = {m.split('.')[0] for m in modules}
installed = {pkg.key for pkg in pkg_resources.working_set}
missing   = required - installed

if missing:
    subprocess.check_call([sys.executable, '-m', 'pip', 'install', '--upgrade', 'pip'])
    subprocess.check_call([sys.executable, '-m', 'pip', 'install', *missing])

for module in set.union(required, modules):
    globals()[module] = __import__(module)

Tests:

print(pandas.__version__)
print(lxml.etree.LXML_VERSION)

Upvotes: 1

arshovon
arshovon

Reputation: 13661

Apart from using the importlib one can also use exec method to import a module from a string variable.

Here I am showing an example of importing the combinations method from itertools package using the exec method:

MODULES = [
    ['itertools','combinations'],
]

for ITEM in MODULES:
    import_str = "from {0} import {1}".format(ITEM[0],', '.join(str(i) for i in ITEM[1:]))
    exec(import_str)

ar = list(combinations([1, 2, 3, 4], 2))
for elements in ar:
    print(elements)

Output:

(1, 2)
(1, 3)
(1, 4)
(2, 3)
(2, 4)
(3, 4)

Upvotes: 4

prossblad
prossblad

Reputation: 944

I developed these 3 useful functions:

def loadModule(moduleName):
    module = None
    try:
        import sys
        del sys.modules[moduleName]
    except BaseException as err:
        pass
    try:
        import importlib
        module = importlib.import_module(moduleName)
    except BaseException as err:
        serr = str(err)
        print("Error to load the module '" + moduleName + "': " + serr)
    return module

def reloadModule(moduleName):
    module = loadModule(moduleName)
    moduleName, modulePath = str(module).replace("' from '", "||").replace("<module '", '').replace("'>", '').split("||")
    if (modulePath.endswith(".pyc")):
        import os
        os.remove(modulePath)
        module = loadModule(moduleName)
    return module

def getInstance(moduleName, param1, param2, param3):
    module = reloadModule(moduleName)
    instance = eval("module." + moduleName + "(param1, param2, param3)")
    return instance

And everytime I want to reload a new instance I just have to call getInstance() like this:

myInstance = getInstance("MyModule", myParam1, myParam2, myParam3)

Finally I can call all the functions inside the new Instance:

myInstance.aFunction()

The only specificity here is to customize the params list (param1, param2, param3) of your instance.

Upvotes: 0

keen
keen

Reputation: 850

spent some time trying to import modules from a list, and this is the thread that got me most of the way there - but I didnt grasp the use of ___import____ -

so here's how to import a module from a string, and get the same behavior as just import. And try/except the error case, too. :)

  pipmodules = ['pycurl', 'ansible', 'bad_module_no_beer']
  for module in pipmodules:
      try:
          # because we want to import using a variable, do it this way
          module_obj = __import__(module)
          # create a global object containging our module
          globals()[module] = module_obj
      except ImportError:
          sys.stderr.write("ERROR: missing python module: " + module + "\n")
          sys.exit(1)

and yes, for python 2.7> you have other options - but for 2.6<, this works.

Upvotes: 6

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