I have a string, say: abc.def.ghi.jkl.myfile.mymethod. How do I dynamically import mymethod?
Here is how I went about it:
def get_method_from_file(full_path):
if len(full_path) == 1:
return map(__import__,[full_path[0]])[0]
return getattr(get_method_from_file(full_path[:-1]),full_path[-1])
if __name__=='__main__':
print get_method_from_file('abc.def.ghi.jkl.myfile.mymethod'.split('.'))
I am wondering if the importing individual modules is required at all.
Edit: I am using Python version 2.6.5.
From Python 2.7 you can use the importlib.import_module() function. You can import a module and access an object defined within it with the following code:
from importlib import import_module
p, m = name.rsplit('.', 1)
mod = import_module(p)
met = getattr(mod, m)
met()
You don't need to import the individual modules. It is enough to import the module you want to import a name from and provide the fromlist argument:
def import_from(module, name):
module = __import__(module, fromlist=[name])
return getattr(module, name)
For your example abc.def.ghi.jkl.myfile.mymethod, call this function as
import_from("abc.def.ghi.jkl.myfile", "mymethod")
(Note that module-level functions are called functions in Python, not methods.)
For such a simple task, there is no advantage in using the importlib module.
For Python < 2.7 the builtin method __ import__ can be used:
__import__('abc.def.ghi.jkl.myfile.mymethod', fromlist=[''])
For Python >= 2.7 or 3.1 the convenient method importlib.import_module has been added. Just import your module like this:
importlib.import_module('abc.def.ghi.jkl.myfile.mymethod')
Update: Updated version according to comments (I must admit I didn't read the string to be imported till the end and I missed the fact that a method of a module should be imported and not a module itself):
Python < 2.7 :
mymethod = getattr(__import__("abc.def.ghi.jkl.myfile", fromlist=["mymethod"]))
Python >= 2.7:
mymethod = getattr(importlib.import_module("abc.def.ghi.jkl.myfile"), "mymethod")
from importlib import import_module
name = "file.py".strip('.py')
# if Path like : "path/python/file.py"
# use name.replaces("/",".")
imp = import_module(name)
# get Class From File.py
model = getattr(imp, "classNameImportFromFile")
NClass = model() # Class From file
It's unclear what you are trying to do to your local namespace. I assume you want just my_method as a local, typing output = my_method()?
# This is equivalent to "from a.b.myfile import my_method"
the_module = importlib.import_module("a.b.myfile")
same_module = __import__("a.b.myfile")
# import_module() and __input__() only return modules
my_method = getattr(the_module, "my_method")
# or, more concisely,
my_method = getattr(__import__("a.b.myfile"), "my_method")
output = my_method()
While you only add my_method to the local namespace, you do load the chain of modules. You can look at changes by watching the keys of sys.modules before and after the import. I hope this is clearer and more accurate than your other answers.
For completeness, this is how you add the whole chain.
# This is equivalent to "import a.b.myfile"
a = __import__("a.b.myfile")
also_a = importlib.import_module("a.b.myfile")
output = a.b.myfile.my_method()
# This is equivalent to "from a.b import myfile"
myfile = __import__("a.b.myfile", fromlist="a.b")
also_myfile = importlib.import_module("a.b.myfile", "a.b")
output = myfile.my_method()
And, finally, if you are using __import__() and have modified you search path after the program started, you may need to use __import__(normal args, globals=globals(), locals=locals()). The why is a complex discussion.
This website has a nice solution: load_class. I use it like this:
foo = load_class(package.subpackage.FooClass)()
type(foo) # returns FooClass
As requested, here is the code from the web link:
import importlib
def load_class(full_class_string):
"""
dynamically load a class from a string
"""
class_data = full_class_string.split(".")
module_path = ".".join(class_data[:-1])
class_str = class_data[-1]
module = importlib.import_module(module_path)
# Finally, we retrieve the Class
return getattr(module, class_str)
Use importlib (2.7+ only).
The way I tend to to this (as well as a number of other libraries, such as pylons and paste, if my memory serves me correctly) is to separate the module name from the function/attribute name by using a ':' between them. See the following example:
'abc.def.ghi.jkl.myfile:mymethod'
This makes the import_from(path) function below a little easier to use.
def import_from(path):
"""
Import an attribute, function or class from a module.
:attr path: A path descriptor in the form of 'pkg.module.submodule:attribute'
:type path: str
"""
path_parts = path.split(':')
if len(path_parts) < 2:
raise ImportError("path must be in the form of pkg.module.submodule:attribute")
module = __import__(path_parts[0], fromlist=path_parts[1])
return getattr(module, path_parts[1])
if __name__=='__main__':
func = import_from('a.b.c.d.myfile:mymethod')
func()
How about this :
def import_module(name):
mod = __import__(name)
for s in name.split('.')[1:]:
mod = getattr(mod, s)
return mod
Related
I have some code in the form of a string and would like to make a module out of it without writing to disk.
When I try using imp and a StringIO object to do this, I get:
>>> imp.load_source('my_module', '', StringIO('print "hello world"'))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: load_source() argument 3 must be file, not instance
>>> imp.load_module('my_module', StringIO('print "hello world"'), '', ('', '', 0))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: load_module arg#2 should be a file or None
How can I create the module without having an actual file? Alternatively, how can I wrap a StringIO in a file without writing to disk?
UPDATE:
NOTE: This issue is also a problem in python3.
The code I'm trying to load is only partially trusted. I've gone through it with ast and determined that it doesn't import anything or do anything I don't like, but I don't trust it enough to run it when I have local variables running around that could get modified, and I don't trust my own code to stay out of the way of the code I'm trying to import.
I created an empty module that only contains the following:
def load(code):
# Delete all local variables
globals()['code'] = code
del locals()['code']
# Run the code
exec(globals()['code'])
# Delete any global variables we've added
del globals()['load']
del globals()['code']
# Copy k so we can use it
if 'k' in locals():
globals()['k'] = locals()['k']
del locals()['k']
# Copy the rest of the variables
for k in locals().keys():
globals()[k] = locals()[k]
Then you can import mymodule and call mymodule.load(code). This works for me because I've ensured that the code I'm loading does not use globals. Also, the global keyword is only a parser directive and can't refer to anything outside of the exec.
This really is way too much work to import the module without writing to disk, but if you ever want to do this, I believe it's the best way.
Here is how to import a string as a module (Python 2.x):
import sys,imp
my_code = 'a = 5'
mymodule = imp.new_module('mymodule')
exec my_code in mymodule.__dict__
In Python 3, exec is a function, so this should work:
import sys,imp
my_code = 'a = 5'
mymodule = imp.new_module('mymodule')
exec(my_code, mymodule.__dict__)
Now access the module attributes (and functions, classes etc) as:
print(mymodule.a)
>>> 5
To ignore any next attempt to import, add the module to sys:
sys.modules['mymodule'] = mymodule
imp.new_module is deprecated since python 3.4, but it still works as of python 3.9
imp.new_module was replaced with importlib.util.module_from_spec
importlib.util.module_from_spec
is preferred over using types.ModuleType to create a new module as
spec is used to set as many import-controlled attributes on the module
as possible.
importlib.util.spec_from_loader
uses available loader APIs, such as InspectLoader.is_package(), to
fill in any missing information on the spec.
these module attributes are __builtins__ __doc__ __loader__ __name__ __package__ __spec__
import sys, importlib.util
def import_module_from_string(name: str, source: str):
"""
Import module from source string.
Example use:
import_module_from_string("m", "f = lambda: print('hello')")
m.f()
"""
spec = importlib.util.spec_from_loader(name, loader=None)
module = importlib.util.module_from_spec(spec)
exec(source, module.__dict__)
sys.modules[name] = module
globals()[name] = module
# demo
# note: "if True:" allows to indent the source string
import_module_from_string('hello_module', '''if True:
def hello():
print('hello')
''')
hello_module.hello()
You could simply create a Module object and stuff it into sys.modules and put your code inside.
Something like:
import sys
from types import ModuleType
mod = ModuleType('mymodule')
sys.modules['mymodule'] = mod
exec(mycode, mod.__dict__)
If the code for the module is in a string, you can forgo using StringIO and use it directly with exec, as illustrated below with a file named dynmodule.py.
Works in Python 2 & 3.
from __future__ import print_function
class _DynamicModule(object):
def load(self, code):
execdict = {'__builtins__': None} # optional, to increase safety
exec(code, execdict)
keys = execdict.get(
'__all__', # use __all__ attribute if defined
# else all non-private attributes
(key for key in execdict if not key.startswith('_')))
for key in keys:
setattr(self, key, execdict[key])
# replace this module object in sys.modules with empty _DynamicModule instance
# see Stack Overflow question:
# https://stackoverflow.com/questions/5365562/why-is-the-value-of-name-changing-after-assignment-to-sys-modules-name
import sys as _sys
_ref, _sys.modules[__name__] = _sys.modules[__name__], _DynamicModule()
if __name__ == '__main__':
import dynmodule # name of this module
import textwrap # for more readable code formatting in sample string
# string to be loaded can come from anywhere or be generated on-the-fly
module_code = textwrap.dedent("""\
foo, bar, baz = 5, 8, 2
def func():
return foo*bar + baz
__all__ = 'foo', 'bar', 'func' # 'baz' not included
""")
dynmodule.load(module_code) # defines module's contents
print('dynmodule.foo:', dynmodule.foo)
try:
print('dynmodule.baz:', dynmodule.baz)
except AttributeError:
print('no dynmodule.baz attribute was defined')
else:
print('Error: there should be no dynmodule.baz module attribute')
print('dynmodule.func() returned:', dynmodule.func())
Output:
dynmodule.foo: 5
no dynmodule.baz attribute was defined
dynmodule.func() returned: 42
Setting the '__builtins__' entry to None in the execdict dictionary prevents the code from directly executing any built-in functions, like __import__, and so makes running it safer. You can ease that restriction by selectively adding things to it you feel are OK and/or required.
It's also possible to add your own predefined utilities and attributes which you'd like made available to the code thereby creating a custom execution context for it to run in. That sort of thing can be useful for implementing a "plug-in" or other user-extensible architecture.
you could use exec or eval to execute python code as a string. see here, here and here
The documentation for imp.load_source says (my emphasis):
The file argument is the source file, open for reading as text, from the beginning. It must currently be a real file object, not a user-defined class emulating a file.
... so you may be out of luck with this method, I'm afraid.
Perhaps eval would be enough for you in this case?
This sounds like a rather surprising requirement, though - it might help if you add some more to your question about the problem you're really trying to solve.
I want to let the user to choose which file to open using a string variable. Basically I want to learn how can I tell Python to use a variable in a code section.
I have the following code:
def call_file(fn1):
import fn1
filename = input("Name of the file to import")
call_file(filename)
And inside the same folder i have the helloWorld.py with:
print("hello world")
As you have found, the import statement does not accomplish what you need. Try this instead:
from importlib import import_module
def call_file(fn1):
return import_module(fn1)
filename = input("Name of the file to import: ")
usermodule = call_file(filename)
The import_module function allows you to import a module given as an argument. The python docs have more information on this function.
Example
Running under ipython, we can use the code above to import the os module and access it under the name usermodule:
In [3]: run t.py
Name of the file to import: os
In [4]: usermodule.stat('t.py')
Out[4]: os.stat_result(st_mode=33200, st_ino=97969455, st_dev=2066, st_nlink=1, st_uid=5501, st_gid=5501, st_size=196, st_atime=1462081283, st_mtime=1462081283, st_ctime=1462081283)
Improvement
If the file the user asks for cannot be imported, the code should handle the error, possibly like this:
try:
usermodule = call_file(filename)
except ImportError:
print('Sorry, that file could not be imported.')
Alternative
It is also possible to import modules from a variable name using __import__:
>>> mod = 'math'
>>> new = __import__(mod)
>>> new.cos(0)
1.0
Note, however, that the python documentation frowns on this:
Direct use of __import__() is also discouraged in favor of
importlib.import_module().
You could also use the sys module to achieve the same effect as importing a module as some other name.
import sys
def my_import(name):
__import__(name)
return sys.modules[name]
module = my_import('random') #just for testing
print module.randint(0,1) #just for testing
Below code can be used to grab module at certain depths!
def my_import(name):
m = __import__(name)
for n in name.split(".")[1:]:
m = getattr(m, n)
return m
m = __import__("xml.etree.ElementTree") # returns xml
m = my_import("xml.etree.ElementTree") # returns ElementTree
I have some code in the form of a string and would like to make a module out of it without writing to disk.
When I try using imp and a StringIO object to do this, I get:
>>> imp.load_source('my_module', '', StringIO('print "hello world"'))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: load_source() argument 3 must be file, not instance
>>> imp.load_module('my_module', StringIO('print "hello world"'), '', ('', '', 0))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: load_module arg#2 should be a file or None
How can I create the module without having an actual file? Alternatively, how can I wrap a StringIO in a file without writing to disk?
UPDATE:
NOTE: This issue is also a problem in python3.
The code I'm trying to load is only partially trusted. I've gone through it with ast and determined that it doesn't import anything or do anything I don't like, but I don't trust it enough to run it when I have local variables running around that could get modified, and I don't trust my own code to stay out of the way of the code I'm trying to import.
I created an empty module that only contains the following:
def load(code):
# Delete all local variables
globals()['code'] = code
del locals()['code']
# Run the code
exec(globals()['code'])
# Delete any global variables we've added
del globals()['load']
del globals()['code']
# Copy k so we can use it
if 'k' in locals():
globals()['k'] = locals()['k']
del locals()['k']
# Copy the rest of the variables
for k in locals().keys():
globals()[k] = locals()[k]
Then you can import mymodule and call mymodule.load(code). This works for me because I've ensured that the code I'm loading does not use globals. Also, the global keyword is only a parser directive and can't refer to anything outside of the exec.
This really is way too much work to import the module without writing to disk, but if you ever want to do this, I believe it's the best way.
Here is how to import a string as a module (Python 2.x):
import sys,imp
my_code = 'a = 5'
mymodule = imp.new_module('mymodule')
exec my_code in mymodule.__dict__
In Python 3, exec is a function, so this should work:
import sys,imp
my_code = 'a = 5'
mymodule = imp.new_module('mymodule')
exec(my_code, mymodule.__dict__)
Now access the module attributes (and functions, classes etc) as:
print(mymodule.a)
>>> 5
To ignore any next attempt to import, add the module to sys:
sys.modules['mymodule'] = mymodule
imp.new_module is deprecated since python 3.4, but it still works as of python 3.9
imp.new_module was replaced with importlib.util.module_from_spec
importlib.util.module_from_spec
is preferred over using types.ModuleType to create a new module as
spec is used to set as many import-controlled attributes on the module
as possible.
importlib.util.spec_from_loader
uses available loader APIs, such as InspectLoader.is_package(), to
fill in any missing information on the spec.
these module attributes are __builtins__ __doc__ __loader__ __name__ __package__ __spec__
import sys, importlib.util
def import_module_from_string(name: str, source: str):
"""
Import module from source string.
Example use:
import_module_from_string("m", "f = lambda: print('hello')")
m.f()
"""
spec = importlib.util.spec_from_loader(name, loader=None)
module = importlib.util.module_from_spec(spec)
exec(source, module.__dict__)
sys.modules[name] = module
globals()[name] = module
# demo
# note: "if True:" allows to indent the source string
import_module_from_string('hello_module', '''if True:
def hello():
print('hello')
''')
hello_module.hello()
You could simply create a Module object and stuff it into sys.modules and put your code inside.
Something like:
import sys
from types import ModuleType
mod = ModuleType('mymodule')
sys.modules['mymodule'] = mod
exec(mycode, mod.__dict__)
If the code for the module is in a string, you can forgo using StringIO and use it directly with exec, as illustrated below with a file named dynmodule.py.
Works in Python 2 & 3.
from __future__ import print_function
class _DynamicModule(object):
def load(self, code):
execdict = {'__builtins__': None} # optional, to increase safety
exec(code, execdict)
keys = execdict.get(
'__all__', # use __all__ attribute if defined
# else all non-private attributes
(key for key in execdict if not key.startswith('_')))
for key in keys:
setattr(self, key, execdict[key])
# replace this module object in sys.modules with empty _DynamicModule instance
# see Stack Overflow question:
# https://stackoverflow.com/questions/5365562/why-is-the-value-of-name-changing-after-assignment-to-sys-modules-name
import sys as _sys
_ref, _sys.modules[__name__] = _sys.modules[__name__], _DynamicModule()
if __name__ == '__main__':
import dynmodule # name of this module
import textwrap # for more readable code formatting in sample string
# string to be loaded can come from anywhere or be generated on-the-fly
module_code = textwrap.dedent("""\
foo, bar, baz = 5, 8, 2
def func():
return foo*bar + baz
__all__ = 'foo', 'bar', 'func' # 'baz' not included
""")
dynmodule.load(module_code) # defines module's contents
print('dynmodule.foo:', dynmodule.foo)
try:
print('dynmodule.baz:', dynmodule.baz)
except AttributeError:
print('no dynmodule.baz attribute was defined')
else:
print('Error: there should be no dynmodule.baz module attribute')
print('dynmodule.func() returned:', dynmodule.func())
Output:
dynmodule.foo: 5
no dynmodule.baz attribute was defined
dynmodule.func() returned: 42
Setting the '__builtins__' entry to None in the execdict dictionary prevents the code from directly executing any built-in functions, like __import__, and so makes running it safer. You can ease that restriction by selectively adding things to it you feel are OK and/or required.
It's also possible to add your own predefined utilities and attributes which you'd like made available to the code thereby creating a custom execution context for it to run in. That sort of thing can be useful for implementing a "plug-in" or other user-extensible architecture.
you could use exec or eval to execute python code as a string. see here, here and here
The documentation for imp.load_source says (my emphasis):
The file argument is the source file, open for reading as text, from the beginning. It must currently be a real file object, not a user-defined class emulating a file.
... so you may be out of luck with this method, I'm afraid.
Perhaps eval would be enough for you in this case?
This sounds like a rather surprising requirement, though - it might help if you add some more to your question about the problem you're really trying to solve.
I'm using Python for a weeks now and i'm confronted to an issue with dynamic import.
I have a file Test.py that in which a class is defined. I would like to use this class after the dynamic import of Test.py from another file.
My final goal is more complex but I simplified it but i still get the same problem.
File : Test.py
class Test :
def __init__ ( self ) :
print ( "instance" )
File : Main.py
def allImports ( ) :
__import__ ( "Test" )
What i get :
>>> import Main
>>> Main.allImports()
>>> myInstance = Test ()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'Test' is not defined
I cannot specify in the fromlist which element from Test.py i have to import because i'm not supposed to know them.
What should i do ?
For a solution closer to your intent:
import importlib
def allImports(globals):
mod = importlib.import_module('Test', globals['__name__'])
try:
keys = mod.__all__
except AttributeError:
keys = dir(mod)
for key in keys:
if not key.startswith('_'):
globals[key] = getattr(mod, key)
# …
allImports(globals())
Test # should work, you can also look into dir(Test) to find the class.
If your module doesn't have an __all__ the code above /will/ clobber your namespace something fierce. Either make sure you define __all__, or modify allImports() to only import the things you want. (E.g. only classes, or only classes defined in the module. This part really depends on your use case.)
When using __import__() to load a module, you have to look it up in sys.modules:
>>> import sys
>>> import Main
>>> Main.allImports()
>>> myInstance = sys.modules['Test'].Test()
instance
>>>
More information in the documentation and here, here, and here.
this code makes __import__ ( "Test" ) a local variable, so you can't access it outside the function.
def allImports ( ) :
__import__ ( "Test" )
try:
def allImports ( ) :
test= __import__ ( "Test" )
return test #return the module
>>> import Main
>>> x=Main.allImports() #store the returned module in x
>>> myInstance = x.Test ()
instance
>>>myInstance
<Test.Test instance at 0x011D7F80>
__import__ doesn't modify magically neither global nor local namespaces.
Modules and classes are first class citizens in Python i.e., you can use them as any other object in Python (bind to a name, pass as a parameter to a function, return as a value from a function).
def allImports():
return __import__("Test")
Test_module = allImports()
Test = Test_module.Test # class
test_instance = Test()
If the above code is inside a function then to put Test into global namespace: globals()['Test'] = Test. Note most probably you don't need it and there are better ways to do whatever you want without modifying global namespace inside a function.
Usage of __import__() is discouraged use importlib.import_module() instead.
If the name of the module and the class are known you could just write at the module level:
from Test import Test
Given a string of a Python class, e.g. my_package.my_module.MyClass, what is the best possible way to load it?
In other words I am looking for a equivalent Class.forName() in Java, function in Python. It needs to work on Google App Engine.
Preferably this would be a function that accepts the FQN of the class as a string, and returns a reference to the class:
my_class = load_class('my_package.my_module.MyClass')
my_instance = my_class()
From the python documentation, here's the function you want:
def my_import(name):
components = name.split('.')
mod = __import__(components[0])
for comp in components[1:]:
mod = getattr(mod, comp)
return mod
The reason a simple __import__ won't work is because any import of anything past the first dot in a package string is an attribute of the module you're importing. Thus, something like this won't work:
__import__('foo.bar.baz.qux')
You'd have to call the above function like so:
my_import('foo.bar.baz.qux')
Or in the case of your example:
klass = my_import('my_package.my_module.my_class')
some_object = klass()
EDIT: I was a bit off on this. What you're basically wanting to do is this:
from my_package.my_module import my_class
The above function is only necessary if you have a empty fromlist. Thus, the appropriate call would be like this:
mod = __import__('my_package.my_module', fromlist=['my_class'])
klass = getattr(mod, 'my_class')
If you don't want to roll your own, there is a function available in the pydoc module that does exactly this:
from pydoc import locate
my_class = locate('my_package.my_module.MyClass')
The advantage of this approach over the others listed here is that locate will find any python object at the provided dotted path, not just an object directly within a module. e.g. my_package.my_module.MyClass.attr.
If you're curious what their recipe is, here's the function:
def locate(path, forceload=0):
"""Locate an object by name or dotted path, importing as necessary."""
parts = [part for part in split(path, '.') if part]
module, n = None, 0
while n < len(parts):
nextmodule = safeimport(join(parts[:n+1], '.'), forceload)
if nextmodule: module, n = nextmodule, n + 1
else: break
if module:
object = module
else:
object = __builtin__
for part in parts[n:]:
try:
object = getattr(object, part)
except AttributeError:
return None
return object
It relies on pydoc.safeimport function. Here are the docs for that:
"""Import a module; handle errors; return None if the module isn't found.
If the module *is* found but an exception occurs, it's wrapped in an
ErrorDuringImport exception and reraised. Unlike __import__, if a
package path is specified, the module at the end of the path is returned,
not the package at the beginning. If the optional 'forceload' argument
is 1, we reload the module from disk (unless it's a dynamic extension)."""
import importlib
module = importlib.import_module('my_package.my_module')
my_class = getattr(module, 'MyClass')
my_instance = my_class()
If you're using Django you can use import_string.
Yes i'm aware OP did not ask for django, but i ran across this question looking for a Django solution, didn't find one, and put it here for the next boy/gal that looks for it.
# It's available for v1.7+
# https://github.com/django/django/blob/stable/1.7.x/django/utils/module_loading.py
from django.utils.module_loading import import_string
Klass = import_string('path.to.module.Klass')
func = import_string('path.to.module.func')
var = import_string('path.to.module.var')
Keep in mind, if you want to import something that doesn't have a ., like re or argparse use:
re = __import__('re')
def import_class(cl):
d = cl.rfind(".")
classname = cl[d+1:len(cl)]
m = __import__(cl[0:d], globals(), locals(), [classname])
return getattr(m, classname)
Here is to share something I found on __import__ and importlib while trying to solve this problem.
I am using Python 3.7.3.
When I try to get to the class d in module a.b.c,
mod = __import__('a.b.c')
The mod variable refer to the top namespace a.
So to get to the class d, I need to
mod = getattr(mod, 'b') #mod is now module b
mod = getattr(mod, 'c') #mod is now module c
mod = getattr(mod, 'd') #mod is now class d
If we try to do
mod = __import__('a.b.c')
d = getattr(mod, 'd')
we are actually trying to look for a.d.
When using importlib, I suppose the library has done the recursive getattr for us. So, when we use importlib.import_module, we actually get a handle on the deepest module.
mod = importlib.import_module('a.b.c') #mod is module c
d = getattr(mod, 'd') #this is a.b.c.d
OK, for me that is the way it worked (I am using Python 2.7):
a = __import__('file_to_import', globals(), locals(), ['*'], -1)
b = a.MyClass()
Then, b is an instance of class 'MyClass'
If you happen to already have an instance of your desired class, you can use the 'type' function to extract its class type and use this to construct a new instance:
class Something(object):
def __init__(self, name):
self.name = name
def display(self):
print(self.name)
one = Something("one")
one.display()
cls = type(one)
two = cls("two")
two.display()
Python has an inbuilt library importlib to get the job done. :, How to access module method and class method dynamically bypassing package name as a param. An example is given below.
Module 1:
def get_scenario_data():
return "module1 scenario data"
class Module1:
def module1_function1(self):
return "module1_function"
def module1_function2(self):
return "module2_function"
Module 2:
def get_scenario_data():
return "module2 scenario data"
class Module2:
def module2_function1(self):
return "module2_function1"
def module2_function2(self):
return "module2_function2"
ModuleTest:
Will access the module methods dynamically based on the package name as param
Will access the class methods dynamically based on the package name as param.
ModuleTest
import importlib
module = importlib.import_module('pack1.nestedpack1.module1')
print(module.get_scenario_data())
modul1_cls_obj = getattr(module, 'Module1')()
print(modul1_cls_obj.module1_function1())
print(modul1_cls_obj.module1_function2())
module = importlib.import_module('pack1.nestedpack1.module2')
modul2_cls_obj = getattr(module, 'Module2')()
print(modul2_cls_obj.module2_function1())
print(modul2_cls_obj.module2_function2())
print(module.get_scenario_data())
Results
module1 scenario data
module1_function
module2_function
module2_function1
module2_function2
module2 scenario data
PyPI module autoloader & import
# 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)
Adding a bit of sophistication to the existing answers....
Depending on the use case, it may be somewhat inconvenient to have to explicitly specify the full path (E.g. package.subpackage.module...) of the class/method you want to import. On top of importlib, we can leverage __init__.py to make things even cleaner.
Let's say I have a python package, like so:
├── modes
│ ├── __init__.py
│ ├── bar.py
│ ├── foo.py
│ ├── modes.py
foo.py, say, have some class/functions we'd like to use somewhere else in our program:
from modes.modes import Mode
class Foo(Mode):
def __init__(self, *arg, **kwargs):
super(Foo, self).__init__(*arg, **kwargs)
def run(self):
self.LOG.info(f"This is FOO!")
With a command line argument, I can pass an argument that corresponds to a mode that I want to run. I'd like to be able to so something like this:
def set_mode(mode):
""" """
import importlib
module = importlib.import_module('modes.foo')
getattr(module, mode)().run()
which outputs:
>> set_mode("Foo")
>> engine_logger:INFO - This is FOO!
That works fine, however what we'd REALLY want to get at is this:
def set_mode(mode):
""" """
import importlib
module = importlib.import_module('modes') # only import the package, not modules explicitely
getattr(module, mode)().run()
Which raises an error:
>> set_mode("Foo")
>> AttributeError: module 'modes' has no attribute 'Foo'
However, we can add the following to /modes/__init__.py:
from .foo import Foo
from .bar import Bar
Then, we can do:
>> set_mode("Foo")
>> engine_logger:INFO - This is FOO!
>> set_mode("Bar")
>> engine_logger:INFO - This is BAR!
In other worlds, all sub modules/functions/classes we import in init.py will be found directly with importlib.import_module(...), without having to specify the full path from outside.
In Google App Engine there is a webapp2 function called import_string. For more info see here:https://webapp-improved.appspot.com/api/webapp2.html
So,
import webapp2
my_class = webapp2.import_string('my_package.my_module.MyClass')
For example this is used in the webapp2.Route where you can either use a handler or a string.
module = __import__("my_package/my_module")
the_class = getattr(module, "MyClass")
obj = the_class()